Rough Idle after warm-up
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Junior Member
Joined: May 2008
Posts: 36
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Car: 88' Trans-AM WS6
Engine: 305 TPI
Transmission: Auto
Rough Idle after warm-up
hey guys.
I have a stock TPI 305 in a 88 Trans-am, which I could not get to start - due to very bad plugs.(I also changed out the HEI module) After the struggle to change em, it starts fine now but after it starts it idles smoothly at about 1000rpm (according to the factory guage) the after a few minutes, the rpms start dropping down to around 500 and rebounding, very rough.
The first time it did it I had the dash and cluster off, I was thinking it was overheating ( I never noticed either fan coming on ) but this time I had the cluster on, and the car was not excessivly hot 150-170, but the fans never came on this time either - Im not sure about that but the idle conserns me
Any thoughts
I have a stock TPI 305 in a 88 Trans-am, which I could not get to start - due to very bad plugs.(I also changed out the HEI module) After the struggle to change em, it starts fine now but after it starts it idles smoothly at about 1000rpm (according to the factory guage) the after a few minutes, the rpms start dropping down to around 500 and rebounding, very rough.
The first time it did it I had the dash and cluster off, I was thinking it was overheating ( I never noticed either fan coming on ) but this time I had the cluster on, and the car was not excessivly hot 150-170, but the fans never came on this time either - Im not sure about that but the idle conserns me
Any thoughts
TGO Supporter
Joined: Oct 2002
Posts: 9,067
Likes: 1
From: Saskatoon, SK, Canada
Car: '83 Z28, '07 Charger SRT8
Engine: 454ci, 6.1 Hemi
Transmission: TH350, A5
Axle/Gears: 2.73 posi, 3.06 posi
Re: Rough Idle after warm-up
Sounds to me like its ECM related, having some kind of problem after it goes into closed loop (? I always get closed/open terminology mixed up lol).
I would check the wiring to the CTS and O2 sensor, see if thats ok. Maybe also hook up a timing light after it gets into closed loop and see where its putting the timing.
I would check the wiring to the CTS and O2 sensor, see if thats ok. Maybe also hook up a timing light after it gets into closed loop and see where its putting the timing.
TGO Supporter
Joined: Oct 2002
Posts: 9,067
Likes: 1
From: Saskatoon, SK, Canada
Car: '83 Z28, '07 Charger SRT8
Engine: 454ci, 6.1 Hemi
Transmission: TH350, A5
Axle/Gears: 2.73 posi, 3.06 posi
Re: Rough Idle after warm-up
Poor running will not always cause the SES light to come on. The ECM has to first actually notice that its not running correctly, by what some sensor or sensors are telling it, and then it has to decide that it cannot correct it by itself before it will turn that light on.
Sometimes an engine can run very poorly, and not turn the SES light on. My '97 did this a few months after I go it, actually - had a dead short on the #6 plug wire (rubbed through on a pulley, grounded out) but it did not turn on the SES light. It just ran really rough and shakey and had no power. Traced it to #6 not getting a spark, and replaced the wire that was almost cut in half. Problem solved. It should have turned the SES light on, because of all the unburned fuel going out the exhaust, but it didn't.e
On the other hand, a sensor giving the ECM bad information can also make this stuff happen, without causing the SES light to come on. All the info looks good to the ECM, but with something not working correctly, the information is wrong, and the ECM doesn't know, and will think its all peachy, even though it runs like crap. O2 sensor and CTS are common causes of problems like yours.
Sometimes an engine can run very poorly, and not turn the SES light on. My '97 did this a few months after I go it, actually - had a dead short on the #6 plug wire (rubbed through on a pulley, grounded out) but it did not turn on the SES light. It just ran really rough and shakey and had no power. Traced it to #6 not getting a spark, and replaced the wire that was almost cut in half. Problem solved. It should have turned the SES light on, because of all the unburned fuel going out the exhaust, but it didn't.e
On the other hand, a sensor giving the ECM bad information can also make this stuff happen, without causing the SES light to come on. All the info looks good to the ECM, but with something not working correctly, the information is wrong, and the ECM doesn't know, and will think its all peachy, even though it runs like crap. O2 sensor and CTS are common causes of problems like yours.
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