'01 GTP, from a warm start it idles very rough

New to the forum, but a long time reader.

I've got an '01 GTP, 3800 S/C, only mod is a Wizaired CAI, 88,000k, owned it for 4 years. Having a problem with the idle now, it's very rough, but seemingly only from a warm start. Service Engine Soon light is on.

I'd driven it home from work last Tuesday, turned it off and went inside. An hour later I'd started it again to leave, but the check engine light came on and it was idling very rough. I decided to change the oil (it needed done anyway), and then started the car again. It was still idling rough for a minute or two, but then it normalized and idled fine. I'd let it run for about 5 minutes and then did a restart, and it instantly began the rough idle again. The following morning I had started it, and it idled just fine so I drove it for 30 minutes. Got home, did a restart, and the rough idle was back again.

If the car is started after it's been warmed up, it will idle very rough with almost no power (while the SES light blinks at me). But... if the car is started when it's cold (not warmed up), it will idle normally and run great.

Has anyone heard of this before?

Welcome to AF.

Sounds like a secondary ignition issue meaning spark plugs, wires or coil pack. A blinking SES light indicates a misfire condition so get it scanned with an odb-ii scanner and post the code.

A misfire condition can damage the CAT converter if not corrected soon.

Thanks for the reply. Went to Advance last night. The code is "p0203 injector ckt open" (cylinder 3). Does this mean that it's stuck open, meaning a faulty injector? If that's the case, I could replace the injector in that cylinder. What do you think?

it means the PCM has detected a circuit fault with injector 3. I would ohm out the injector with a mulitmeter. It could be a bad injector (internally open), bat wiring to, or even a bad PCM. Do you use a lot of gas line anti freeze / fuel additives?

I've got an ohm meter I'm gonna use right now, and I've never used additives. The PCM wiring is stretched a little tight from when I'd installed the Wizaired cai box, but it's been like that for 4 years now. If it is the PCM, that'll be a great excuse for me to get the Digtal Horsepower PCM, and also do the Ubend delete and pulley! I'll check it now and see what it does.

I think you're well into troubleshooting. However, one diagnostic trick is to unplug one injector at a time when idling.

On newer vehicles with the coil-on-plug ignition, the injectors are hard to get to, but the coil packs are not.

Essentially by doing either of the above, you are "disabling" one cylinder at a time. When you disabled a good-running cylinder, it will really get rough and probably stall. When you disabled a non-firing cylinder, then there should be no change in the general engine operation.

It takes a certain bit of luck to be able to catch the vehicle in the "problem state" where you can jump out, open the hood and try the above. However it sounds like your problem was one of those occasions.

On newer vehicles with the coil-on-plug ignition, the injectors are hard to get to, but the coil packs are not.

:confused:

The fuel injectors are not hard to get at on this engine so I do not see the relevancy of this comment. You can even connect a noid light to check the fuel injector pulse width modulation signal. Also, you can run a resistance check of the fuel injector coil. 11-12 ohms.

The best thing to do is check for the 12 volts at the respective injector electrical connector. Ignition to ON. And then connect a noid light or a multimeter and check the injector pulse from the PCM module while engine is running. It will pulse high then low, etc.

This DTC is normally related to an internal PCM module fault related to the output driver module similar to the old quad driver faults on older odb vehicles so the PCM module is a possibility.

Why would he need to do a cylinder drop test? He's got the code that points to the exact cylinder/injector/wiring. He's just got to do some basic electronic checks from here on out to determing if the problem is an open in the circuit, a bad injector, or a bad PCM. I'll be willing to bet that since he last posted at 3:00 yesterday, and said that he has a multimeter, that he has already found and repaired the problem. He just hasn't gotten back to us yet.

:confused:

The fuel injectors are not hard to get at on this engine so I do not see the relevancy of this comment.


Exactly. It is not relevant to a 3800, hence the comment was directed at "newer GM engines which have the coil-on plug ignition" (3800 does not have this with 3 remote coil packs and 6 plug wires). Consider trying to disconnect a single injector on the Line-6 or on the L-850 4-cylinder family.

It's important in my mind to recognize various techniques you can use for engine troubleshooting. I've worked on cars, trucks, forklifts, boats and farm tractors all which use GM engines and while each application is different with different controls, the basic principles still apply. The above is one that can usually be done with few to no tools.

Hey guys. I hadn't fixed it yet, but only because the ohm meter that I'd had broke when I went to adjust it. So I'd bought a multimeter last night, and don't know which setting to set it on to test it! There's so many settings on there, anyone know which one? It's a cheapy from Harbor Frieght (I hate buying from them, but was right by one). And where on the injector connector would I touch with the multimeter prongs? I'd assume I'll need to disconnect the injector first, and then touch the prongs into each of the two openings on the plug?

Exactly. It is not relevant to a 3800, hence the comment was directed at "newer GM engines which have the coil-on plug ignition" (3800 does not have this with 3 remote coil packs and 6 plug wires). Consider trying to disconnect a single injector on the Line-6 or on the L-850 4-cylinder family.

It's important in my mind to recognize various techniques you can use for engine troubleshooting. I've worked on cars, trucks, forklifts, boats and farm tractors all which use GM engines and while each application is different with different controls, the basic principles still apply. The above is one that can usually be done with few to no tools.

Crazyhorst,

I think you missed my point. As a Moderator I read, screen or review all posts in this forum and reading your posts is getting very boring and/or tedious. I find your posts although sometimes insightful consist of more diatribe or irrelevant comments than clear, concise technical help relating to a specific platform or in this case a Pontiac Grand Prix. Please get to the point on your replies and cut the fluff. You may consider this as a friendly warning.

And where on the injector connector would I touch with the multimeter prongs? I'd assume I'll need to disconnect the injector first, and then touch the prongs into each of the two openings on the plug?

To ohm out the injector coil remove the electrical connector. Then with meter on ohms and low resistance scale go across the two pins with the red and black meter leads at the injector. Reading should be around 11-12 ohms.

To check voltage take meter selector to DC voltage and the appropriate scale to read low voltage or in this case 12 volts. Take black lead to a good chassis ground like the battery negative terminal or engine/tranny ground stud. With red lead probe each of the two pins at the injector electrical connector (Note: Not at injector). One will read 12 volts with ignition to ON. The other pin is the pulse that comes from the PCM module. When high the injector does not fire. When pulses low injector fires or sprays.

Crazyhorst,

I think you missed my point. As a Moderator I read, screen or review all posts in this forum and reading your posts is getting very boring and/or tedious. I find your posts although sometimes insightful consist of more diatribe or irrelevant comments than clear, concise technical help relating to a specific platform or in this case a Pontiac Grand Prix. Please get to the point on your replies and cut the fluff. You may consider this as a friendly warning.

Hi, with respect for your position, I will continue to post as I see fit. If you feel that I'm giving out erroneous information then please say so and we can debate like educated adults. I can say with confidence that none of my posts are made without respect nor without some technical merit.

I doo recognize that many of my posts have to do with how systems work and the various strategies that are baked into the engineering thereof....so anyone would probably get the idea that all the details can't be hit in a single sentence and the posts can get longer than "change the TPS sensor". I don't feel like everyone necessarily has the full understanding of how the mechanicals and/or controls systems work and that's the type of information I'm trying to (hopefully) simplify and divulge.

Again, I try to give another viewpoint and a different problem-solving philosophy. If it is not needed here then I believe you are armed with the necessary powers.