Stumped on 86 Buick idle!

86 Buick park ave.,engine good,new timing chain and gears,new coils and base,new crankshaft sensor,new camshaft sensor, new IAC,lots of power BUT when i start it and this happened 95% of the time it will go to high idle,cold or warm makes no difference,have to drive at least a block or more before it will slow down to regular idle.no vacum leaks,no fuel leaks,no codes,fuel pressure to specs,new fuel pump in tank.I dont know what to do,any ideas, Thanks in advance.

It sounds like the PCM does not know whether the motor is hot or cold! Check the CTS; you would be amazed what effect that little baby has on proper engine operation.

It sounds like the PCM does not know whether the motor is hot or cold! Check the CTS; you would be amazed what effect that little baby has on proper engine operation.
Whats a CTS?

CTS = coolant temperature sensor. I works sort of like the TPS by sending variable voltage to the PCM, however it depends on coolant temperature to vary voltage. You should have 5.0V in and variable out. (less than 5.0V)

CTS = coolant temperature sensor. I works sort of like the TPS by sending variable voltage to the PCM, however it depends on coolant temperature to vary voltage. You should have 5.0V in and variable out. (less than 5.0V)
Thanks ill try it.

CTS = coolant temperature sensor. I works sort of like the TPS by sending variable voltage to the PCM, however it depends on coolant temperature to vary voltage. You should have 5.0V in and variable out. (less than 5.0V)
I have 2.50 v on the white wire and 0.02 on the black wire,this is with the key on,engine stopped and cold and the wiring harness plugged into the sensor.These readings are with one lead from the tester to the black or white sensor wire and the other lead to ground.

Now, unplug the wiring connector and check voltage on the white wire to ground. The black is ground and should be infinite to ground.

Now, unplug the wiring connector and check voltage on the white wire to ground. The black is ground and should be infinite to ground.

Unplugged,ingnition switch on,white to ground=5.02 V,black to ground=0.02.

The CTS sensor tells the computer what the operating temperature of the engine is. When the engine is cold, it needs more fuel to operate correctly, and if it is cold enough, it will inject a large quantity of fuel to start the car. This is called Cold Start Enrichment and replaces the function of a choke. The 2.5V reading you are getting when cold is suspicious.

The computer sends a 5-volt reference signal to the CTS sensor. As temperature changes in the coolant, the electrical resistance of the CTS sensor also changes. Start the engine, and check voltage with the wiring connector hooked up the same as you did in post #6. The only wire you need to check is the white wire. Record the voltage changes as the engine warms up to operating temp. i.e. 120, 140, 160 & 180. You should see close to the full range of voltage (0-5.0) during this test. If it does not change from 2.5V, replace the sensor! The sensor costs a wopping $8 at Advance and takes about 15 minutes to change.
Part # TSU81 GP Sorensen

The CTS sensor tells the computer what the operating temperature of the engine is. When the engine is cold, it needs more fuel to operate correctly, and if it is cold enough, it will inject a large quantity of fuel to start the car. This is called Cold Start Enrichment and replaces the function of a choke. The 2.5V reading you are getting when cold is suspicious.

The computer sends a 5-volt reference signal to the CTS sensor. As temperature changes in the coolant, the electrical resistance of the CTS sensor also changes. Start the engine, and check voltage with the wiring connector hooked up the same as you did in post #6. The only wire you need to check is the white wire. Record the voltage changes as the engine warms up to operating temp. i.e. 120, 140, 160 & 180. You should see close to the full range of voltage (0-5.0) during this test. If it does not change from 2.5V, replace the sensor! The sensor costs a wopping $8 at Advance and takes about 15 minutes to change.
Part # TSU81 GP Sorensen

Key on-motor off, wire harness disconnected,white wire=5.02 V.
Key on-motor off,wire harness connected,white wire=2.50 V.
Motor running,wire harness connected,3.60 V,as motor warms up V keeps going down to 1.06 V then jumps up to 3.50 V then V starts to go down and stops at 2.50,as motor gets warmer it will flucuate up and down,never below 1.00 V and never above 3.60 V,and sometimes as its idling it will stay steady at 2.50 V. I had another sensor from another engine i put in and it stayed at 2.50 V,never moved up or down.

Key on-motor off, wire harness disconnected,white wire=5.02 V.
Key on-motor off,wire harness connected,white wire=2.50 V.
Motor running,wire harness connected,3.60 V,as motor warms up V keeps going down to 1.06 V then jumps up to 3.50 V then V starts to go down and stops at 2.50,as motor gets warmer it will flucuate up and down,never below 1.00 V and never above 3.60 V,and sometimes as its idling it will stay steady at 2.50 V. I had another sensor from another engine i put in and it stayed at 2.50 V,never moved up or down.

Update,i also checked the TPS,0.45 V at throttle closed, engine stopped,when i moved the throttle to full open V increased accordinly to a high of 3.52 V.

Key on-motor off, wire harness disconnected,white wire=5.02 V.
Key on-motor off,wire harness connected,white wire=2.50 V.
Motor running,wire harness connected,3.60 V,as motor warms up V keeps going down to 1.06 V then jumps up to 3.50 V then V starts to go down and stops at 2.50,as motor gets warmer it will flucuate up and down,never below 1.00 V and never above 3.60 V,and sometimes as its idling it will stay steady at 2.50 V. I had another sensor from another engine i put in and it stayed at 2.50 V,never moved up or down.

Based on the information above, I would say you need a new CTS sensor!! Also, the TPS idle voltage is good, WOT should be 4.5V to 5.0V.

Key on-motor off, wire harness disconnected,white wire=5.02 V.
Key on-motor off,wire harness connected,white wire=2.50 V.
Motor running,wire harness connected,3.60 V,as motor warms up V keeps going down to 1.06 V then jumps up to 3.50 V then V starts to go down and stops at 2.50,as motor gets warmer it will flucuate up and down,never below 1.00 V and never above 3.60 V,and sometimes as its idling it will stay steady at 2.50 V. I had another sensor from another engine i put in and it stayed at 2.50 V,never moved up or down.

Based on the information above, I would say you need a new CTS sensor!! Also, the TPS idle voltage is good, WOT should be 4.5V to 5.0V.

My TPS is at closed throttle=0.45 V, at wot its at 3.52, you say it should be at 4.5 V to 5.0 V at wot, you say my V is "good",did you mean to say "no good", just checking with you,im at 11/2 V lower than it should be,what do you think,and Thanks for your help.

The TPS idle voltage is ideal, (.45V)
WOT should be 4.5V to 5.0V. Usually they check 4.2 to 4.5V at WOT. I wonder why you only get 3.52V @ WOT. Check the white wire on the TPS to see if you have 5.0V.

The TPS idle voltage is ideal, (.45V)
WOT should be 4.5V to 5.0V. Usually they check 4.2 to 4.5V at WOT. I wonder why you only get 3.52V @ WOT. Check the white wire on the TPS to see if you have 5.0V.

Yep i have 5.01 V on white wire with harness plugged in ,motor off,key on.

On the CTS, i put new one on,it showed 3.80 V motor stopped,harness plugged in,motor running it went down to 1.00 V and stayed there,key on,harness unplugged=5.01

Sounds like you have tackled the CTS problem.
You have the 5-volt reference signal from the PCM to the TPS and good voltage at idle or closed throttle. I would not worry about the WOT reading if you have a linear increase throughout the range. How is the idle acting now?

Sounds like you have tackled the CTS problem.
You have the 5-volt reference signal from the PCM to the TPS and good voltage at idle or closed throttle. I would not worry about the WOT reading if you have a linear increase throughout the range. How is the idle acting now?

Actually its doing a whole lot better but mind you the motor is warm and its hot out,i will see tomorow morning when the engine is cold,when i start it in the morning that will tell the tale,i will post the results tomorrow morn,Thanks again.

Sunday morning update on Buick idle,its 51 degrees F here, just went out and started the car,it went to a nice high idle,better than it did for a long time,then after 1 min and 30 secs. it went to normal idle in 2 stages, sat there and idled perfect, i only hope it keeps on like this, time will tell. Thanks again for all the help.

I love it when a plan comes together! Unfortunately it is difficult to diagnose problems without the car in front of you. You were able to give all the correct information and made the diagnoses less complicated. If all people were as persistent as you, diagnosis over this forum would be a lot easier. Thanks for all the information you provided!! I hope all goes well.

:cheers: :rofl: