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HELP! Subaru Forrester '99 idle problem

For the past 1 year I have gone to two mechanics and the Subaru dealer to figure out why my Forrester idles up and down while at a stop light. Occasionally it will stall, but usually just idles up and down. Things have been cleaned and reprogrammed. Lately, the ISC valve was replaced (at $375 for the part!) and the O2 sensor was replaced to no avail! Since the last mechanic checked it out, the idle will run up past 5-6 rpm when I push in the clutch on the highway. At a stoplight, the car idles at 2rpm. No one has any idea why the idle is so funny and what to do about it, even the dealer themselves!


Since it is not possible for a car to idle at 5-6 rpm, I have to ask:
Is the “5-6 rpm” idle speed actually 500-600 rpm, or is it 5000-6000 rpm?
(For your sake, I REALLY hope that it is not 5,000-6,000!)

Similarly, is the “2 rpm” idle speed actually 200 rpm, or is it 2,000 rpm?

I think its 500-600 and 200rpm. when i look at my rpm gauge, i was reading the big number which said 5-6. As you can tell, Im not super car savvy. Does this make more sense? I am not as concerned about this because I think my mechanic was messing around with my car yesterday and probably just didnt put something back right. Thats a new problem. The bigger and more annoying problem is that is idles up and down every time I am at a standstill, and only when the car is warmed up. I was told that the O2 was too rich, but I thought the ISC valve would have fixed that. Everyone I have taken the car to (and have spent a few thousands of dollars) is scratching their heads over this.

the big numbers are thousands - so 5-6 is 5-6,000 - that would be bad. first I would make sure that you’re not actually getting revs that high.

but if the problem is really just the up and down idle - first I’ll assume that by “ISC” you meant “IAC” (idle air control). if that was just replaced I would first wonder if anyone has looked for vacuum leaks. that is what I would look for.

Thank you. The idle only reved up that high when I put in the clutch and I could get it to come back down when I put the car back into gear and let go of the clutch.

They replaced the Idle Speed Control Valve. I had the dealer look for exhaust leaks because of the horrible smell the exahust is giving off, and I didnt know if that was related. The car spent the whole day at the dealer and I assumed they looked at the most common things.

as far as looking at “most common things” I wouldn’t necessarily assume that a careful check for vacuum leaks has necessarily been done. these can be really hard to find and by and large most of the “parts” in vacuum systems are really cheap - like measured in pennies or a few dollars. sometimes the “fix” is just a dislodged connector or hose and the parts cost is zero. especially when thinking about what a dealer’s service dept would do - there’s not a lot of money in diagnosing & fixing vacuum leaks.

I would ask each mechanic what they have done to check for one - the ideal would be a “smoke test”

I’m also not saying it is a vacuum leak. its just the kind of thing that can a) cause this loping idle and b) be hard to find/diagnose

Thank you, I will try that. I am not sure if they have done a smoke test, but it sounds familiar. Do you have any other ideas or have you seen this before?

I would also suspect a vacuum leak if the idle speed keeps bouncing up and down, and a vacuum leak is usually a very minor repair in terms of cost. However, if the mechanic is looking for big-ticket items to replace, he may not want to look for the cheap stuff.

If the engine is really revving to 5,000-6,000 rpm, that is perilously close to the “redline”, which I think is 6,500 rpm on that engine. Running an engine consistently at revs this high will cause major damage in short order, so this needs to be attended to by someone competent a.s.a.p., and from the sound of things, this means going to a different shop. Lucy–Take a look at the tachometer, and tell us how close the “6” is to the red area of the tachometer.

Just to clarify things for Lucy–once you engage the clutch and the car is moving, the engine is under load, and is not “idling” at that point. So, if I am interpreting things correctly (and I’m not confident that I am), after warm-up the engine idles at 5k-6k, and when the engine is put under load, the engine speed goes down to 2k. Is that correct Lucy?

Also, if the exhaust has a nasty smell, I suspect that there is something amiss in the ignition system. A scan of the OBD system (free-of-charge at many auto parts retailers) would be in order here.

When was the last time that the spark plugs were replaced? For that matter, when was the last time that things like the air filter and the pcv were replaced? Is this vehicle up to date with maintenance?

Thanks again. I cannot look at my car right now because I am at work. After warming up the engine, the idle was between 1000-2000 when I engaged the clutch. After traveling about 60mph on the highway, I tried to downshift, so I engaged the clutch and the rpms went up to 5000 and I quickly put it into a lower gear and the revs came back down. This continued to occur until I was in the city and only traveling at 35mph, then the revs only went up to 2000 with the clutch engaged. I plan to drive it home and try to not let it rev up that high as I didnt think that was good either. I am taking it back to the mechanic as I suspect it was something he did yesterday while trying to fix the other idle problem. Maybe something is sticking or wasnt put back together right?

The dealer did do a OBD system scan and only came up with needing a new O2 sensor, which was replaced. I know the air filter was replaced recently, but I am not sure about the spark plugs. I can bring these to the attention of my mechanic. I will also suggest checking the vacuum. I guess I put too much trust into the dealer figuring out the problem since they charge so much, but I am growing very weary of anyone who fixes my car now…seems like something else goes wrong when I get it back.

I really appreciate all your insight.

it sounds to me like the high revving probably did come from something sticking - like the throttle - and may have been related to something the mechanic did.

for the erratic idle, here is a rather basic list of low cost things that one ought to start with - each one is more like “maintenance” than “repair” and each is good to do even if it doesn’t solve the problem. many of these may have been done already:

  • check/replace air filter (seems to have been done)
  • replace fuel filter
  • use/perform some manner of fuel injection system cleaning
  • clean throttle body, IAC (you replaced); EGR valve (and test)
  • clean MAF sensor
  • check spark plugs/wires/ignition coil/distributor
  • check/clean PCV valve

those are common kinds of things that many people would start with. if it has all been done by now, then I would be likely to ask about 3 things - 1) the check for vacuum leaks - this needs to include check for leaks around intake manifold; 2) fuel pump testing; 3) check of the engine’s condition (compression & leak down tests)

if 2 mechanics & a dealer haven’t gotten you sorted out yet, you might think about going to engine condition first - just in case you have internal engine problems and decide to stop putting $$ into it.

Thanks again! I will try those things and hopefully we can figure it out so I can keep the car! I really appreciate the suggestions.