Mechanics

HIGH IDLE ISSUE AND INTERMITTANT SHIFTING CAUSE IAC, TPS?

1999 Buick Century • 170,000 miles

Hello, I have a 99 Buick Century and recently I have experienced very high idle speeds and erratic shifting issues. I have cleaned the IAC valve, and did notice my throttle body was rather dirty on the inside. I had cleaned the throttle body with a cleaner, cleaned the IAC, and started the car. The idle was still high. I disconnected the IAC, the TPS, the air box sensor, the sensor on the air intake tube and the EGR sensor, I thought the car should have shuddered or something, so it ran high still. I connected all the sensors back, started the car and still high. My next thought is to disconnect the IAC alone, then the TPS alone. Is there anything I am missing? I have seen others with the same problem, but have not seen any sort of resolution. I thank you for your assistance.
Dennis
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Dlvaughn1963
September 24, 2012.



So do you have any codes right now in the computer?Also did you check for vacuum leaks?If so how did you check for them?

Saturntech9
Dec 18, 2012.
Still have the random misfire.I don't have a smoke generator, I don't smoke a cigar or cigarette ( the cheap smoke generator so I read : ) ) I have sprayed the top of every seam and gasket of the car with a mild soapy solution to chase a leak, and have not found one. I have read that 99 Buick centuries are famous for clogging fuel injectors, I've run two cans of seafoam in my tank and there has been a slight increase in performance. The ransom misfire and idle stumbling does not happen when the engine is first started. It runs fine for about 20 sec. Then as the engine runs, I guess the electrical signals from whatever goes through the engine computer. The engine then starts the cycle of high revs with the first one higher revving then tapers about every 10 seconds for roughly 5 cycles. Then the engine settles with the misfire. When the engine throttle is moved instead of a smooth acceleration, it stumbles and at a certain point will quickly accelerate. So I think one of two things is going on. Either I have a vac leak still, or I have a horribly clogged fuel injector. The misfire does seem to go away, but not entirely.

Tiny
Dlvaughn1963
Dec 18, 2012.
I really dont see how soapy water can find a vacuum leak since a vacuum is the absence of air pressure so no bubbles are going to form. You use carb cleaner and look for rpms going up or down stumbling etc.

Saturntech9
Dec 19, 2012.
Will try again but got no rise in rpms

Tiny
Dlvaughn1963
Dec 19, 2012.
Iam thinking vacuum leaks with the unstable idle have you tried cleaning the idle air control valve?

Saturntech9
Dec 19, 2012.
Replaced the idle air control valve

Tiny
Dlvaughn1963
Dec 19, 2012.
Merry Christmas.
I'm really at a loss. It seems like what ever I think maybe the issue is not. I am thinking now it is some sort of seal leak. When the engine is cold initially it runs nicely for about a minute, then the random misfires occur, and stay until the engine reaches optimum temperature. Once the engine is allowed to cool, then the misfires come back, then goes away when the engine has reached the optimum temp. When the misfires occur, the engine is very sluggish to the point of stalling. Then once warm, the acceleration is normal, and so is the idle. I'm thinking intake manifold from the throttle body? My intake manifold is NOT plastic, but looks like cast alluminum. Is this gasket normally an issue or is it somewhere else?

Tiny
Dlvaughn1963
Dec 26, 2012.
I have seen clogged fuel injectors cause random miss fires a smoke test is the best way to find vacuum leaks.
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Saturntech9
Dec 27, 2012.
Can a bad O2 sensor cause this problem or is it truly a leak?

Tiny
Dlvaughn1963
Dec 31, 2012.
You are gonna love this.
Had run out of options and tired of buying parts so. I looked at routing of spark wires and all of them were good plug to right coil. However. I did find the #3 plug wire NOT clipped I the top of the plug. So I checked them all and started the car and I had a single hiccup and surge and immediately the idle went away. So for anyone who reads this thread to the end random misfire check all cables for connection. Random multiple misfires DO typically point to something in the ignition/timing discrepancy. It took three months to get to this point and I believe that if I would have changed the ICM I may still have not fixed this. Truly the craziest and most challenging of problems always have the simplest corrective action. I did have some minor initial drive rumbles with acceleration but the ECM quickly returned the engine to normal operating parameters. Thanks for your assistance and suggestions for fixing this idle/rpm misfire dilemma.
Dennis

Tiny
Dlvaughn1963
Dec 31, 2012.
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