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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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If you replaced the wires and plugs.Then I take it has
a cap and rotor. inspect the cap and rotor.replace as necessary.
check your spark at each plug the work back to the coil to cap.
use an adjustable spark tester as per this video.Hi collegeman, thanks for answering. The car does not use a distributor, it just has a central ignition coil for the 4 cylinders. The next step for testing is to use a spark tester and see what the coil is putting out but I wanted to make sure I’m on the right track and see if anybody has any other suggestions. Thanks!
Why would the coil be going after warming up though?
Maybe I’m off on a bad guess here as to what to look at, but that’s another centralized/overall issue and the coil doesn’t seem logical [to me], I’d think this would have more to do with closed loop, or a leak (exhaust or intake)from heat expansion, as this occurs after it’s warmed up; as in it’s fine so long as the 02 sensor doesn’t get involved. Does it fall on it’s face at all at WOT when it throws it back into open loop?
[quote=”RyanS93″ post=39219]Hi collegeman, thanks for answering. The car does not use a distributor, it just has a central ignition coil for the 4 cylinders. The next step for testing is to use a spark tester and see what the coil is putting out but I wanted to make sure I’m on the right track and see if anybody has any other suggestions. Thanks![/quote]
Try doing a power balance test.
Truly not trying to be a smart-ass here, but how does this test help when multiple misfire points are occurring?
From the description it sounds [to me] to be pretty solid to investigate the o2 sensor(pre-cat). From my limited knowledge open loop is usually set by the PCM programming for idle, using just the MAF readings to then add more or less fuel, then when running past a certain rpm range and other criteria are met, closed loop(using o2 sensor readings to do trims) is used until a certain programmed TP.
The above test is only good to my knowledge if you suspect a specific piece of the ignition or otherwise something localized to a cylinder.
The power balance test will see what cylinders are
down on power.which we know 3 of the 4 are misfiring.
If the wires are disconnected at the coil for each cylinder
then we can determine that the problem is either the ignition
coil primary or the secondary. The o2 is getting raw fuel dumped
at it from the misfires.I would suggest diagnosing the misfires
first and see if the o2 problem straitens out.*shrug*
I’m just not seeing how this would diagnose that given the stated conditions of the misfires(cold engine everything’s fine, under or not under load, then misfiring after warming up), or the fact that it’s multiple with I’m guessing a COP setup? Flooding cylinders with fuel during closed loop for a sensor perceived lean condition makes more sense to me to cause it. I would expect a code if the sensor was faulty(but I have heard of stranger even if unlikely), so I’d probably lean a bit further toward looking for a sizable exhaust leak(manifold to cat).
The misfires are always there.cold or hot. The fuel trim
is -30 that says we are subtracting fuel.Why? because of the
misfires.The engine runs better cold because the o2 is in open
loop cold and is not being monitored until heated up. If at this
stage.You were to replace the o2 sensor.You would still have the
misfire condition.which you would have the same condition. The pbt
eliminates using a spark tester for now and brings us right to the
ignition coil.[quote=”RyanS93″ post=39149]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.[/quote]
The above [to me] is stating it’s fine until the o2 sensor becomes part of the equation. As this is a COP system, I’m still not seeing how this would diagnose multiple differing plug misfires. But anyhow, I’m sure he’ll test this out and check the others later as well and hopefully tell us. I don’t mind being wrong, everybody is regularly, I just want to know why.I guess I should clarify. It begins the misfires very shortly after starting, so I don’t think it’s enough time for it to go into closed loop with the O2 sensor in the equation. I mean that right off of a cold start it seems okay but after a couple minutes it goes into the problems. And yes, if you try downshifting and giving it WOT it just sits there and has almost no power and then will all of the sudden go. We also tried unplugging the upstream O2 to get it to go to the default value at the computer and the car ran exactly the same. And with the car smelling rich and the computer telling me it is trying to pull fuel away it seems like the upstream O2 is doing its job right? I’m just thinking maybe after the coil has started firing for a minute or two and heated up inside that it begins having issues. He told me it still runs decent not great when cold but not nearly as bad as the other times. It very well could be missing right from startup too but not as bad as when warmed up…I only have what my friend says to go on. Also this car does NOT use a coil-on-plug setup, it uses one coil, mounted on the intake, with 4 leads from it to each cylinder. So that’s why I was suspecting it in this case. If this was COP I would not be looking in this direction.
That makes things much more clear to me. If you have not already done so I would in addition yank the CMP sensor connection during the rough period, see if it smooths out.
Does your scan tool have any graphing capabilities?
Any idea when the timing belt was last serviced?
Did you ever check fuel pressure and stress test the ignition system?
Mileage?
[quote=”dreamer2355″ post=39298]Does your scan tool have any graphing capabilities?
Any idea when the timing belt was last serviced?
Did you ever check fuel pressure and stress test the ignition system?
Mileage?[/quote]
Unfortunately no, my scan tool doesn’t have graphing capabilities, it’s one of those simple Actron PocketScan things. From what I can remember, the timing belt was done a bit after 100k, a little bit before he bought the car. I think that’s what he told me. The car has about 120k now. I did not check fuel pressure or stress test the ignition system yet. If it was fuel pressure wouldn’t it be a more constant thing? Feels very electrical to me, and since plugs and wires are new I’m leaning to the ignition coil. I am going to have him test it before any parts are purchased though.Just wanted to update this thread, my friend called me up and said that the check engine light has turned off and the car has been running really well. All of this just out of the blue one morning when he drove to work. He stopped by the other day, and had the car idling and it sounded pretty smooth and wasn’t having any issues. This is really confusing me and I’m not sure where to go next. Just seems strange for it to go from barely able to keep an idle and missing horribly to running pretty smoothly and no CEL within a week or so. Any thoughts?
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