Thread: RE: CES Light Rough Idle

Hello all...
Problem 2003 Grand Prix GT 3.8..
Within the past week of driving, I've noticed a stall when the engine returns to idle. If I increase throttle to around 1500 to 200 RPM then quickly return to idle the engine will stall down to about 100 to 200 RPM then back up to normal idle. It dose this constantly, but more frequent after a long drive.
Now, there are codes "DTC P0300 Engine Misfire Detected - ECU $10" and "P0302 Cylinder 2 Misfire Detected - ECU $10", this is according to what I found in Palmer ScanXL Pro.

Maybe a bad gas cap? Would this cause a misfire on one Cylinder? I've been trying to come up with a solution based on these two issues are related (Stall and now new codes)..

I can clearly hear the misfire at the rear of the car but can't determine which side the misfire is on (seems to be coming from both pipes).

I've recently replace plugs (Iridium's) and wires (GM OEM).

Thoughts?

Well P0300 is random misfires and P0302 is specifically cylinder 2. I wonder if the car threw a P0300 and then narrowed it down to just cylinder 2. Or if it started with P0302 and then found more misfiring.

Misfiring will probably be heard in both pipes since these cars aren't 'true duels' meaning both manifolds meet right in the back of the engine to a single pipe out the back to the split before the mufflers

Interesting..
I pulled the connector off the MAF.. The idle issue (stalling) seemed to clear itself, but I think I need to drive the vehicle at least another 5 to 10 miles to confirm this. But so far so good.. After I pulled the connector I cleared the codes. I still hear a very slight misfire but not noticeable if you didn't know the vehicle well. There are no more codes, but I'm going to take the vehicle out on the highway to test.. Thanks for the replies, I'll be back..

I recommend cleaning the MAF with MAF sensor cleaner. Then try it

Ok... The engine threw another code which I expected due to the MAF was offline.. But I looked back at the history of misfires in the last few months, I have random misfire data on CYL's 1, 2 and 6.. Most misfires on CYL. 2. I'm going to replace all the plugs. If I have any other misfires after the plug replacement, I'll begin with the coil pack swap.
Any recommendations on which spark plugs? Iridium's? I've read much here about which plugs should go in. Perhaps Iridium's aren't the plug of choice any longer?

Thanks again..

I didn't mention, I did clean the MAF with MAF cleaner back when the stalling started.. No change.

Hello all..
Could use some more help..
The short version:
No more codes.
Engine seems to have a very slight rhythmic miss, hardly noticeable unless your ear is near the exhaust.. Maybe not a miss at all. Otherwise smooth running at all throttle ranges.
Exception (main problem): Raising the idle to around 1200 to 1300 RPM and quickly returning back to idle, the engine stalls and sometimes quits, never throws a code when it does that.
Here's what I have done as maintenance and my attempt to eliminate the stall:
Spark plug change, Iridium's. The plugs have about 10,000 Mi. on them
Spark Plug Wire change (done at the same time the plugs were changed). GM OEM Premium wire sets.
Fuel Filter. GM (OEM)
Fuel pressure check. From Idle to 4000 RPM = 42 to 45 LBS. steady.
I Tank of Seafoam. 1 OZ to each Gallon)
New Thermostat. GM OEM 195 degree
New Water Pump
New Radiator (needed one anyway)
Temp. ranges right around 200 to 210 degrees
New PCV Valve. GM OEM
I've disconnected the MAF - No change/still stalls/perhaps worse. Palmer ScanXL Pro is not indicating a faulty MAF. The range is from 0.6 LB/MIN to 3.9 LB/MIN (Real time).
I have not used carb spray to check for vacuum leaks, although vacuum is -19 Hg (-64kpa) average on a 60 Mile drive.
So, summary.. When the engine stalls whether the RPM/Engine recovers or stops, there are never any codes thrown.

I'm running out of ideas and I'm not a parts swapper..

Any help/suggestions obviously very much appreciated.

I forgot to mention.. I've pulled the throttle body and thoroughly cleaned it.

Might want to check for vac leaks anyway, and maybe check the IAC valve. Did the PCV have new o-rings and the spring on it when it went back in?

Maybe not your problem, but I had a misfire on cylinder 2......ended up being the #2 fuel injector on mine....maybe something to consider if you exhausted the coil pack ..plugs and wires ..maf...cleaning throttle body etc.