2003 Subaru Outback with an intermittent rough idle

Problem started last week, I could swear with a bad fuel load, but I wouldn’t stake my limited reputation on it …

Symptoms are:

- Idle very rough, varying between 200-500rpm.

- Only happens when engine is warm. Engine can run 30 minutes or 2 hours without the problem. Three days of clean running over the weekend, happened again this morning.

- When condition triggers: slowing to a stop, sounds like timing chain has skipped a couple of teeth. Running on a couple of cyls, car bucks, wants to and can stall. Doesn’t set MIL, no codes.

- Occasionally, revving the engine to 3000-4000 a few times and it will settle down to a normal 700 rpm clean quiet idle again. Problem may or may not reoccur afterwards.

- If the situation is occuring, shutting down the engine and attempting a restart is hard: engine cranks like it’s starving for fuel, but it eventually starts.

Just before this: car originally threw a P0068 MAP / TPS mismatch, so started the diag for that. Found the MAP was a little dirty, so cleaned it with MAF sensor spray first. Slight improvement. Replaced MAP and air temp sensor per diag, problem a little better again, no solution.

Exercised throttle position sensor through full ranges on ECU monitor, satisfied it probably works, reset MIL, end P0068 diagnosis.

Seeing the dirty MAP, and pulling the PCV hose and noting what I think of as a whole lot of oil, cleaned K&N filter, cleaned the throttle plate, and did a Seafoam of the intake. Seafoam / throttle cleaning seemed the most effective, as this got me the three days of clean living / running that I alluded to previously (or maybe I’m dreaming). Went around the vac lines with an unlit propane bottle looking for leaks, didn’t find any. Plus, I saw no smoky Seafoam in the engine compartment.

I don’t (yet) have a vac gauge to test fuel pressure or other vacuum related things, but that may be next.

Fuel filter replaced last year, plugs about due, wires two years old and no visible arcing in the dark.

Does anyone else have any thoughts? What’s next?

Did you clean the MAF?
Some people who use K & N filters have reported an oil-fouled MAF.


This car doesn’t have a MAF … first year for the MAP sensor. Both cleaned and eventually replaced.

I’m familiar with the K&N oiling issue: as long as you don’t over oil the thing, it seems to work fine. Never mind the three opportunities for engine oil to migrate up there behind the air filter from the PCV system … that’s the one that has and keeps me suspicious.

Thanks for the reply!

Just to continue on this thread …

tried a few more things … but cleaning out the EGR valve by soaking it up to its pintle in throttle body cleaner overnight and scrubbing it out made the most difference so far. But, while rebolting the EGR back in, happened to look down at the underbody tray, and noticed what looked like fresh oil …

Put the car up on ramps, took out the tray, and confirmed my worst fears: oil leaking from cylinder head gaskets in a Subaru. I’ve been bitten.

Hard to believe that Subaru can’t design a head gasket that doesn’t leak after all these years with this problem. Who knows, perhaps this has been the problem all along. No overheating, no smell of coolant in the exhaust, no oil slicks in the coolant bottle
(but then, I have a terrible sense of smell …). But I have been losing coolant ever so slightly lately.

Sigh. Off to spend some real money now. I’ll post the results when a real mechanic gets through with it.

And to end the saga …

Had the head gaskets replaced (not leaking yet, but on these cars, I think it pays not to wait). Mechanic also found that oil was getting past defective seals and wetting the spark plugs / wires, replaced them too. My car should be all clean and happy now, no more leaking oil.

But the real culprit may be the idle air control valve (IACV).

I had taken it out and cleaned the carbon off of it, but noticed when I applied power to it, it moved pretty slowly and may have been binding. I then reset the ECU and watched it attempt to step this motor to try and find a place where the engine would run … slowly …

It would explain everything if this was it … computer unable to correct air mix fast enough to prevent stalls, idle all over the place … so on a lark, I found one used (new, this is a $350 part. Why, i’m not quite sure). It came in the day I got my car back from the mechanic, so I installed it in the morning.

Knock wood: situation hasn’t happened again. It may also be the warm weather we’ve had, so I’ll wait a week, but right now, I’m going with I finally found it.

Hope this helps you other Subaru guys out there.