1989 Plymouth Horizon Repair Question
Plymouth Horizon Thermostat Problem
Look at the old and / or new thermostat to see if there's a small bleed hole in the middle of it. That will allow a small amount of circulation when the thermostat is still closed so the heat that is generated "over there" by the temperature gauge's sending unit can migrate over to the thermostat "over here" faster and cause it to open sooner. If there is no bleed hole, you can drill a 1/16" hole to solve this problem. I ran into this with a replacement thermostat in my '88 Grand Caravan 3.0L about ten years ago. The problem did not show up for many years, then one day started doing this. After living with it for about two years, I had a student drill the hole and that problem has not occurred once since, and that van is still my daily driver with just over 400,000 miles.
For the high idle, first look at the carburetor base gasket. They tend to shrink with age and cause vacuum leaks. Also be sure the bolts are tight and the carburetor isn't loose. There's an idle speed solenoid on the back of it that tends to become stuck from corrosion. People will turn the adjusting screw in the middle of it to get the idle speed up to where it should be but then if it moves and sticks later in the high idle position, it may not retract when you stop the engine. It is supposed to pop out to increase idle speed when you turn on the ignition switch, but it's not strong enough to move the throttle. It's only strong enough to HOLD the throttle at the higher speed, so you have to tap the gas pedal after turning on the ignition switch to allow that plunger to pop out. When you turn the ignition switch off, that plunger retracts to lower idle speed a lot to prevent engine dieseling or run-on.
Look for any other vacuum leaks too. That will increase idle speed without a corresponding increase in power.
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