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I have a 2004 Subaru Forester XS (manual transmission). A few times a month, the engine will not start or idle without feathering the accelerator. During normal driving, everything works fine until I press the clutch in or put the car in neutral. It feels like there just isn't enough fuel being fed into the engine. Most of the time, though, I have no issues. This issue has appeared and disappeared for a couple of years now. It happens in hot summer weather and in the dead of winter. It happens when the car has been sitting for a day or two, and it happens while I'm out and about.

I pulled out and cleaned the idle air control valve as well as the air intake system. I have used a couple different kinds of fuel injector cleaner. I checked for vacuum leaks. The timing belt was changed recently, and they checked the fuel pump while they were in there and said it was fine.

Any ideas? Maybe something I should double-check or test more thoroughly? It is more of a minor inconvenience than a catastrophic issue, which is why after a couple of years, I still haven't fixed the issue. Whenever it happens, I just give it some gas as I'm starting it and when the engine isn't in gear, and it works fine.

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  • Have you replaced the fuel filter? I assume since you didn't mention it that the car isn't throwing any codes when this happens?
    – MooseLucifer
    Jul 28, 2016 at 21:40
  • I haven't checked the filter. I'll do that this afternoon. And no, there are no codes or anything coming up.
    – BlakeG
    Jul 28, 2016 at 21:44
  • Would it be possible to get us the fuel trims as well? You should be able to get them if you have access to an OBDII reader
    – Zaid
    Jul 28, 2016 at 21:49
  • Re-re-reading your question, I believe this is the most important sentence that I somehow kept glossing over: "everything works fine until I press the clutch in or put the car in neutral." This means that intermittently when the car recieves the signal to return to idle, the problem occurs. This re-re-validates my confidence in the fuel filter suggestion and the rest of @Zaid's answer.
    – MooseLucifer
    Jul 29, 2016 at 16:21
  • I was going to check the filter yesterday, but I didn't get a chance. When I have some time tomorrow, I'll open the hood and see what shape it's in. I'll report back when I have more info.
    – BlakeG
    Jul 29, 2016 at 16:25

1 Answer 1

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Why would the car stumble occasionally?

Based on what's been described, I'd say it's probably because the air-fuel mixture isn't right. It's either running too rich or too lean.

  • If it is running lean enough, you should be able to hear the engine ping/"cough".

  • Too rich, and the engine will feel like it's getting bogged down.

Depressing the clutch or shifting to neutral acts as a near-instantaneous load reduction on the engine, which changes the amount of air flowing into the cylinders. If the fuel management doesn't keep up, it is very easy for the air-fuel ratio to go beyond the engine's intended range of operation.

Possible culprits

  • an under-reading MAF sensor

    positive fuel trims would indicate that this is a plausible root cause.

    If you're confident that there are no vacuum leaks, MAF's can give lower-than-expected measurements over time due to fouling of the sensor.

    Cleaning the MAF sensor with electronic cleaner may help restore the sensor's functionality. Bear in mind that a clogged air filter can also cause this.

  • insufficient fuel pressure

    Again, a positive fuel trim would corroborate this possibility.

    You've said that the fuel pump was deemed to be fine, but there are other players on the fuel side of the engine. MooseLucifer suggested a clogged fuel filter, and a malfunctioning fuel pressure regulator can also do this.

    You would need to measure fuel rail pressure to narrow down possible reasons in this category.

  • too much fuel pressure

    This root cause would be evidenced by a negative fuel trim.

    One reason for this would be a malfunctioning fuel pressure regulator.

  • lazy lambda feedback

    If an O2 sensor is not able to keep up with what the actual AFR's are, the fuel management will not be able to keep up with changing loads.

    This one is less likely given that you intermittently face issues even when starting the vehicle, when open-loop mode is active.

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  • Not disagreeing, as I'm still stumped - Which of these would only cause the engine to stumble when the clutch is depressed and not when it is engaged in neutral, all without throwing a code?
    – MooseLucifer
    Jul 28, 2016 at 23:24
  • @MooseLucifer if I understand correctly the OP's problem is that it just won't idle sometimes, regardless of clutch position. Opening the throttle a bit prevents stalling.
    – I have no idea what I'm doing
    Jul 29, 2016 at 8:17
  • @Zaid I'm interested in the "Bear in mind that a clogged air filter can also cause this". Could you explain how a clogged filter can cause MAF underreading?
    – I have no idea what I'm doing
    Jul 29, 2016 at 8:18
  • @Zaid also about the insufficient fuel pressure part. If the fuel pressure is reduced by a weak pump or clogged filter, shouldn't problems arise under hard load when fuel pressure provided becomes insufficient for the airflow, rather than idle?
    – I have no idea what I'm doing
    Jul 29, 2016 at 8:20
  • @IhavenoideawhatI'mdoing a clogged air filter will have a greater pressure drop across it. This means the engine would be consuming less air. If the air filter is clogged enough, the resultant air-fuel mixture could be too rich for fuel management to rectify, resulting in sputtering/stalling. Regarding your suggestion about insufficient fuel pressure, the way I read the question the sputtering is not happening at stable idle, but sudden load reduction.
    – Zaid
    Jul 29, 2016 at 10:15

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