How to correct extreme idle speed on 1987 GMC R1500 with HEI and 5.0L engine?


Asked by Apr 27, 2015 at 02:59 PM about the 1988 GMC Sierra 1500 C1500 Standard Cab LB

Question type: Maintenance & Repair

Been doing this for over 35 years, never had this problem before.
Got truck cheap, No maintain history. Idles about 2000rpm. Have check for vacuum leaks, stalls when air is blocked. Base timing correct. When EST connection unplugged, idle speed near specs. Have replaced fuel pump/filter (pressures within specs), ESC module, overhauled TBI and ECU. Only thing left I think is distributor and/or ignition module.
Any ideas? Am too old to climb back there unless there is a good reason.
Thanks. Cliff

6 Answers


Were all of the sensors on the throttle body replaced?

Best Answer Mark helpful

Is the throttle stop tab bent, click on picture

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Did not replace MAP. Tried short test. Disconnected and plugged MAP vacuum hose. Idle dropped below spec idle then returned to extreme idle speed. Connected vac hose and disconnected MAP electrical, got same results, dropped to spec idle and then returned to high idle. Forgot to mention that engine cranks slow like ignition timing is too far advanced but does the same with the ESC spout connected/disconnected. Throttle stop tab not bent and screwing the stop screw out does nothing. Throttle plates in TBI are closed completely. This usually points to vacuum leak but tested fine there plus it would be one heck of a leak..


Have you scanned it yet?


First thing. no DTCs. What data is available looks close to specs exc for rpm and timing. Tomorrow I am going back to basics: Manual check of timing, rotor position, wire position. Maybe, Maybe, some one pulled dist/cap and maybe misaligned rotor (prob not, fits one way) and plug wires then tried to correct their mistake by juggling wires to at least make engine run . Electronics no good if mechanics are no good. will repost tomorrow evening. Thanks for you ideas so far.


Sorry took so long to get back. After checking everything I could think of, decided to go ahead and replace intake manifold gasket. Had to force dist out of intake with crowbar and chisels (last removed was during engine assembly in 1987). Needless to say, destroyed the dist. Got new one and installed the intake gaskets. But "may have" found the problem in the process. Take a look at the photo of the dist pu coil and reluctor. completely rusted. the reluctor was dragging on the pu coil's every tooth, not just touching but actually dragging across the individual teeth. Don't know for sure but this is the only thing that is oblivious.

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