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Hi everyone, I wanted to get some thoughts on the problems a friend of mine is having with his 2002 Subaru Forester (2.5 SOHC 4-cylinder, 5-speed manual, 120k miles). For a little while he has been having issues with rough running and odd behavior by the car. Basically, it runs okay when started right from cold but after a bit when it’s warmed up a little, it begins running very poorly. I drove the car too, and basically what happens is when you take off from a start there is very reduced power, then all of the sudden it will “kick in” and have normal power. Also, you can be driving along normal, and it will start to buck and shake pretty bad (like when you’re learning manual), and if you push in the clutch then let it out again or change gears it will smooth back out but usually start back into the bucking and shaking. It definitely feels like pretty bad misfires.
He had a check engine light for an O2 sensor that turned itself off and has not returned (he did not get the exact code). For a while it has not set a CEL at all. The other day I changed out the plugs and wires for him – plugs were old and wires were aftermarket so I figured it needed them anyway and it would give us a clean baseline. Put in new NGK plugs and OEM Subaru wires, and the problem is still there. We drove it and then while it was idling, the CEL went on and the car stalled. We pulled the codes and had P0301, P0302, and P0304. So misfire on those 3 cylinders. What is weird is that no code had been set prior to that even though we could clearly hear the engine missing. Leaving it idling you can see the engine shaking pretty badly and the exhaust popping and moving around, and it struggling to maintain an even idle. The car has also sounded like it backfired a couple times while driving. I looked at the freeze frame data and the ST fuel trim was about -30% and LT fuel trim was about -6% which would tell me the car is detecting a rich condition from the misfires and is pulling fuel away right?
The car smells rich, has misfires, and is running very poorly, with reduced power and bucking and stalling. The problem is relatively intermittent as you’re driving but happens every time you drive it.
My guess is the ignition coil (this car uses one central coil for all 4 cylinders), and as the coil warms up it has issues delivering spark and since that there is a misfire and unburned fuel the car is running rich. I didn’t want to have him spend $150-$200 on a new coil before getting a second opinion and testing the coil however.
Any help would be very much appreciated!
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Given that the misfire was there when it was cold and in closed loop I would say that the O2 is not likely to be the cause. I was on board with the power balance test because I suspected a problem with one of the coils. I know it LOOKS like it’s just one coil but it’s actually 2 put together. A power balance test might have show 2 cylinders that were effected and if they were related to a particular ignition coil you could replace the coil and recheck for the condition.
Keep us posted if things change.
Thanks for the reply Eric, I tend to agree with you. However, the car seems to still be running okay for now, so I’m obviously not going to do anything until the problems resurface.
For my own knowledge and in case there is an issue again in the future, how would I check that the feed going into the coil was okay, in case it’s getting a bad signal and the coil itself is fine? Due to how intermittent it is I’m wondering if that may be an issue, or is that rare and in most cases it’s the coil? Thanks everybody.
Could be a bad MAF sensor, try cleaning it and reset the memory, that’s important. If not, try replacing it. Multiple misfires can be set by a faulty neutral position sensor on the manuals. Subaru ECUs are a bit slow to learn new values so don’t give up on the MAF until after a reset and a good roadtest.
The best way is with a lab scope so you can see the switching pattern but a quick and dirty way is to check the coil negative with a test light. It really doesn’t tell you much if I’m honest. Perhaps these videos will shed some light.
Hey guys, just wanted to update this thread and close it out. After battling with this issue for a long time off and on, he finally brought it to a professional who diagnosed it and replaced the ignition coil and both oxygen sensors. I’m not so sure the O2 sensors really needed replacement, I think the coil was the main problem, but this is a reputable shop so I trust them. Either way, the car is running great now, and it looks like my suspicion of the coil was correct. Wish I could have helped him do the coil and saved him some money but it just didn’t work out that way. Anyway, thanks everybody for the help!
Thanks very much for the update. It will go on to help others with similar issues. Thanks also for using the ETCG forum.
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