This is normally the way it is suppose to work. You need to have the RPM measured for High RPM and duration. What you are describing is how an electronic choke is suppose to work.
The errors would be how high the RPM goes and for how long it lasts. When cold the RPM, fuel mix and timing are changed to give a rich fuel mix. As the O2 sensors warm(Could be defective) they signal the engine to lean up. The water thermostat also has something to do changing the engine to a lean running slow condition.
If obtaining heat from your heater seems to take Forever, the thermostat could be weak. This would coincide with the engine continuing at a high RPM longer because it is not warming up.
SOURCE: will not drop down from fast idle when first started
It sounds like an automatic choke system. Let it warm up for 5 minutes and then give it gas up to 2000rpm and release.
SOURCE: 87 Ford F150 idles too fast
either a vacuum leak or an idle air control valve problem most likely.the iac is on the side of the throttle body.cylinder shaped with a small harness.
SOURCE: 1987 F150 302. No trouble codes but engine idles too fast
The IAC (idle air control) may be stuck from dirt or malfunctioning.It's located in the intake near the throttle body.Remove it clean with carb cleaner and reinstall
Testimonial: "Thank you for response. Actually had IAC off and was clean inside. Resistance within range 9 ohms but voltage signal at 12.5 V rather than 10.5 V"
SOURCE: 86 Ranger 4X4 2.9 fuel inj runs rough when warmed up
1988 ranger 2.9 runs great cold then after a few minutes when warmed up hesitates with intermittent black smoke from the tail pipe until i hit the the throttle then clears and will repeat this behavior...
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