Mechanics

FAST IDLE SPEED

1990 Buick Century

Engine Performance problem
1990 Buick Century 6 cyl Front Wheel Drive Automatic 71500 miles

1990 Buick Century 3300 V-6 idle speed is very fast.
The car goes 30 mph by itself, foot off the gas pedal.
When first started cold, it idles slow and normally.
I can idle the car in the driveway until it warms up and the idle speed is still slow and normal.
The moment I drive the car down the block the idle speed begins speeding up and after five minutes of driving, the car goes 30 mph by itself, foot off the gas.
I checked for vacuum leaks, replaced the idle speed control valve, computer, O2 sensor, MAF sensor, Coolant Temp sensor, fuel injectors and this car has no EGR valve.
The Check Engine comes on when acellerating and shifting around 30 mph to 50 mph sometimes, but no code is set.
After driving for 20 minutes I can shut off the engine and wait 5 minutes and then restart engine, it idles a bit fast and then slows down to normal. Then as I drive down the block the idle speeds up and goes 30 mph foot off the gas.
Also sometimes the engine pings as if the knock sensor is being ignored.
The speedometer works and transmission shifts normally.
What would make the computer overpower the engine?
Avatar
Mmirro
July 21, 2010.




I'm answering my own question. The century is fixed, all I did was adjust the TPS to 0.46 volts at closed throttle. What was wrong is that I had adjusted the idle stop screw to low and put the TPS out of range. The Throttle Position Sensor measured 0.01 volts and this caused the Computer to use it's own default voltage which caused a very fast idle speed. Now I never got a code 22 in the months of driving before, so the computer is likely to go to a default in a borderline situation and not set any code, even though the check engine did come on.
Also the pinging noise was only the catalytic converter rattling.

Tiny
Mmirro
Jul 27, 2010.


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