454 W/ VERY HIGH IDLE AT INITIAL STARTUP

[tjwolf]

1991 P-30 that runs great except for the initial start when cold (as in overnight). It starts immediately hot or cold. Races only when cold, and after several minutes it gradually returns to normal idle. Have replaced the Idle air control, the Throttle position sensor, and have been over all of the vacuum lines. Any suggestions?? Have been to two local dealers w/o results. The "logical" GMC truck repair center refuses to touch a unit this old. Anyone remember back to the days of the 454 with a throttle body?

Thanks,
Tom

[tjwolf]

1991 P-30 that runs great except for the initial start when cold (as in overnight). It starts immediately hot or cold. Races only when cold, and after several minutes it gradually returns to normal idle. Have replaced the Idle air control, the Throttle position sensor, and have been over all of the vacuum lines. Any suggestions?? Have been to two local dealers w/o results. The "logical" GMC truck repair center refuses to touch a unit this old. Anyone remember back to the days of the 454 with a throttle body?

Thanks,
Tom

[TandW]

If the throttle body has three bolt mounting pattern to manifold you likely have a failing gasket. See if you have any take-up available on these bolts. If this is the problem, and you decide to change out the gasket, retorque these bolts 30 days later and thereafter at every filter maintenance interval. Good luck!!

[Mutha]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by tjwolf:
1991 P-30 that runs great except for the initial start when cold (as in overnight). It starts immediately hot or cold. Races only when cold, and after several minutes it gradually returns to normal idle. Have replaced the Idle air control, the Throttle position sensor, and have been over all of the vacuum lines. Any suggestions?? Have been to two local dealers w/o results. The "logical" GMC truck repair center refuses to touch a unit this old. Anyone remember back to the days of the 454 with a throttle body? Thanks, Tom </div></BLOCKQUOTE>

Tom, If your motor has a large black motor with 4 or more harness wires plugged into the back of it attached to the left side of the throttle body, you may have a ECM or ISC MOTOR failure. The black motor I referenced is a Idle Speed Controller (ISC) and can be adjusted with the screw that the motor operates by running the screw in and out to control the idle speed via the ECM. These ISC motors were used on the 4 barrel Rochester Carbs and some of the big block throttle bodys, and were very prone to failure and irradic operation. If your engine does not have the ISC motor, then check for a coolant temperature vacuum switch with three or four rubber vacuum lines plugged into it. It is usually located in the front of the intake manifold near the thermostat housing. Unplug the Vacuum lines from the switch and plug them, then start your engine when it is cold and see if your idle is very low, if it is then the vacuum switch is bad and needs to be replaced. Hope this helps you find your problem.

Good luck - Glenn

[tjwolf]

Have replaced both of the gaskets under and within the throttle body. No change in the idle speed. Do not have the motorized speed control system. Will go on to address the vacuum system, as suggested. Thanks to all for the suggestions. May just have to live with unit as is. Just don't like the sound of the engine racing when cold and also concerned about the initial oil-up of components.

Thanks again!
Tom

[Bucky1]

Hope you find the problem and share the solution. My 90 does the same thing.

[Argosy]

It is probably the fan clutch engaged making the engine sound like it is running at a higher rpm than it actually is. When the clutch disengages you don't hear the roar of the air and think it has dropped back to a lower rpm.

Lift the doghouse cover and watch it on a cold start.

[Bucky1]

It is the engine for sure. I can hear it standing by the exhaust, which is a Flowmaster. So it is really easy to hear.

[tjwolf]

Argosy, Your clutch fan diagnosis was right on. I disconnected the engine belts and the electric fan, then started the engine. No noise or apparent high idle now, only normal engine operational sounds. Then reconnected the belts and the noise was back.
I had considered the clutch and electric fans early on. Had disconnected the electric several times with no change in the noise level. Since I had replaced the clutch fan as part of spring air conditioning service, I had discounted that probability.
Thanks again and thanks to all others who responded.

Tom

[Argosy]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Since I had replaced the clutch fan as part of spring air conditioning service, I had discounted that probability. </div></BLOCKQUOTE>
Glad it was such an easy fix for you, I like those.

One of the tests for a fan clutch failing is if it doesn't run at cold start for a minute or so. As long as you hear that initial roar the clutch is probably good.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bucky1:
It is the engine for sure. I can hear it standing by the exhaust, which is a Flowmaster. So it is really easy to hear. </div></BLOCKQUOTE>
On COLD starts the engine RPM will increase over normal idle. To meet emissions standards it only has a certain amount of time to go into closed loop when the computer controls kick in, the catalyst starts working. It operates similar to a choke but is done electronically.

Does the rpm drop back to normal after a couple of minutes? If you run it until normal temp and restart it is the rpm normal?