Car talk: sonata MPG dropped a lot

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AQ
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Car talk: sonata MPG dropped a lot

Post by AQ »

I drove a Hyndai Sonata 2011. I have been pretty happy with it but with one question recently. Its fuel efficiency had been decent until a couple of months ago. It used to be about 30 mpg with a combination of highway / locals.. But starting a few months ago, its MPG appeared to have a cliff drop to about 20 mpg.. Otherwise the car seems running perfect and no complaints.. Is this to be expected when a car aged? I thought it'd be a more gradual process. Anything I need to do or not? Thanks.
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just frank
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Re: Car talk: sonata MPG dropped a lot

Post by just frank »

Happened to me with an older car....turned out to be the engine thermostat....causing the cooling system to keep the engine too cool all the time. Watch your temp gauge.
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Re: Car talk: sonata MPG dropped a lot

Post by livesoft »

Release the emergency brake all the way.

Is the air pressure in the tires too low?
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DSInvestor
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Re: Car talk: sonata MPG dropped a lot

Post by DSInvestor »

I am the original owner of a 97 Camry and the car gets 30MPG highway for as long as I've owned it.

Here's a link to a car talk post on a sudden drop of MPG which mentions a few possible causes:
http://community.cartalk.com/discussion ... its-or-gas
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Watty
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Re: Car talk: sonata MPG dropped a lot

Post by Watty »

Is it still under the emission or power train warranty? If so I would have the dealer check it out.
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Re: Car talk: sonata MPG dropped a lot

Post by bottlecap »

It would not happen so quickly. Assuming you are doing very similar driving, something is wrong. There are a number of things that could do this suddenly, but the thermostat is a good guess.

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Re: Car talk: sonata MPG dropped a lot

Post by FrugalInvestor »

No, it's not normal. I would expect a vehicle to gradually gain mileage for the first few thousand miles, stay steady for 150,000 or perhaps many more, and then begin to degrade slowly as critical parts begin to wear out. This all assumes a good program of preventative maintenance is followed. Any major change without a change in driving habits or conditions would suggest a problem that should be addressed.
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telemark
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Re: Car talk: sonata MPG dropped a lot

Post by telemark »

It is not to be expected. I have a 2003 Mazda still getting close to its original mileage. I would take it in for a tuneup and mention the problem. With a drop that sudden any competent shop should be able to find the cause.
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Re: Car talk: sonata MPG dropped a lot

Post by Cindyjrn »

That is not normal at all. You have some type of issue requiring a tune up or possibly repair/replacement of an emissions component. There's no way that under inflated tires or bad gas can cause that type of MPG drop. Improper tire pressure along with something like needing an alignment could cause a big drop, but you would likely know that this was an issue by the way your car handles. My bet is something like catalytic converter or O2 sensor or just a run of the mill tune up.
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Re: Car talk: sonata MPG dropped a lot

Post by AQ »

Thanks to all of your comments! Sounds I need to make a trip to the dealership. Hyndai offers 10 year power train warranty and I hope this is covered. Will see.
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Re: Car talk: sonata MPG dropped a lot

Post by bottlecap »

AQ wrote:Thanks to all of your comments! Sounds I need to make a trip to the dealership. Hyndai offers 10 year power train warranty and I hope this is covered. Will see.
Not likely to be covered under a power train warranty, unfortunately. Are there any independent mechanics in your area? I hate to see you have to go to a dealership. It's something to consider that will save you some bucks.

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Re: Car talk: sonata MPG dropped a lot

Post by strafe »

On most modern cars, anything that would affect fuel economy to that degree would trigger the MIL (check engine light).

May 1 is when the fuel supply switches over the summer blend. Some states have very strict requirements. Is that when this started?

Are you calculating the MPG manually (accurate) or relying on the onboard computer (not so accurate)?
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Re: Car talk: sonata MPG dropped a lot

Post by Cindyjrn »

strafe wrote:On most modern cars, anything that would affect fuel economy to that degree would trigger the MIL (check engine light).

May 1 is when the fuel supply switches over the summer blend. Some states have very strict requirements. Is that when this started?

Are you calculating the MPG manually (accurate) or relying on the onboard computer (not so accurate)?
Except that gas mileage is very slightly worse with winter blend, not vice versa.
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Re: Car talk: sonata MPG dropped a lot

Post by fognyc »

I have an Optima GDI that is likely the same engine as you.. the most swing I ever saw was a 6 months loss of mpg from about 32mpg to about 28mpg.. This happened about 60k miles and the bulk of my usage was hwy miles. I resolved that easily with 2 treatments of Techron to clean out the injectors. I would agree with others that there is something significantly wrong.. I would also calculate properly (there are many phone apps out there) and not use the computer which is based on a fuel flow meter built in the car.
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Re: Car talk: sonata MPG dropped a lot

Post by furnace »

strafe wrote:On most modern cars, anything that would affect fuel economy to that degree would trigger the MIL (check engine light).

May 1 is when the fuel supply switches over the summer blend. Some states have very strict requirements. Is that when this started?

Are you calculating the MPG manually (accurate) or relying on the onboard computer (not so accurate)?

This may sound outlandish, but did someone in the OP's family fill up with diesel instead of gasoline? You'll see a lot of smoke coming out. Does the car pass emissions?
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Re: Car talk: sonata MPG dropped a lot

Post by jharkin »

I agree with all the responses so far - A modern car should never see such a dramatic sudden dropoff in mileage. Something is definitely wrong.

A common culprit is a sensor such as the primary O2, MAF (air flow), MAP (manifold pressure), IAT (intake temp) going bad. When one or more of those goes out, the computer cant get all the readings it needs to dynamically optimize fuel delivery on the fly and reverts to a default pre-programmed fuel map. That default map is extra-rich for safety and this is what causes your mileage to drop off.

The thing is, a full on failure of any of those would throw a CEL. I would take the car to a shop and have somebody plug in a code reader to check for errors- maybe the CEL light itself is burned out (rare but stranger things have happened). Lots of auto parts chains (AutoZone, Advance, Napa, CarQuest) will scan it for free.
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Re: Car talk: sonata MPG dropped a lot

Post by jharkin »

furnace wrote:
This may sound outlandish, but did someone in the OP's family fill up with diesel instead of gasoline? You'll see a lot of smoke coming out. Does the car pass emissions?
Should be impossible - the Diesel fuel nozzle is larger than the filler on a gasoline car (on purpose) - this mistake usually happens the other way around.

If you somehow pulled it off, the engine would stop running altogether once diesel made its way to the injectors leaving him stranded on the road side.
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Re: Car talk: sonata MPG dropped a lot

Post by jabberwockOG »

Good idea to take it to parts place and ask them to scan for codes. Assuming no obvious fault codes show up then you need to work through checking easy items first. Like others have posted I'd check and replace as necessary these items in this approximate order:

1. Check air pressure in tires.
2. Replace engine air filter if dirty.
3. Clean MAF with CRC spray MAF cleaner.
4. Clean throttle body and butterfly value with CRC TB spray cleaner and rag.
5. Jack up car and check that each wheel/tire rotates easily with minimal to no noise. Noise or significant grinding sound on rotation or excessive reluctance to move and spin indicates possible bad hub/bearing or caliper/brake pad dragging issue.
6. Replace engine cooling thermostat - engine running too cool with also lower mpg.
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Re: Car talk: sonata MPG dropped a lot

Post by Cindyjrn »

furnace wrote:
strafe wrote:On most modern cars, anything that would affect fuel economy to that degree would trigger the MIL (check engine light).

May 1 is when the fuel supply switches over the summer blend. Some states have very strict requirements. Is that when this started?

Are you calculating the MPG manually (accurate) or relying on the onboard computer (not so accurate)?

This may sound outlandish, but did someone in the OP's family fill up with diesel instead of gasoline? You'll see a lot of smoke coming out. Does the car pass emissions?
You're right, it is outlandish and unhelpful. Diesel nozzles won't fit in any US car made in the last 20 or so years, by design. Plus, diesel doesn't evaporate like gasoline, so the engine would never start as the plugs would have nothing to ignite. Diesel engines work by using extremely high compression to super heat the air in the chamber and directly ignite the diesel fuel. Completely different than gasoline engines. You could get away with maybe putting a very small amount of diesel fuel in a gasoline engine, on accident and provided you could figure out a way to actually get it in the tank. You couldn't start or operate a gasoline vehicle completely filled with diesel.
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Re: Car talk: sonata MPG dropped a lot

Post by Cindyjrn »

The more I think about it, the more I would check tire inflation and alignment. The few things listed that could possibly be bad without throwing a code would never kill the MPG by 33%. More like 2-5%.
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Re: Car talk: sonata MPG dropped a lot

Post by MGBGTV8 »

Could be a bad temperature sender. I had a problem with mine and it failed (open circuit), so it lit the light, and caused problems idling when car was warmed up. If your sender is only "kinda bad" the output could be incorrect leading to poor fuel economy, but reasonable enough to not light the light. Does your exhaust smell a bit "gassy?" It would be more pronounced when the car was warm. On top of all that, the sender for the computer (Bosch Motronic in a Volvo with OBD-I) was separate than the input to the dashboard gauge, so I had normal temp indications, even thought the computer was getting bad input.

You may need to go a bit beyond the presence or absence of a check engine light code.

Let us know how it turns out!
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Re: Car talk: sonata MPG dropped a lot

Post by FrugalInvestor »

Cindyjrn wrote:The more I think about it, the more I would check tire inflation and alignment. The few things listed that could possibly be bad without throwing a code would never kill the MPG by 33%. More like 2-5%.
Tire pressure or alignment wouldn't impact mileage to that extent. If it did you'd be driving on flats and off the road.
Last edited by FrugalInvestor on Mon Jun 27, 2016 12:55 pm, edited 1 time in total.
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Re: Car talk: sonata MPG dropped a lot

Post by SimonJester »

I am having a similar issue with my 2002 corolla, went from low 30s MPG to mid 20s MPG. Connecting an ODBII system to the car I can see long term fuel trim is 23%. Its right below the threshold to throw the MIL code for system lean. I have replaced O2, MAF, check for intake leaks and exhaust leaks. There are two more items I need to check and one is fuel injector o rings and the ECU. For now I am just living with it.

My point is you can be right below the threshold for the ECU to turn on the MIL.


Another possibility switch fuel stations. A friend of mine is a auto mechanic for a large dealer, he has found vehicles with 20%+ ethanol in their fuel when we are only supposed to have something around 10%.
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Re: Car talk: sonata MPG dropped a lot

Post by MindBogler »

There is a lot of good advice in this thread, but I would narrow my search to a problem with low engine temperature.

Of the potential causes of low engine temp, two obvious ones come to mind:

Thermostat - stuck open (these are a simple spring with a copper pellet and they tend to fail either full open or fully closed).
Coolant temp sensor - incorrect (low) temp value

The reason this is most likely to be the case is a story of "open loop" vs "closed loop" engine operation. All fuel injected gasoline vehicles in the last almost 30+ years that are controlled by a computer (engine control module) use these two modes of operation. Open loop is what your car runs in when it is first turned on and the engine coolant temp is below a certain threshold. This mode consists of a pre-programmed, generally static table of fuel values based on RPM. In this mode, the computer ignores many external engine inputs (O2 sensors, etc.) when making fueling decisions. Once the coolant temp reaches a desired value, the ECM drops into closed loop and begins to adjust its fuel tables based on the external inputs. This is how the car determines the optimal amount of fuel for a given workload. If your thermostat is stuck open, it will never get into closed loop or it may be moving in and out of closed loop based on the load on the engine. For example, coasting down a hill you end up with low coolant temp while climbing a hill you end up with higher coolant temp.

I believe your car is stuck in open loop due to one of the above causes. This problem is also the least likely to throw an engine code.
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Re: Car talk: sonata MPG dropped a lot

Post by MindBogler »

I'll add that if you are feeling technically inclined, you can probably diagnose a stuck open thermostat by yourself (easier with 2 people).

1. Ensure the engine is cold (first start of the day)
2. Remove the radiator cap
3. Start engine
4. Look at the coolant (static or moving around)
5. Rev the engine to 2-3k RPM and observe the behavior of coolant (does it drop or start "swirling")?

If the coolant is moving around on step 4 and/or drops on step 5 the thermostat is stuck open. Your water pump is circulating the coolant through the engine which it can only do at cold temp if the thermostat is stuck open.
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Re: Car talk: sonata MPG dropped a lot

Post by Epsilon Delta »

MindBogler wrote:There is a lot of good advice in this thread, but I would narrow my search to a problem with low engine temperature.

Of the potential causes of low engine temp, two obvious ones come to mind:

Thermostat - stuck open (these are a simple spring with a copper pellet and they tend to fail either full open or fully closed).
Coolant temp sensor - incorrect (low) temp value

I believe your car is stuck in open loop due to one of the above causes. This problem is also the least likely to throw an engine code.
I'm surprised it doesn't throw a code. It's worth hooking up a scanner and seeing if it's got a diagnostic that doesn't trip the check engine light. It seems like they should be detecting the failure to warm up and throw a code that leads to a very short checklist:
  • Are you in Antarctica? -- ignore the code.
  • Replace the temperature sensor.
(That's somewhat humorous but I've worked on software for industrial equipment and we aimed to diagnose every single point failure, so we could remote diagnose the problem and send the mechanic out with the right parts. It seems that car manufacturers could put in the same effort.

Occasionally we'd only be able to localize to two possibilities, in which case there was an extra sensor on the next years model. And occasionally our equipment would be used in an industrial freezer, the manual would say to ignore certain codes at 40 below)
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Re: Car talk: sonata MPG dropped a lot

Post by MindBogler »

Epsilon Delta wrote:I'm surprised it doesn't throw a code. It's worth hooking up a scanner and seeing if it's got a diagnostic that doesn't trip the check engine light. It seems like they should be detecting the failure to warm up and throw a code that leads to a very short checklist:
  • Are you in Antarctica? -- ignore the code.
  • Replace the temperature sensor.
(That's somewhat humorous but I've worked on software for industrial equipment and we aimed to diagnose every single point failure, so we could remote diagnose the problem and send the mechanic out with the right parts. It seems that car manufacturers could put in the same effort.

Occasionally we'd only be able to localize to two possibilities, in which case there was an extra sensor on the next years model. And occasionally our equipment would be used in an industrial freezer, the manual would say to ignore certain codes at 40 below)
I think you're giving auto manufacturers too much credit and I would assume your industry is much more rigorous in this regard. :D

At least as recently as 2009 model year vehicles have been capable of this failure mode with the only indication being a drop in mileage.

I agree that taking it for a free code scan would be a quick and wise move given that check engine lights in many models are still a simple incandescent bulb.

The alternative, if neither of these is true, is a malfunctioning engine temp sensor. Diagnosing this requires backprobing the ECT sensor with a multimeter and reading the value when the engine is cold / warm. In order to properly diagnose if the value is correct will require purchasing a repair manual or searching for one available online that contains a table of values per a given temperature.
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Re: Car talk: sonata MPG dropped a lot

Post by Epsilon Delta »

MindBogler wrote: The alternative, if neither of these is true, is a malfunctioning engine temp sensor. Diagnosing this requires backprobing the ECT sensor with a multimeter and reading the value when the engine is cold / warm. In order to properly diagnose if the value is correct will require purchasing a repair manual or searching for one available online that contains a table of values per a given temperature.
I may still be giving the automotive industry too much credit, but the temperature sensor output should be available to an OBDII scanner. Quicker, safer and cleaner than sticking a multimeter in the engine bay. I suppose it doesn't quite localize the problem to the sensor, since there remains the slight possibility of bad wires or the ATOD in the ECU but it certainly narrows it down.

I did deal with ossified mechanics who refused to use the scanner and tried to fix all problems with a lump hammer. Ironically ossified mechanics are one problem that could be fixed with a lump hammer, although I think OSHA frowns on it.
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Re: Car talk: sonata MPG dropped a lot

Post by MindBogler »

Epsilon Delta wrote:I may still be giving the automotive industry too much credit, but the temperature sensor output should be available to an OBDII scanner. Quicker, safer and cleaner than sticking a multimeter in the engine bay.
This is true and you are correct...if you have a more expensive OBD II scanner. I'm not sure if the free scan from the auto supply shop is going to do this. My cheapo scanner only reads and clears codes. Everyone with a car should buy one of these anyway. :beer
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Re: Car talk: sonata MPG dropped a lot

Post by MindBogler »

I just remembered the Torque Pro. I think you can get the OBD2 plug + an android app for under $25. This is a much better solution than using a multimeter.

This is what I'm talking about:

OBD 2 plug in bluetooth sensor
https://www.amazon.com/Panlong-Bluetoot ... torque+pro

Torque Pro app
https://play.google.com/store/apps/deta ... rque&hl=en
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Re: Car talk: sonata MPG dropped a lot

Post by bottlecap »

Epsilon Delta wrote:I'm surprised it doesn't throw a code.
I'm not an expert on this, but have dealt with a similar issue lately and had to troubleshoot the t-stat. My guess is something like a stuck t-stat is unlikely to throw a code because it can be a a mechanical failure of the part. The ECU can't detect that, but simply reads the engine temp at 160 degrees (or some amount less than the normal operating range) and adjusts the fuel mixture for that temperature. The actual temperature may be 160 degree because of the stuck t-stat. The ECU isn't smart enough to say, "Wait a minute, we should be at normal operating temperature by now," and never throws the code. Unless the problem causes something else to go wrong, you get no code.

I could be wrong, but this is my guess.

You can get an OBDII scanner and monitor the coolant temp. Rather than open the radiator cap, you can use the upper radiator hose to see if the t-stat is stuck. Touch it when cold (in the morning after a whole evening) - it should be cold. Then start the car and monitor the coolant temp. Once it gets to 150 degrees or so, touch the hose again. It should be cold. If its warm, you have a problem (stuck "open" I think). If its not warm at 190 degrees, it is stuck closed.

JT
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Re: Car talk: sonata MPG dropped a lot

Post by MindBogler »

bottlecap wrote:You can get an OBDII scanner and monitor the coolant temp. Rather than open the radiator cap, you can use the upper radiator hose to see if the t-stat is stuck. Touch it when cold (in the morning after a whole evening) - it should be cold. Then start the car and monitor the coolant temp. Once it gets to 150 degrees or so, touch the hose again. It should be cold. If its warm, you have a problem (stuck "open" I think). If its not warm at 190 degrees, it is stuck closed.

JT
The problem with diagnosing in this manner is that the water will all eventually reach the same temperature whether it is stuck open or closed. The coolant circulates through the block via the upper and lower radiator hoses. The thermostat blocks the water pump from cycling water through the system at the upper radiator hose, but it will still heat up the entire system via heat exchange from the lower hose to the radiator. For someone who is not experienced, it will be a subtle difference. Opening the radiator cap and following the procedure I listed a few posts back will produce an obvious result, even to someone inexperienced in auto maintenance.
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Re: Car talk: sonata MPG dropped a lot

Post by Cindyjrn »

FrugalInvestor wrote:
Cindyjrn wrote:The more I think about it, the more I would check tire inflation and alignment. The few things listed that could possibly be bad without throwing a code would never kill the MPG by 33%. More like 2-5%.
Tire pressure or alignment wouldn't impact mileage to that extent. If it did you'd be driving on flats and off the road.
You'd be surprised. I'd bet you could easily get close to 7 MPG drop from 30 with very underinflated tires and a poor alignment. You might research what I'm saying, you might see I'm right and that you're off base.
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Re: Car talk: sonata MPG dropped a lot

Post by Epsilon Delta »

Cindyjrn wrote:
FrugalInvestor wrote:
Cindyjrn wrote:The more I think about it, the more I would check tire inflation and alignment. The few things listed that could possibly be bad without throwing a code would never kill the MPG by 33%. More like 2-5%.
Tire pressure or alignment wouldn't impact mileage to that extent. If it did you'd be driving on flats and off the road.
You'd be surprised. I'd bet you could easily get close to 7 MPG drop from 30 with very underinflated tires and a poor alignment. You might research what I'm saying, you might see I'm right and that you're off base.
Yes, I'd be surprised. That's a 23% reduction. A really bad alignment might cost 10% of your milage. A 13% reduction from tire pressure would require tires to be something like 32 psi low. The problems would be visibly obvious, and the handling effects darn scary. Basically pushing flat tires sideways down the road.
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Re: Car talk: sonata MPG dropped a lot

Post by FrugalInvestor »

Cindyjrn wrote:
FrugalInvestor wrote:
Cindyjrn wrote:The more I think about it, the more I would check tire inflation and alignment. The few things listed that could possibly be bad without throwing a code would never kill the MPG by 33%. More like 2-5%.
Tire pressure or alignment wouldn't impact mileage to that extent. If it did you'd be driving on flats and off the road.
You'd be surprised. I'd bet you could easily get close to 7 MPG drop from 30 with very underinflated tires and a poor alignment. You might research what I'm saying, you might see I'm right and that you're off base.
According to the U.S. Dept. of Energy (see here: https://www.fueleconomy.gov/feg/maintain.jsp)......
Under-inflated tires can lower gas mileage by about 0.2% for every 1 psi drop in the average pressure of all tires.
So going from 32 psi to 10 psi may degrade mileage by 4.4% (0.2 x 22). That would drop 30mpg to 28.68mpg ((100-4.4)x30) so 2.3 mpg. Most drivers would notice handling/ride issues before the tires got this low.

I can't find reliable statistics on exactly how much alignment can impact mileage but I think another 4.7mpg would be next to impossible without it being very obvious. So back to my original statement - "you'd be driving on flats and off the road (because with alignment that poor you wouldn't be able to stay on the road) to cut your mileage by 33%."
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Re: Car talk: sonata MPG dropped a lot

Post by MindBogler »

FrugalInvestor wrote:I can't find reliable statistics on exactly how much alignment can impact mileage but I think another 4.7mpg would be next to impossible without it being very obvious. So back to my original statement - "you'd be driving on flats and off the road (because with alignment that poor you wouldn't be able to stay on the road) to cut your mileage by 33%."
I agree. The loss of mileage from underinflated or poorly aligned tires would be small enough that it wouldn't be noticeable within the range of natural variability of MPG -- noise, that's all, and nothing more. I'm not saying tire inflation doesn't matter, it's just that it couldn't be responsible for a drop of this magnitude without introducing driveability concerns that the OP would have noticed long ago.
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Re: Car talk: sonata MPG dropped a lot

Post by tacster »

Does the car have a locking gas cap?
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Re: Car talk: sonata MPG dropped a lot

Post by AQ »

tacster wrote:Does the car have a locking gas cap?
Yes.

This is OP here. Thanks again for all the comments so far!

Update: I'm not a handyman at all so I went to a dealer today. The mechanic just turn on the engine and found no error codes, and then was reluctant to take my car, suggesting nothing is wrong and I could be mistaken somehow. I plan to use the car for a long distance trip in the long weekend so I'd like to get some assurance and gas efficiency isn't my biggest concern for now. So nothing was done and I came home.

MPG numbers were taken from a meter on the dashboard. Every time I filled up Sonata reset the meter. That was how I saw the big drop in MPG recently.. Maybe I'll try to do estimates myself using the mileage meter combined with fuel purchased.

I'm positive it's not a flat tire problem (again Sonata has a warning meter if air pressure drops; and it's not a hand brake issue, as clumsy as I am 8-)
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Re: Car talk: sonata MPG dropped a lot

Post by toast0 »

jharkin wrote:
furnace wrote:
This may sound outlandish, but did someone in the OP's family fill up with diesel instead of gasoline? You'll see a lot of smoke coming out. Does the car pass emissions?
Should be impossible - the Diesel fuel nozzle is larger than the filler on a gasoline car (on purpose) - this mistake usually happens the other way around.

If you somehow pulled it off, the engine would stop running altogether once diesel made its way to the injectors leaving him stranded on the road side.
The size of the diesel fuel nozzle is unregulated. They are often the size of a standard leaded gasoline nozzle, unleaded gasoline is dispensed from a smaller nozzle, and cars taking unleaded gasoline only have a smaller filler so you can't put leaded gasoline in. In 2013, VW started putting contraptions in the fillers for their TDI cars which make it more difficult to fill up with an unleaded sized nozzle (to try to prevent misfueling) -- after we got that installed, I started noticing (and avoiding) the stations with the smaller nozzles.
donfairplay
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Joined: Mon Oct 06, 2008 8:16 pm

Re: Car talk: sonata MPG dropped a lot

Post by donfairplay »

AQ wrote:
tacster wrote:Does the car have a locking gas cap?
Yes.

This is OP here. Thanks again for all the comments so far!

Update: I'm not a handyman at all so I went to a dealer today. The mechanic just turn on the engine and found no error codes, and then was reluctant to take my car, suggesting nothing is wrong and I could be mistaken somehow. I plan to use the car for a long distance trip in the long weekend so I'd like to get some assurance and gas efficiency isn't my biggest concern for now. So nothing was done and I came home.

MPG numbers were taken from a meter on the dashboard. Every time I filled up Sonata reset the meter. That was how I saw the big drop in MPG recently.. Maybe I'll try to do estimates myself using the mileage meter combined with fuel purchased.

I'm positive it's not a flat tire problem (again Sonata has a warning meter if air pressure drops; and it's not a hand brake issue, as clumsy as I am 8-)
I had a fuel pump go bad on my car and it never threw a check engine code. Went from 20'ish MPG to less than 5 MPG. Dealership had a little trouble diagnosing it at first, asking how often that happens, then they filled up the tank and drove it around to confirm.

AQ, consider taking it to an independent mechanic instead of the dealership that apparently does not want to work on your car.
jharkin
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Location: Boston suburbs

Re: Car talk: sonata MPG dropped a lot

Post by jharkin »

donfairplay wrote: I had a fuel pump go bad on my car and it never threw a check engine code. Went from 20'ish MPG to less than 5 MPG. Dealership had a little trouble diagnosing it at first, asking how often that happens, then they filled up the tank and drove it around to confirm.
That's one Ive never heard of. Usually when the fuel pump goes bad you loose fuel pressure and the engine starves for fuel and wont run at all or stumbles and dies under load.

Was it the actual pump or some sort of pressure regulator that failed ?
donfairplay
Posts: 217
Joined: Mon Oct 06, 2008 8:16 pm

Re: Car talk: sonata MPG dropped a lot

Post by donfairplay »

jharkin wrote:
donfairplay wrote: I had a fuel pump go bad on my car and it never threw a check engine code. Went from 20'ish MPG to less than 5 MPG. Dealership had a little trouble diagnosing it at first, asking how often that happens, then they filled up the tank and drove it around to confirm.
That's one Ive never heard of. Usually when the fuel pump goes bad you loose fuel pressure and the engine starves for fuel and wont run at all or stumbles and dies under load.

Was it the actual pump or some sort of pressure regulator that failed ?
Fuel pump and the fuel sending unit both failed. Oil sending unit failed about 1,000 miles before the fuel pump and fuel sending unit, though I don't think it was related.

Now that you mention it, I'm glad my engine ran at all or ran long enough to get me to the dealership.

The fuel pump going bad is somewhat common on PT Cruisers. Nowadays I take it to the mechanic and use the dealership for oil changes. A repair every 6 years or so, that's more than fair.
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wander
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Re: Car talk: sonata MPG dropped a lot

Post by wander »

AQ wrote: MPG numbers were taken from a meter on the dashboard.
If you own a smartphone, download one of the free apps to monitor mpg. If it does match with what seen on your dashboard (20 mpg), then look for a solution.
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grabiner
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Re: Car talk: sonata MPG dropped a lot

Post by grabiner »

wander wrote:
AQ wrote: MPG numbers were taken from a meter on the dashboard.
If you own a smartphone, download one of the free apps to monitor mpg. If it does match with what seen on your dashboard (20 mpg), then look for a solution.
And if you don't have an app, an easy way to check the MPG is to reset the trip odometer every time you buy gas. If the trip odometer shows 270 miles and you buy 9 gallons of gas, then you got 30 MPG on that tank, regardless of what the car's MPG monitor says you got.
Wiki David Grabiner
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