1986 Chevrolet Astro, 4.3L, 4 speed automatic.
It idles really high, around 1100-1300 rpms, when cold. Sometimes it settles
down to 750 rpms after warm up. Sometimes not.
Basic idle is set at 650-700. All vacuum lines going to the intake manifold
have been checked. There are no air leaks. The check engine light is not lit.
There are no codes in the ECM. The throttle linkage is not binding.
The PROM in the ECM does predate the updated PROM identified in TSB 01/23/1987
Chevrolet 86-T-218, which corrects a cold high idle problem. However, the Astro
did not have a previous cold high problem.
Can a defective temperature sender cause the ECM to produce a high idle?
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