(SOLVED!) VERY rough Idle.

[SHOspazz92]

This weekend we installed a PNP/BBB/Painted intake on Ashley's car. We had issues getting the thing to start and later found that it was probably a ground issue (The strap for the DIS). Anyways, Got it fixed and the car started right up.

When you start the car it Idles fine, When you drive it, It drives fine. However, If let to sit at idle (Wether your just sitting in the driveway, Or at a stop light) for more then about 20-30 seconds the idle gets very rough, Like 4-500 RPM rough. The thing feels like a Cammed V8. I now have 2 Grounds for the DIS, I can't hear any Vacuum leaks and I've tried 2 different IAC's with no change. I'm at my whits end with this car and I only have about 15 days before I have to it drive 2400 Miles across country. Any help is GREATLY appreciated.

-Sam

 

[SHOspazz92]

Well, If I Disconnect the MAF while the car is doing this the Idle Jumps back up to 1,000 and it idles smooth, With no signs of the rough idle returning. If I plug it back in it eventually goes back to idling rough....as soon as I unplug it...no problem.

Does that make sense to anyone? Wouldn't a bad MAF cause drive ability issues?

-Sam

 

[SHOspazz92]

Swapped out MAF's, This was not the problem.

-Sam

 

[Wakeboy1337]

Swap pcms. It could have gone bad.

 

[frosho]

Stock MAF? Did you try cleaning it? I know you said you swapped them and it didn't seem to be the problem, but the fact that it idles OK with the MAF unplugged would make me want to investigate it further before crossing it off the list.

Did you have the battery disconnected recently? If so, did you reset the idle? You need to do this if you swap PCM's, too.

Have you checked for vacuum leaks?

Does it run any better while driving?

 

[SHOspazz92]

Stock MAF? Did you try cleaning it? I know you said you swapped them and it didn't seem to be the problem, but the fact that it idles OK with the MAF unplugged would make me want to investigate it further before crossing it off the list.

Did you have the battery disconnected recently? If so, did you reset the idle? You need to do this if you swap PCM's, too.

Have you checked for vacuum leaks?

Does it run any better while driving?

The car runs absolutely perfect while driving. It's only if your stopped for more then 30 seconds that it starts to do this. I've searched for Vacuum leaks but can not hear anything leaking. I've done the idle reset procedure as well.

-Sam

 

[frosho]

Hmm... I'd try cleaning the MAF if you haven't already. It certainly seems like a MAF issue... I'll think about it some more and let you know if I have any other ideas, gotta get back to work for now though.

 

[SHOspazz92]

Here's a thought. It almost does this like clockwork. It will idle fine for about 45 seconds or a minute or so, Then it will do it's rough idle buisness. So, Does something Cycle....or chance per say, After the car is at it's normal idle speed for a certain amount of time?

-Sam

 

[Bob Gervais]

Sam, all connectors plugged in? TPS, etc? When you swapped intakes, did you use your original throttle body, or did the PNP intake come with one? (thinking throttle stop screw may be out of position)

 

[SHOspazz92]

Sam, all connectors plugged in? TPS, etc? When you swapped intakes, did you use your original throttle body, or did the PNP intake come with one? (thinking throttle stop screw may be out of position)

Nope, Same throttle body. I leave the throttle body in the car when I pull intakes.

-Sam

 

[jonheese]

My suspect list, in order from highest probability to lowest:
1. DIS (transistors aging, may be fried)
2. IAC (leaky gasket maybe?)
3. Vac leak
4. MAF (two in a row?)
5. ECU (don't usually go bad, especially coincidental right after swapping intake components)

You can use an unlit MAPP or propane torch to probe for vacuum leaks. If you hear the idle bump up for a sec, the vacuum leak is near wherever your propane is pointing.

 

[kevinspann]

I'd have to guess vacuum leak. My car will do the same thing, kinda. First start, its fine, then it'll start to idle poorly, but if you rev it, it's okay.

 

[rubydist]

I don't think its a vacuum leak - typical vacuum leaks make the car idle faster, not slower.

Do any codes get set? It sounds to me more like a misfire situation - like a bad plug, bad wire, bad coil pack, bad dis kind of thing. It could be that a plug wire got damaged during the intake r/r or that a plug wire is partially disconnected.

The other thing I would take a real good look at is the wiring harness in the area of the mafs - since it runs smoothly when you unplug the mafs, its possible that there is some wire/connector damage that is showing up. iirc, the mafs, iac and tps all use a common feed and issues are therefore inter-related in some cases.

And, you might do the idle reset again -leaving the battery disconnected overnight. the red 94 that I had for many years would take several hours of battery disconnected before it would lose its kam, whereas many cars will only take a couple of minutes.

 

[SHOspazz92]

I don't think its a vacuum leak - typical vacuum leaks make the car idle faster, not slower.

Do any codes get set? It sounds to me more like a misfire situation - like a bad plug, bad wire, bad coil pack, bad dis kind of thing. It could be that a plug wire got damaged during the intake r/r or that a plug wire is partially disconnected.

The other thing I would take a real good look at is the wiring harness in the area of the mafs - since it runs smoothly when you unplug the mafs, its possible that there is some wire/connector damage that is showing up. iirc, the mafs, iac and tps all use a common feed and issues are therefore inter-related in some cases.

And, you might do the idle reset again -leaving the battery disconnected overnight. the red 94 that I had for many years would take several hours of battery disconnected before it would lose its kam, whereas many cars will only take a couple of minutes.

I'll check, But the car is running a lot more rough then as if there was a misfire on just one cylinder, I've had that issue before. I'll have to post a video of what it's doing. It's pretty amazing how one second it will be idling relatively smooth and then just go to **** with no warning.

-Sam

 

[jonheese]

I still say the DIS is my #1 suspect. I've seen a lot of them going bad recently, especially if it's factory (which, of course, you have no way of knowing for sure).

I have heard of at least one failing DIS causing this exact symptom-- stumble/low RPM after idling for a short time.

 

[Frisbeeguy]

I may be way off base here... try wriggling the wires leading to the cam sensor when it's idling rough. My '89 will do what your describing ocasionally and it "fixes" it immediately.

 

[frosho]

I still say the DIS is my #1 suspect. I've seen a lot of them going bad recently, especially if it's factory (which, of course, you have no way of knowing for sure).

I have heard of at least one failing DIS causing this exact symptom-- stumble/low RPM after idling for a short time.

+1

The DIS is easy to swap, so you might as well give it a shot. Yank one off of one of your other SHO's (one that's running well) and try it out.

 

[SHOspazz92]

Swapped out DIS's....

Started it up and it idled fine for the first 4 Minutes. I thought it was fixed...then it went to shit again. Yes, I have heat sink on the back of the DIS's. Wiggled around the MAF wiring, The Cam sensor wiring...no change.

-Sam

 

[jonheese]

Crap, I thought I might get one right for once.

Have you checked the IAC gasket? Do you have a propane torch you can use to search for vacuum leaks? How 'bout a spare ECU from your other car (it's a long shot, but it sounds like we're getting desperate...)?

 

[Devin]

Why'd it work for four minutes?

Put the DIS on your car, how does it respond?