Help! odd idle issue

[82883t]

Hi folks. I have an odd idle issue on my '87. It's a Procharged L-98, with the manual trans. I just installed a new 58MM throttle body. The car idles fine and drives fine.....UNTIL it reaches normal operating temperature. Once the coolant temp has gone above 205, when coming to a stop, the idle climbs from 8 hundred rpm all the way to 3K. I then have to blip the throttle to bring it back down. This will occur on most stops. So I gave the throttle return spring an additional rotation to increase the return tension. But the symptom remains. Any thoughts?

 

[ecss]

Hi folks. I have an odd idle issue on my '87. It's a Procharged L-98, with the manual trans. I just installed a new 58MM throttle body. The car idles fine and drives fine.....UNTIL it reaches normal operating temperature. Once the coolant temp has gone above 205, when coming to a stop, the idle climbs from 8 hundred rpm all the way to 3K. I then have to blip the throttle to bring it back down. This will occur on most stops. So I gave the throttle return spring an additional rotation to increase the return tension. But the symptom remains. Any thoughts?

If the problem began after the new throttle body install, reinstall the original and see what happens.

Doesn't make sense the rpm increases after dropping down to idle unless something is wrong with the super charger bypass valve or the IAC (idle air control valve) is acting weird or air is some how getting around the throttle plate.

Someone had a new BBK throttle body and the throttle plate would stick when the engine reached operating temperature. His solution was to bypass the throttle body coolant circuit but you are saying the rpms increase from 800 to 3K when coming to a stop.

 

[DonB]

As ecss said, I'd have to agree that air could be somehow getting into the plenum. Be it past a sticky(partially open) throttle plate(s), leaking gasket or seal. Also could there be an issue with your TPS, like it's mounting being loose?

Are you pulling any codes?

 

[82883t]

Pulling a code 36.... Don't think it can be related to this issue. I'm going to pull the throttle body and inspect the bores and remove the iac body to inspect. Thanks you all your help..

 

[ecss]

Pulling a code 36.... Don't think it can be related to this issue. I'm going to pull the throttle body and inspect the bores and remove the iac body to inspect. Thanks you all your help..

DTC 36 is a Mass Air Flow burn off problem. After you shut the engine off, the ECM enables the Burn off relay which passes 12 volts through the MAF sensing wire to burn off any impurities on the sensing wire.

Look closely at the wires at the relay sockets. On the older C4's the wire insulation would shrink back and allow the wires to short to each other.

In most cases replacing both relays used in the MAF circuit fixes DTC36.
The two relays are mounted on the firewall near the driver side A pillar. I believe the two relays are not interchangeable on your year vehicle.

Below is a block diagram showing the MAF circuit.

Also did you set the Throttle Position sensor to .54 volts DC when you moved it over to the new throttle body?
After that you should disconnect the negative battery terminal to reset the ECM. You measure from the TPS Black wire to the MAF Dark Blue wire with the ignition On engine not running. As you manually operate the throttle body you should see a smooth voltage rise from .54 volts to over 4.6 volts at wide open throttle.

From the Black wire to the Gray wire you should measure 5 volts. To take these measurements you need to pierce the wire insulation with something sharp like a sewing needle.

 

[Hib Halverson]

If the problem began after you installed the TB, you may have a calibration issue. I'd solve the code 36 and then, if the idle problem continues, find a calibration engineer who can record some data as to how the engine is idling then burn you a new cal.

 

[82883t]

If the problem began after you installed the TB, you may have a calibration issue. I'd solve the code 36 and then, if the idle problem continues, find a calibration engineer who can record some data as to how the engine is idling then burn you a new cal.

Hi folks. Thought I'd give you an update for anyone's future reference. After checking everything I could think of and more (as a result of all your help), I was left only suspecting the new throttle body. I ordered a new one (different brand) and installed it. Re-set the IAC and TPS and idle stop on the new unit. Not only did the symptom disappear, but the car runs better than it EVER has before. Thanks to everyone for all your suggestions. I really appreciate it.