Rough Idle Pontiac Grand Prix


Old 07-24-02, 08:23 AM
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Exclamation Rough Idle Pontiac Grand Prix

Subject vehicle is a 1990 Pontiac Grand Prix 3.1 V-6 with 311,000 miles. Yes the mileage is correct. Previously this car was the vangaurd of reliability with the exception of the total repair of the rear disc brakes on an annual basis.
The other day I was in the process of running errands in 95* and about 65% humidity. Coming out to start the engine in order to go home, the car would not start at all. The engine turned over without a problem, but the engine just would not "go". AAA was experiencing critical mass and it would be hours before I could even think about the calvary arriving, so I called my wife who was only 30 minutes away. Upon her arrival, I attemptted to restart the Pontiac. It started, but idled extremely rough and the smell of unburned gas could be smelled. I left the car and returned later in the evening. It started once again with the same rough idle. Much like that of a .518 lift / .310 lift cam. I attempted to drive the car home. When the car is staionary, I can increase the RPM to really whatever I want, but under a load it bogs down and I found myself pumping the accelarator for power and backing off to keep the car from stalling.
I made it home, although I did have to restart the car on the fly (restarting in neutral while the car is moving). I was able to get the car up on ramps, as I originally thought that it might be the famous crankcase sensor making itself known (an annual replacement, although this one is only 6 mos. old) I have replaced the IAC Valve. Idle Air Control valve.
Initially it seemed to do the trick, as I idled at about 3k for a few minutes, hoping that the RPM at normal operating temp might just 'trick' the sensor to reset itself. I still have the problem.
I just had the engine tuned about 3k miles ago (although it has been 6 months) The car is used as a spare - meaning that if either of my other two cars go 'down' then this is the temporary replacement, as my wife and I head in opposite directions for work.
Any I deas?
The maintenance on this auto is up to date and I take extreme care of it, as otherwise I would not have been able to get the mileage I have out of it. Also, it still has the Texas look to it, although it has been in Wisconsin for 5+ years = no rust.

Thank you.
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Old 07-24-02, 08:30 AM
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Might have missed the basics

How's your fuel pressure? Could have a bad pressure regulator at this mileage.

Since you have started with ignition, I'll ASSUME it's ok. You could also have a bad coil or module. Any trouble codes in the computer?

See my links and posts below for more ideas.
Old 07-24-02, 08:41 AM
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Rough Idle Pontiac

The fuel pump, filter etc have all been replaced within the last year, also when sustaining an idle of 3k there is no hesitation at all, something I think would be evident if the pressure was malfunctioning. I hope it is not the coils, as I have 3 and the cost is around $800.00.
Old 07-24-02, 10:20 AM
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Nope, still have to check the fuel pressure. It's one of the basics.

Again unless you work in progression, you will exceed the value of the vehicle EASILY by throwing parts at the problem. Have to work at the basics and rule out what is good and what is not before going further.
Old 07-24-02, 11:39 AM
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Rough Idle Pontiac Grand Prix


I am afraid that I am way past the point of economical return on investment. I have recently had it repainted and a set of wheels installed. It has become more of an issue of pride. The overall condition of the car is great and I still feel (or would if I can get this problem fixed) comfortable in making another trip from Wisconsin to Texas - obvioulsy from the mileage I have made a few trips already. I will check the fuel issue and advise, altghough it might be a couple of days before I can advise you.

Best regards,

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