Q. My 1991 GMC pickup has two problems I hope you can help with.
In the morning, or anytime it has been parked for several hours, it cranks for a long time before starting (three to five seconds).
When used frequently, it starts right up. It also idles too fast (jerks when putting it in drive and is a bit hard to slow down).
It had a code for the idle motor, so I replaced it, but it didn’t do any good.
My manual says the idle speed isn’t adjustable, as it’s controlled by the computer. Could it be bad? I’ve always done my own work. Please help me salvage my pride and not take it into the dealer.
— Marc Moeller, Kansas City, Mo.
A. Your GMC’s long cranking time is likely caused by a faulty fuel-pump relay. Its fuel-injection system uses an electric fuel pump, mounted in the fuel tank, to deliver fuel to the fuel injectors.
The system computer (ECM) requests continuous pump operation via an electrical relay whenever the engine is cranking and/or running. To enhance reliability, GM adds an engine oil pressure switch to provide a duplicate current path to the pump.
Why does the GMC crank long only sometimes? If the relay, or its request circuit should fail to function, the fuel pump will still spring to life — after engine oil pressure occurs.
This can take several seconds during a cold start, but occur almost instantly when the engine has recently run.
How can you determine if the problem is the relay or a missing ECM request? Stand near the right front corner of the truck (hood open) while a helper turns the ignition key from off to the run (only) position. If a click is heard from the rear of the engine compartment (relay commanded on), then a second click two seconds later (relay operation canceled due to no engine activity), we know the request is occurring. This leaves the relay — an inexpensive, easy to replace part — as the prime suspect.
Your fast idle condition is likely caused by a vacuum leak that exceeds the idle air control (IAC) device’s corrective ability. With the air cleaner removed, locate an L-shaped, dime-diameter hole at the top/passenger side of the throttle body.
This is the IAC system’s air-intake port. With the engine idling — in park or neutral, parking brake firmly applied — try restricting the hole with the tip of your finger.
If the engine slows down, we know the ECM is improperly asking for the high idle speed or the IAC device isn’t responding.
This may require professional help to correct. If no change occurs, a slow-down command and condition is occurring. (Air is entering the engine elsewhere, likely due to a vacuum leak.)
Common causes of vacuum leaks are disconnected or leaking vacuum hoses.
Try spraying a stream of glass cleaner or other nonflammable liquid around the base of the throttle body with the engine idling.
If a speed reduction momentarily occurs, you’ve found the leak. Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif. E-mail him at under-the-hood@juno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.
