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Rough idle

7point62

7point62

New member
First my transmission and now this. My '01 GT just started rough idling 2 days ago.... Starts fine, then idle will be very rough, vibrating the car. Tach bounces up and down and feels like its going to stall. After maybe 3-5 seconds of this it smooths out to just under 1000rpm idle like normal and it's fine until next startup.

- Normal idle at stoplights
- Revs to 4000rpm in park fine, once it smooths out that is
- Temp is fine (gauge never gets higher than about 1/3)
- No hesitation (driven maybe 120km city/highway since this started and did not notice any hesitation or loss of power)
- No CEL

In fact other than the first few seconds at startup it seems totally fine. I did change the air filter the other day, needed to be done anyway, and it had no effect on the problem.

2000 grand prix problem starting - YouTube looks exactly like this except it never does stall, just tach bounce like that between around 500-1500rpm then smooths out to normal in a few seconds.

MAF? CPS? Plugs/wires? Something else?
 
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I would clean the MAF and check the IAC (idle air control valve). If its sticky or defective it will screw w/ the idle. And yes, plugs/wires can cause this as well. So can a vacuum leak but usually a vac leak will cause it to be this way all the time.

Might be time to check the PCV as well. If you replace it be sure to install the o-rings properly. One goes on the base of the PCV itself, and the other goes inside the lip of the UIM where the MAP sensor sits on top. This can cause issues if not sealed.
 
Is it possible for bad gas to do this? Reason I ask is, I had the same issue this morning at idle, then went to get gas at my usual station and found it was closed and out of business. I just got gas there three days ago and today everything is gone. Since filling up at a different station I've had the car on/off at least 10 times and the rough idle hasn't come back. I'm thinking my old station just pumped the tanks dry the other day before they closed down and I got some bad gas out of the bottom or something. Crossing my fingers....
 
Very likely you got some junk in the tank then. Probably had some water in it. I'd run a full tank through it and maybe throw some Seafoam in as well and see what you get at the next fillup.
 
I'll be keeping an eye on it, probably stick some seafoam in the tank next weekend. So far the issue hasn't been back since gassing up at a different station. Can I still get codes without CEL on?
 


I grabbed some TB cleaner too, figure I may as well clean the TB while I'm at it; any particular procedure for removing the intake screen thats in front of the MAF? I don't want to break anything and I've never cleaned TB on one of these.
 
Only one- DO NOT REMOVE THAT SCREEN!!

Besides that, nothing special, but you will need a TB gasket in order to bolt it back on. Remove the MAF before you clean the TB and take out the IAC and clean it as well. Make sure to get the butterfly clean as possible as this is where it likes to stick open a bit causing wierd idle issues and whatnot.
 
7point62 said:
any particular procedure for removing the intake screen thats in front of the MAF?
Click to expand...

You don't need to remove it. But it's no biggie if you want to as long as you're not a klutz that will drop a hammer on it or something. You just poke your finger through the other side of the maf tube on the side without the wires and the screen pops out - it's held in by friction.
 


I don't know what's in there, I haven't done anything yet. I was just wondering how to get the screen out so I could give the TB a quick clean out while I'm doing the MAF, without unbolting the whole TB from the engine... I'm just concerned with possible sticky valve from buildup (and I don't have 10mm socket handy anyway). If it's as simple as popping it out with my finger from the MAF hole and doesn't require a new gasket or something around the screen, then that's all the info I need. I've never cleaned a TB that had a screen there before that's why I ask.
 
sticky throttle blade?

You need to remove the snap ring without damaging the screen.

Or IAC valve?

IAC passage mostly has to be cleaned from the outside.
 


Just one more question before I begin; do I need to replace IAC o-ring? Because if that's the case I may as well just get a new IAC for $20...
 
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