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Old 03-17-2009, 06:12 PM   #142
Cinnabuns
Scooby Newbie
 
Member#: 173912
Join Date: Mar 2008
Location: Terre Haute, IN
Vehicle:
2002 WRX
Pearl Blue

Default

I've been fighting the P0171 for about 2 months now and I've been reluctant to take the car to the dealership because I knew it was something simple.

Like other posters I've tried everything on the OPs list and still no dice.

I also had another symptom and this may/may not be a good way to check for this particular issue. I explain below.

I have the P0171 that comes on when the car is idle. I get off the highway, come down to a stop light, sit there for a couple minutes and boom, CEL would come on 0171. I'm driving around town, going to work or whatever, hit the drive through to grab breakfast and sitting in the drive through, 0171 would come on. I kept resetting it with my AP because the car ran completely fine. Only at idle would I experience this condition. I reset it because once the light came on the ECU would start dumping gas in to the engine and I'd get about 200 miles per tank of gas.

I thought the 0171 issue and the below issue were separate but it turns out they were the same issue and this 'test' actually helped point right at the problem (next paragraph).

Whenever I would go to a 'touchless' car wash, the kind with just the sprayers that spray high pressure water under/around/at the car, my CEL would blink which indicates a misfire. The idle would get rough then smooth then rough and the light would flash but after the wash was over, the light would shut off and all was well again. The AP showed no code for this misfire.

As it turns out, and as I've read in a couple others diagnosis of this issue, the passenger side intake manifold-to-TGV has 6 bolts/nuts holding it down. My car has 68k miles and apparently these loosen over time. They were all about 1/2 to 1 full turn NOT hand tight. These 10mm bolts require 6.1 ft/lbs torque which isn't alot and they can loosen over time.

I torqued them all down and now the CEL does not come on anymore. P0171 gone, and the car wash no longer makes the CEL blink for misfire. Seems that at idle under vacuum it was pulling in the water misted air causing the misfire and under normal conditions was pulling in additional unmetered air causing the lean condition at idle.

When you are on WOT, this small amount of air does not matter because the intake is under pressure, not vacuum, so some of the metered air would get 'out' but the unmetered air entering at idle is what caused the code to be thrown.

So check both sides of where the intake manifold bolts down to the TGVs on both sides. On my car, the driver side was exactly to spec, 6.1 ft/lbs.
Its curious that both I and another guy had this issue on the passenger side only.

He said his intake manifold gasket was blown and it had to be replace but mine just required re-torqueing the bolts/nuts located here.

Probably saved me a few to many hundreds of dollars over taking it to the dealership.
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Last edited by Cinnabuns; 05-06-2009 at 11:45 AM.
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