Suddenly bad milege on my Elantra!

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Hi,

I have 39700 miles on my MINT 2005 Hyundai Elantra GLS sedan.
The car and engine are in EXCELLENT condition since day one.
I have been doing 5000 OCI with OEM filter since day one also.

At 30,000, did complete auto trans fuild exchange with Amsoil
and coolant flush and refill, new air filter, new Kumho tires
that are also properly inflated to 32 PSI and checked every 2 weeks, ALSO rotated EVERY 5000 miles and the car tracks completely straight without ANY pull to either side.

But suddenly, my gas mileage is suffering and I can understand
why!? ALL the maintenance is ALWAYS done on time.

Any possibility is could be either a BAD PVC valve, EGR valve
or MAF sensor??

The car runs beautifully smooth, but its just suddenly the bad gas mileage that has me concerned.

Any other thoughts??
 
At 39k its a bit early but 02 sensors? helped my escapes mileage at 130,000 no codes were showing up. got back about 2 mpg. You did'nt get any e-85 did you?
 
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REALLY?? The thermostat may possibly cause a decrease in mpg??

I really havent noticed any issue with the thermostat.
 
Originally Posted By: flinter
REALLY?? The thermostat may possibly cause a decrease in mpg??

I really havent noticed any issue with the thermostat.


Yes the computer will sense a cold engine and the mixture will be richer. Do you have an temp gauge? watch it to see where it runs.
 
Quote:

REALLY?? The thermostat may possibly cause a decrease in mpg??


Open thermostat will make the computer think the engine is constantly in "warm up" mode. That leads to a too rich mixture which ='s more gas....
 
BAD PVC valve (doubtful)
EGR valve (Highly doubtful)
MAF sensor (If you're not getting a code I don't think so)

Very early for O2 sensors but you never know....
 
The car does not have a EGR valve. It uses variable valve timing.
You are using 5W-30 oil but the car calls for 5W-20 but that probably would not make much of a difference.
Pull for codes.
Also even though you only have 39,000 miles on it the thing is going on 6+ years old and needs a timing belt.

I am thinking though you will find nothing wrong and it is just the winter cold affecting your mpg's
Ken
 
I lost mileage on my Kia when the OEM tires were replaced. The car came with Kumho KH16 205/60R16, and I replaced them with Michelin Primacy MXV4. Not only is the mileage worse by 1-2 mpg, but the tires ride stiff as a board.

The tire guy warned me that the Primacys had a stiffer sidewall, I should have listed. I just wanted to get an H rated tire since the car came with one. In retrospect I don't think it would have made that much of a difference since I will never operate the vehicle at the full envelope that the tire offers.

I can say that, at least the tires are vibration free at 95 mph, not that the car feels like it really wants to go that fast.
 
KEN,

Thanks, but thats NOT true, my owners manual CLEARLY states to use
EITHER 5W-20 OR 5W-30. The car is NOT speced exclusively for 5W-20!
 
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The tires are NOT the issue, as they have been on for over a year now and are SUPER smooth. I replaced the OEM Michilens to the Kumho
Platimum Ecsta...HUGE improvement over the Michelins
 
Are you very sure about the tires? I changed the tires on my 07 Tahoe and the milage dropped 6% (after many miles of testing). The old ones were Bridgestone Dullers and the new ones were Kumhos. Consummer reports rated them both the same for rolling resistance and they both rode very well.
 
Did you hit any large potholes lately? Could be your alignment is thrown off enough to affect your fuel economy.
 
Hi.

How bad is it dropping? If it's miniscule, like 2-3 mpg, it could be due to reformulation of the gasoline you are buying.

If it's significant, then it is definitely your emissions control system. The oxygen sensors are the main culprit. 40,000 is actually not too soon for oxygen sensors; many are only built to last 30,000 miles and then they slowly degrade as they get clogged with deposits. However, this varies by engine design and manufacturer.

The oxygen sensors are the most sensitive piece of equipment on the emissions control system in a car, so you should check that first.

I do not think it has anything to do with the fuel system or ignition/electrical system but it's worth a check. Spark plugs will definitely last longer than your mileage so don't bother with that. Make sure the electrical system is ok, and the battery is ok.
 
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