Thread: Radical Idle Problem - Running Out of Options - HELP

The problem is the car will drop to about 500 RPM, surge to circa 2k, drop to about 400 RPM, surge back to about 2k, and then die. Also tries to die when stopped at lights/stop signs.

Trying various things to fix the idle problem...

Steps taken below:

Sprayed carb cleaner all over engine bay, no change. Problem still occurs.

Take off IAC, idles steady at about 3k. Replace, and problem comes back.

Take off the MAF, no change. Problem still occurs.

Take off the TPS, no change. Problem still occurs.

Any other options please let me know. I know that I have posted a couple times about this in various threads of mine, but i am consolidating it all in one place. I need to get this problem fixed - since I cannot really drive the car until I do. It has been down for a little over a month now during the engine rebuild and it is my only car.

Put a scanner on it so you can see sensor live data as it happens when the engine is running.

Tried a scanning tool and no codes... You mean throw on an actual scanning program?

One person said it may be the crank sensor...

Without knowing much about the car. I had an O2 sensor cause those symptoms. Changed it out and the symptoms went away. Not immediately but prob within 20 mins of driving

I had a 99 LeSabre 3.8L that would do that same thing. Rev up to 2k, down to like 500, up to almost 2500, down to 500 ish, back up to 2000 - 2500, then die. It was the IAC valve. The car would sometimes die even if I was driving as long as I was doing 25 or slower the car would try to die.

My 99 GTP did something prettg similar but that was a dirty mass air flow sensor. I cleaned it with mass air flow sensor cleaner and it ran fine

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I cleaned the MAF and nothing changed... The weirdest thing to me is that no matter what we unplug, nothing changes (except the IAC, in which case it idles steady at ~3k. Why would that be? I would think that everything you unplugged would change something...

Also tried unplugging the boost bypass solenoid with no change.

UPDATE - When I put the car in reverse, the idle evens out for about 30 seconds, then it goes back to the same issue. Does that bring up any ideas?

I know that you said the throttle body is fubar'd, but the posts are separate topics, so I am trying not to intertwine posts too much, as it will become a mess. I read everything and take everything into consideration. It's just that I am trying to do so much to the car right now, I start to lose track of things. (I've worked on it for over 40 hours in the past 3 days.) I am probably going to just get a stock TB if you think the ZZP won't help much.

I haven't listed everything because I'm not entirely sure of everything that has been done, I bought the car pre-modded (yes I know, bad idea).

The car is a 2000 Grand Prix GTP with 102k miles (at least most of it)

The mods I know:

Cam (as far as i can tell intermediate)
Bigger injectors
High flow fuel pump
Headers
Catback exhaust
Stronger springs/Rockers
3.1 Pulley
Lowered
Full size intercooler
Apparently ghetto rigged TB (with spacer!)
Cone filter intake (HAI)
18" wheels
Tuned, but probably tuned like crap
Iridium spark plugs

A lot of electrical work has been done as well, since there used to be an amp and sub hooked up, and there was a gauge overlay hooked up. There are wires running to nowhere, my interior lights are pretty dim, fog lights no longer work, TCS is stuck off, and the DIC blinks.

I also just rebuilt the engine, replacing the crank and the associated bearings.

All of the sensors I tested I unplugged while the car was off, then started it. Then plugged it back in and tried a different one.

I will get pictures of the engine bay (and car if wanted) tomorrow after work.

https://www.youtube.com/watch?v=hvgi0r1I4KA

There is a link to a video of the car's idle issue for reference.

I would probably still replace the IAC. As I said, that was the gremlin in my LeSabre. Or at least find a friend that will let you borrow one to see if that smoothes things out at all. Its a $30 from Autozone. The IAC I know will cause the symptoms of revving way up and then dropping down really low and eventually falling so low the car stalls. When it starts failing it will malfunction, (rev way up), then the car tries to fix itself although it can't since the part is malfunctioning and ends up revving too low and does that until it eventually dies

I watched your video. Thats what my GTP did when my MAF was bad. And my LeSabre did that same thing when my IAC was bad.
Question. When it starts to do that, can you press the gas and save it? Or does it still stall out?
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Alright, replaced the IAC, no change.

Yes, I can rev it up and save it.

Tried unplugging the MAF and starting it, no change. (No change at all, which is weird to me)

The exhaust leak is between the down pipe and the catback... The doughnut was rusted in place so we couldn't get to the bolts to tighten them down all the way. May have to take it to a shop to tighten them, unless someone has an ingenious idea.

Hmm. Alright. My two theories are gone then =P
When my GTP MAF was bad I couldn't save it by revving it up. My Buick could be saved by revving it up when it had the bad IAC. But you ruled that out.

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MAF's can't go partially bad can they?

And there are two vac lines that I have not thought until just now, because I didn't know about them until I looked at a vac diagram. Apparently there is one that branches off to the tranny, and one that goes to the EVAP canister. I cannot find either of these. Any clever ideas on how to test them? It's hard to test the lines with carb cleaner since the car idles so radically.

All of the lines I can see either look fine or were just replaced. Another thing I haven't checked, just assumed worked fine is the PCV valve, would that cause this issue?

Well they can get dirty. Mine worked on and off when it was dirty. After I cleaned it with mass air flow cleaner it worked fine. If you can't find the line, it may not be there. I don't think I have a picture of one branching off of the tranny.
Are you talking about a little silver nipple off of the tranny pointing upward? Located kinda by the passenger wheel well?

Edit:
Found a picture of it.


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I'm not sure where exactly it runs to, the diagram I looked at just showed that there was one. The diagram also showed that it should be coming off the throttle body... It's not. But will all the weird changes under the hood, it may have been rerouted?