HELP! Surging & Poor Idle - but no CEL/DTC

[JOEK39]

Hope someone can help me. 2001 Forester, non-turbo, 80K miles. Started right after a bad tank of gas a couple months ago. Bucking and surging and poor idle. No error codes, but changed the fuel filter and ran some injector cleaner through the system and got immediate improvement.

 

Two weeks ago, started with issues once its warmed up. Runs perfect when cold. Gets to operating temperature and the surging under engine load starts up. Also really bad at idle when stopped - engine will almost die, have to put in neutral to keep it running.

 

No error codes set, no CEL.

 

I've changed the plugs and wires, and reset the ECU a few times. The front O2 sensor was changed out about 11 months ago when it failed at 68K miles.

 

I'm prepared to change out the front O2 sensor again, but the whole business about it running fine until its warmed up got me thinking about the coolant temperature sensor. I have a diagram from the Suby shop manual that shows it just behind the ignition coil/pack and just below the intake manifold on the left center of the engine. I'll be darned if I can see it or find it however.

 

Any thoughts on what the issues could be ?? And where is that darn coolant temperature sensor ?

 

Thanks in advance. - Joe

 

88 GL-10 (deceased)

01 Forester

[Manarius]

Does your car have a MAF? If it does, I'd suggest replacing that. My car acted the same way when the MAF was toast.

[kimokalihi]

I will second that. One of the wires that goes to my maf got fried halfway down the wiring harness because it got too close to the exhaust pipe that went into the intake manifold on my toyota pickup. Took me forever to figure out what was going on but I similar symptoms.

 

I think what happened was it grounded out on the pipe and was sending a signal that told the computer that there wasn't any air coming into the intake and it wasn't giving the engine hardly any gas.

 

Different situation but I can see how it could be the MAF, without the MAF you're screwed.

[skizix]

I had intermittent bucking and surging, occasional idle problems and a couple of stalls. No CEL or codes. Turned out to be an intermittently malfunctioning neutral switch. To properly diagnose, you'll need a real-time scan tool -- in which case is should be pretty obvious.

 

That switch is like a $20 part though, and not tough to replace, so you might just try one and see if it fixes anything.

[JOEK39]

I had intermittent bucking and surging, occasional idle problems and a couple of stalls. No CEL or codes. Turned out to be an intermittently malfunctioning neutral switch. To properly diagnose, you'll need a real-time scan tool -- in which case is should be pretty obvious.

 

That switch is like a $20 part though, and not tough to replace, so you might just try one and see if it fixes anything.

 

skizix - thanks for the post. Thats one idea i would never have come up with. I will certainly look into it, but could you explain how that failure can cause the issues I have, especially only after the engine is warm? Where is the switch located? Is this the one on the transmission ?

 

Thanks!