Occasional trouble idling

Question

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.

Answer 1

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.