vader1

Have a friend at work with a 2002 Forester that stalls and hesitates in cold (10 degrees and below). Dealer, as usual, shows no ability to diagnose problem, tried new o2 sensor, second time new plugs, third time reset ecu. I am not familiar with layout of subie 2.5. My thoughts were maybe throttle position sensor, map sensor? Does not suggest vaccuum leak because behaves this way only in cold. I do not know if this car has auto idle speed motor.

Anyone familair with Subies have any ideas????

Thanks in advance.

shaggy

So, you've found Subaru's customer service deparment, too?

4EAT or 5 speed? Oil grade? EJ25T or simply an EJ25? Is it stalling as you apply throttle or just trying to use the idle circuit? Does it get better as the engine warms up or only when the ambient air temperature warms up?

Without answers to those questions, here's some ideas:

Because of the horizontally opposed layout, these engines are VERY sensitive to oil viscosity. If you have a 10W-40 and you're trying to start it in 10 degree weather, it might balk. 5W-30 would be more appropriate. Also, double check the electrical system. These cars are notorious for poor ground wire connections and weak batteries. Finally, try starting it and initial warm up with the lights off as much as possible. I know the wiring cuts the lights when the ignition is off, but as soon as the switch is turned to ON the lights are lit up again. Its another source of drain on the engine.

Andy H.
Daily Driver: 2000 Impreza 2.5RS

vader1

NA auto with 5w-30. Seems to do it when air temp is cold but car can be warm. At idle or moderate rpm, application of throttle will cause car to lose power abrubtly or stall. The grounding issue might be something to look into.

shaggy

It definitely sounds like a false sensor reading is affecting the A/F ratio. Thoughts...

MAP (but these are substantially more reliable than past versions, like the 1999 MAF fiasco)
Cold air is causing enough water to for in the exhaust to foul the O2 sensors (there's 2 of them, were both changed in step 1?) initially, until it warms up enough to make the water evaporate off.
ECU ~ is it horked?
Circuit Grouding ~ poor grounds cause false sensor readings, too

Has the fuel mileage also gone way down? I'd guess that its running full rich (and then some?) and the fuel economy is miserable. Double check and re-ground as many engine circuits as you can find. Its possible that a combination of false ground readings and MAP sensor readings is making the car run richer than it needs to. Has the situation tripped a CEL also?

Other things to try... Pull the air filter and check it out. If it has any moisture at all in it... well you know.

I hate engine electrical problems.