'91 TBI 5.7l With Idle problem
 

Hey all, new member here. Old mechanic but aircraft and now locomotives, only occasional car maintenance.

I have a '91 C1500 2WD with the TBI 5.7L.

It wont idle fast enough to keep the engine going when it's in gear.
If I unplug the Idle Air motor, it idles so fast that it's hard to keep it from rolling thru stop signs. But at least it doesn't stall.

It has 222,000 miles on it, has good compression, doesn't leak too much oil and gets 21 mpg on the highway. So I'd like to keep it going for a couple more years.

I have changed the Idle air motor, coolant temp sensor(the one for the fuel injection system), spark plugs, plug wires, ignition coil, distributor cap. Previous owner says He changed the fuel filter (which does look new) and the Injector unit (which doesn't look new, but there was a really crusty one in the box when I cleaned it out).

I've changed enough parts. I'm now thinking the idle circuit inputs are being misread by the Engine management computer, so the output to the Idle air motor is not opening (or closing) it far enough to get the proper idle speed. Am I on the right track? How can I check the computer output to the Idle air control?
My VW Factory Maintenance manual has the output signal voltages for each controller posted. Is there a similar listing for Chevrolet? Chilton's doesn't have it.


Thanks in advance for any suggestions!!!

Marc L

Have you cleaned the throttle body?? Seems like something simple but it also can be overlooked

Those throttle bodies are famous for the rod that the throttle plate rides on getting worn out. Especially with those kind of miles. It may be closing too much or be dirty like ChevySilvy already suggested. I would look there first.

Thanks, I'll look into that this weekend.

Marc