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My engine idles fine when it's cold, and more often than not it's fine when it's warmed up too. But sometimes it will dip from regular idle speed (~750 rpm?) to below 500 rpm, sputter, and then bounce back to regular idle speed. It will stay at 750 for a couple of seconds, then dip and repeat.

This used to happen a lot more -- it was a real problem before I got the car and it just sat in my grandparents' garage for most of the time. I remember driving my grandfather to a dentist's appointment over Christmas, and he said to use the Legacy instead of the car that I was driving at the time (1992 Infiniti G20) because the Legacy hadn't been driven in about a month and needed some "exercise." It ran fine for a couple of miles while it was warming up, and then started doing the idle-sputter-idle thing at every stoplight.

As I said, it's only an issue some of the time now that I've put 3500 miles on it in the past couple of months (that was how much my grandparents put on in the average year before I got it). But it is sort of disconcerting. Any ideas as to what would be causing it?

I'm thinking it's your IAC valve.

Mine does this once in a while...I rev it a little, and it goes away. I'm not worried, but if it gets unbearable like you're indicating, I might consider pre-emptive matinence.

Others will most likely say the same thing.

Is there a way to clean thse out? Just maybe some brake parts cleaner blasted in there?

no idea at all...I wonder if it closes when the throttle is open...if you could run seafoam through it...

Gah. It started happening again today on the way back from the grocery store... Practially every time I depressed the clutch for more than a fraction of a second it would do it. I'm wondering whether my debacle with an ECU wire last night made it worse. The wire is taped back up, but it might have exacerbated the situation somehow.

How much is an IAC valve? How hard is it to replace?

It might be a vacuum leak.

maybe the purge control valve? when my auto threw a code for that, it would do the same thing. i replaced it and it became fine. u throwin any codes?

Project_Legacy wrote:maybe the purge control valve? when my auto threw a code for that, it would do the same thing. i replaced it and it became fine. u throwin any codes?

I literally got the purge control valve replaced last month. The Check Engine light was blinking on and off; it's not coming on at all now.

I was chatting with Vikash on IRC last night, and he said to get some starting fluid to check for vacuum leaks. I haven't found any by a couple of quick sprays, but I was doing it when it was dark out so I'm going to try again today.

thefultonhow wrote:
I literally got the purge control valve replaced last month. The Check Engine light was blinking on and off; it's not coming on at all now.
.

ahh ic. thats exactly what mine did. other than that i guess it could be a vacuum leak or IAC.

Vikash -- how much is the idle supposed to change when you spray the starting fluid? I heard a slightly different tone a bunch of times when I sprayed it in different places, but I find it hard to believe I have that many vacuum leaks.

Incidentally, my car wasn't doing it today. It will literally idle fine for several weeks and then one day will start doing this out of the blue after a warm start.

WTF?

the IAC does not close when you open the throttle, the duty cycle falls to about 20-30%. the IAC is two portions, an upper electrical portion, and a lower mechanical portion which has the valving, the upper portion uses an electromagnet to drive the valving on the lower portion. the IAC defaults to around 20% open (it doesn't fully close, from what I can tell)

If there's a leak upstream of the throttle plate the change can be very subtle. It might help to unplug the oxygen sensor and IAC valve to help reduce variation unrelated to a leak.

It won't idle at all with the IAC unplugged... that is, if I was unplugging the IAC and not something else. The one I was unplugging had a 3-wire plug like in the Haynes manual and was in the top left corner of the engine when you're facing it (as in, the right side of the car). It's plugged into a cylindrical type thing, which screws onto the engine with two phillips-heads. Is that the IAC, or was I barking up the wrong tree?

Yes, that is the IAC valve.

Was the engine warmed up? If it wouldn't idle with the IAC valve disconnected and the engine warm, that's abnormal.

I think you should try cleaning the IAC valve out. See below.

Mike - There are a few different ways you can clean it out. I have recently had success with this approach:

-Warm the engine up.
-Then shut it off and remove the hose from the IAC valve.
-Attach your own 3/4" hose to the valve (5/8" for pre-facelift NA 5MTs) and plug the stock hose.
-Start the engine again and have a friend use the throttle to keep it running.
-Pour some Sea Foam slowly into your hose (your friend will probably have to work harder at keeping the engine running).
-When the smoke coming out the tailpipe subsides, shut the engine off.
-Pour a little more Sea Foam into the hose and let it sit a few minutes.
-Start the engine up and again run it until the smoke subsides.
-Return the plumbing to stock, reset the ECU, and start the engine. Let it idle for ten minutes or so.

It was warmed up. However, it had been sitting briefly (like 15 minutes). I don't think that should have affected it much, though, and the temp gauge was a bit below halfway.

Hose... I assume you mean the one coming from the big intake pipe thingee (that will have to suffice for the technical term at the moment) and going to below that cylindrical thing? (I guess the IAC valve had more to it than I thought.)

I'll see if I can get a friend to help me with this over the weekend. If I just go into AutoZone and ask for some seafoam, will they know what I'm talking about? To reset the ECU, I disconnect the battery and wait for how many minutes?

Yeah, 15 minutes of sitting wouldn't cool it down enough to make it fail to idle without the IAC valve connected, unless maybe it's really cold where you are.

Yes, I mean the big hose coming from the big black plastic intake pipe and going to the body of hte IAC valve.

It's possible to do this without a friend; I actually did it myself just manipulating the throttle cam directly. If you look where the throttle cables end up, you'll see the throttle cam that they pull on. You can push on the cam yourself. Just try it and you'll see what I mean. I use a big flathead screwdriver.

Last I checked, AutoZone doesn't carry Sea Foam. Advance/Schuck's/Kragen/whateverit'scalledwhereyouare does though. So does NAPA. But if you can't get Sea Foam, you could still probably get good results just using any brand of aerosol fuel injection cleaner.

Vikash -- do I need to get another hose? I was thinking of doing it this way:

-- Remove the end of the hose that comes from the black intake pipe, and use duct tape (would that work?) to cover up the hole.
-- Pour the Sea Foam down the stock IAC hose.

I'd do everything else the same way. Feedback? Is this a terrible idea, and if so, why? I'm planning on doing it today or tomorrow.

Yeah, you can do it that way if the hose's shape is cooperative (I'm not that familiar with non-turbo plumbing). Just don't let any pieces of duct tape go into the engine.

I went through the procedure just now, and it went off without a hitch. I'm resetting the ECU now (the battery is unplugged and will be for half an hour), but I tried unplugging the IAC again now that it's cleaned out, and it still wouldn't idle. Maybe it will do better when the ECU is reset.

Incidentally, I noticed that my right CV boot is torn... d'oh! I'll have to replace it soon, i guess.

Okay, after letting the car sit for an hour with the battery (negative terminal) unplugged, the ECU should be reset, right? Because it still won't idle with the IAC valve unplugged when warmed up.

I haven't had the problem since, so I think cleaning the IAC valve fixed it.