2001 LS Premium Stumble, Misfire, Rough Idle

[Da Gunner]

During warmer weather, this problem was noticed as a miss or stumble at redlights. Now that it is cold here in Jacksonville, and cold for us is anything that results in frost on the ground, the car is a beast to keep running at first startup. Even after running for a while, the car still stumbles mildly at red lights. I am going to pull codes this afternoon. I have replaced the EGR valve, DPFE sensor, IAC motor, and the problem seemed to go away. This is the worst that it has been. I am concerned that I may have an elusive vacuum leak or a problem with the intake.

I will post as soon as I know anything.

12/22/05- I pulled the codes yesterday. I have both left and right bank lean conditions, and misfires on cylinders 1,2 and 3. Five codes in all. I am thinking either a massive vacuum leak, poor fuel pressure, or failed Oxygen sensors.

I can't find a problem with the PCV valve or associated plumbing. Doesn't mean there isn't one, I just don't see broken down rubber at any of the connections. I did not pull the PCV valve.

Fuel pressure looks good, very stable at 55 psi, even during the stumble.

Anybody got any ideas?

[KimMG]

misfire could be caused by bad spark plug wires.

[Da Gunner]

Originally Posted by KimMG

misfire could be caused by bad spark plug wires.

Based on the excessive lean condition on both cylinder banks, I suspect that I will need to replace the plugs and wires at any rate (88k miles), but I think this is caused by the computer trying to richen up the mix. I don't think it is the root cause of the problem. I expect to find fouled plugs due to an excessively rich fuel trim. An excessively lean condition code being latched tells me the computer has used the entire range of injector pulse opening to try to fatten up the mixture, but still failed. So either I am injesting more air into the cylinder than is being detected by the MAF sensor (indicating a leak somewhere after the MAF), or the MAF sensor is malfunctioning. I thought OBD2 was supposed to catch failing sensors.

I am ruling out Oxygen sensors at this point. My understanding is that upon cold start, the system operates in Open loop, and does not use o2 sensor input for fuel trim.

Any other input would be appreciated. And thanks KimMG.

[Da Gunner]

Originally Posted by Da Gunner

Based on the excessive lean condition on both cylinder banks, I suspect that I will need to replace the plugs and wires at any rate (88k miles), but I think this is caused by the computer trying to richen up the mix. I don't think it is the root cause of the problem. I expect to find fouled plugs due to an excessively rich fuel trim. An excessively lean condition code being latched tells me the computer has used the entire range of injector pulse opening to try to fatten up the mixture, but still failed. So either I am injesting more air into the cylinder than is being detected by the MAF sensor (indicating a leak somewhere after the MAF), or the MAF sensor is malfunctioning. I thought OBD2 was supposed to catch failing sensors.

I am ruling out Oxygen sensors at this point. My understanding is that upon cold start, the system operates in Open loop, and does not use o2 sensor input for fuel trim.

Any other input would be appreciated. And thanks KimMG.

Grand revelation today. The scanners that Autozone uses are not able to read / interpret All Codes !! The computer was storing a code for intermittent MAF signal, but I didn't know it was there due to the scanner I was using. The MAF sensor was indicating excessive air flow at startup, so the computer was fattening up the fuel mix, resulting in misfires and fouled plugs. Things ran better after the car warmed up due to the o2 sensors being part of the equation once the car was up to operational temperature.

Right now I am in the middle of replacing the plugs, wires, and repairing the intermittent wiring problem at the MAF sensor connector. That probably is what killed the MAF sensor in the first place.

BTW, I should clarify that I have an excellent mechanic here in Jacksonville that is performing all of the troubleshooting and repairs. If there is anyone here in Jacksonville that has reached the limit of their expertise, I am happy to recommend them to you. They have done excellent work for me in the past on tough problems with my 94 LT1 Corvette. I typically do all of my own work, but know when a problem is beyond my test equipment and skill level. This turned out to be one of those times.

[Da Gunner]

Finally found the culprit. The intake manifold is warped at the mating surface of the throttle body / manifold flange. In my case, it is so severe you can see it. By the way, our throttle bodies are held on with 2 bolts, not 4, which made this very easy to happen.

We pulled the gap closed with a small C clamp and voila, no leak. Now I have to determine if I can find some appropriate gasket material that will not get sucked into the intake, or will I need to replace the intake. My local stealership will provide one for @220.00.

So all you folks that seem to experience severe startup problems in cold weather can spray carb cleaner in that area upon initial startup and see if your stumble clears for a few seconds.

Best of luck.

Until next time,

Da Gunner

[pekin21]

Thanks for the update!

[Da Gunner]

Originally Posted by pekin21

Thanks for the update!

You're welcome, Pekin.

I replaced the intake manifold, which corrected the terrible cold start performance, left and right bank lean conditions, and misfires on cylinders 2 and 3. When I road tested the vehicle, the car had a slight stumble upon accelleration, and I noticed I would feel it miss once in a while when sitting at a redlight. Upon hard accelleration, the check engine light would flash, indicating a misfire condition that could damage the catalytic converter by dumping raw fuel into the exhaust system. I pulled the codes, and I still had a misfire on cylinder 1.

I pulled the plug on cylinder 1, no cracks from the installation, and the plug wire read fine. I moved the injector from cylinder 1 to cylinder 2. The failure stayed on cylinder 1 after another road test. These cars have what is referred to as a distributor-less ignition system, where the computer send a signal to a coil pack that looks just like the old distributors, only there is no coil wire. I replaced this unit, which is on the rear cylinder head by the firewall. I removed the intake to do it, making it an absolute breeze. The intake is also very simple to remove. Long story short, this corrected the intermittent misfire on cylinder 1. The car accellerates perfectly now, under all throttle positions.

[pekin21]

So do you think it was a combination of the intake and the coil pack causing the problem? Obviously your intake needed to be replaced anyway since it was warped but I may try replacing the coil pack first since I had read a cylinder misfire on my codes before. How much for that?

[Da Gunner]

Originally Posted by pekin21

So do you think it was a combination of the intake and the coil pack causing the problem? Obviously your intake needed to be replaced anyway since it was warped but I may try replacing the coil pack first since I had read a cylinder misfire on my codes before. How much for that?

I paid 104.00 including sales tax for the coil pack. If the misfire tends to stay on the same cylinder, I would definitely consider the coil pack as the primary suspect. In my case, I had already replaced the plugs and wires. If you are going to do plugs, wires, and this coil, I can not emphasize enough how simplified this process becomes if you remove the intake. It is entirely too simple to do.

Best of luck,

Da Gunner

[pekin21]

How difficult is the intake to remove? My garage isn't heated and the temp. here in Minnesota is dropping fast; close to the single digits last night. If it's something that's going to take awhile (remoing intake, replacing coil pack) then I'll probably take it somewhere to have done.