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Old 07-03-2014, 09:44 AM   #1
schro311
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Light stumbling at idle and low rpms with decrease in MPG

Howdy folks, first post.

I've been to the local dealership already and they said "many things can affect your mileage, blah blah" Anyhow just wondering if anyone on here has experienced something similar.

Car has 72,200 on the odo. Recently mpg has dropped roughly 3.5 mpg from what it was, with ABSOLUTLEY no change in driving style/conditions/routes fuel types tire pressures etc. Only change that was recently performed was a new air filter. This was done AFTER noticing the problems and has not offered any improvement after a MPG reset and 3 full tanks of fuel.

Along side the MPG drop, sometimes at idle the engine will be rough for a few seconds, causing the needle to bounce around, but then recovers. Not like the A/C compressor kicking in and putting a constant load on the engine, the RPM is variable at this point. This fluctuation in RPM is also noticed under light to moderate loads ie: maintaining speed and transmission gear while going up a hill. It seems the low end torque has decreased too, but maybe I am imagining that.

No CEL.

Thanks for any input!


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Old 07-03-2014, 12:20 PM   #2
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Do you do a lot of short trips or around town driving? If so, sounds like you might benefit from an "italian tune-up".
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Old 07-03-2014, 03:14 PM   #3
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Possible spark plugs (At 72,000 mi would not hurt to replace them)

"Italian tune up": is to go driving in a spirited style on some long curvy back roads.. staying mostly in one gear (best is third) and letting the engine rev up and down a lot. as you drive with some abandon. Some suggest it 'fixes' issues with the PCM and such. Not too bad an idea but i doubt it is your problem.

Question: Do you use regular 87 octane gas? You might try a tank of Premium and see if the stumble goes away (Highly recommended test)

your gasoline may be not as good as it was. Yeah the car will run on 87, but it really does better with 89 or 92/93 octane gas.
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Old 07-03-2014, 03:19 PM   #4
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As soon as I read rpms bouncing around at idle I instantly think idle air control. With these new DI motors it's like a whole new ball game though.
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Old 07-03-2014, 05:02 PM   #5
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Get new Spark plugs!
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Old 07-03-2014, 07:25 PM   #6
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I would check the front o2 sensor , your miles are about there for it to be changed

There is not IACV on the MK3

Bad front o2 would cause cold start mis fire , at idle misfire and maybe stall and poor MPG

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Old 07-03-2014, 08:26 PM   #7
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I stand corrected. Thanks Tom.
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Old 07-07-2014, 09:52 PM   #8
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Sorry for the delayed response, went out of town for the weekend.

Yes most of my driving is city stop and go now, has been since the problem was first noticed. Although its still there after a 1200 mile weekend averaging about 75 mph.

I have run 3 consecutive tanks of premium through it, with no change so I decided it wasn't worth the extra cost.

My tuning experience is with older EEC 4 and 5 stuff so maybe this doesn't apply, but wouldn't the O2 be taken out of the loop once any load above cruise is applied? (going up hill)

I plan on having the fuel injectors cleaned soon, so I will add spark plugs to the list.

I just keep thinking vacuum leak. Again, maybe my knowledge of older systems is irrelevant.
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Old 07-08-2014, 05:25 PM   #9
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Vacuum leak would make sense on a older vehicle but i cant think of where one would occure on these newer engines. I think you just need a tune up.
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Old 07-09-2014, 02:56 PM   #10
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Sounds exactly like what my 2012 SE did. The car ran just like you described and gas mileage went from 38 down to 35. On my way home from work one night it stumbled hard a few times, bucked hard and stalled but never threw any codes. Dealer could not find anything wrong. Three months later the car threw a CEL on my way to work and went into limp mode and I was able to get it to the dealer. The mechanical high pressure fuel pump had failed. Dealer replaced the pump under warranty and the car now runs like new again. My fuel mileage has also return to 38mpg.
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