93 DeVille Engine surging at idle

[mikerader]

Hello,

I have been having this issue for about 6 months now. I was getting E27 Throttle position sensor errors and the engine is surging at idle.

I replaced the Throttle position sensor and set it up with a volt meter. .55 volts at fully closed and just over 4 volts wide open throttle. The voltage on the return wire seems smooth with no drop outs.

My engine still surges at idle and I still get the TPS error. When I am driving and take my foot off the gas the car will drive at 35 to 40 mph with no gas applied. It does not slow down until I hit the gas or brake a few times.

I suspect the Idle speed control motor may be bad and not homing. How can I test this? When I lift the hood and watch what is happening I see the following.

If I pull back on the throttle, the speed motor expands or steps out then when I let go of the throttle the stepper motor slowly retracts and sometimes get stuck. This keeps the car running at a higher speed to long and seems to validate what happens when I drive.

Why does the Idle speed motor take steps out like this when you apply the throttle. Is this normal operation? Do I have a bad Idle Speed Control Motor or is it the Control Module? I am guessing the idle speed motor is not homing and the cause of my problems.

Thanks in advance.

[Dave S Texas]

The ISC may be faulty or only in need of adjustment. The adjustment procedure is covered by posts over in the 4100 forum and in your FSM. When the TPS is removed/adjusted, the next step following that is the ISC adjustment procedure.

[mikerader]

Thanks I will look for that post. I am confused by the operation of the ISC motor though. Why does it expand or take steps out when you apply the throttle?

E27 says "Open throttle signal switch" I am also wondering if I have a bad cable to the TPS. The blue wire, (wire B), measures OK with a meter but may have a faulty cable and the signal may not get to to the PCM.

[mikerader]

I found a post that explains the operation of the ISC motor.

Its says,

"Description of ISC.
When the throttle is released it depresses the plunger on the ISC, this closes a switch in the
ISC to signal the computer that the throttle has been released and to go to the idle mode.
If the switch is not closed when the throttle is released, then the engine "hunts" and doesn't
know what to do, hence the ranging of rpms."

This sounds like my problem, the ISC is hunting because of dirty contacts. I will take it apart and try cleaning the contacts.

[Ranger]

I think a good cleaning may solve your problem. If not, then maybe a new switch. One sign of a bad ISC motor is ratcheting when you shut down.

[mikerader]

Update,

I took apart the ISC motor and cleaned the contacts with 2000 grit wet/dry sandpaper. This seems to have fixed the issue for now. E27 code is also gone for now. Definitely had corrosion on the contact points.

Thanks all for your help.