Help: 1986 Astro high idle

Chevy experts:
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.
Background:
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.
The question:
Can a defective temperature sender cause the ECM to produce a high idle?
TIA
Bill
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Yes, if the coolant or manifold air temperature sensor was reading colder than it was, the idle speed could be too high.
The TSB you mentioned says that the updated PROM reduces the cold idle speed to 1600 RPM (!), so under some conditions it probably is normal for it to idle that high, but I'm guessing it isn't actually that cold out..
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Robert Hancock Saskatoon, SK, Canada
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Bingo! It was a bad thermostat (rusted through, never saw that before) and a bad sender. Idle is nice and stable now at around 800.
Thanks.
wrote:

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