The situation: at idle, truck warms up normally, then after a fer minutes the engine temp gauge moves slowly up into the red zone, then suddenly drops back to normal, and stays there.
The truck: 1985 longbed, 200K miles, 22R-E (fuel injected) engine.
Maintenance so far. Cleaned radiator, replaced hoses, replaced fan clutch, replaced engine temp sensor, replaced thermostat. Coolant is at normal level, no sign of loss.
Any ideas?
Sounds like the thermost (the thing that blocks waterflow until the temp gets up) is opening late.
It has a little hole in it to let water flow enough so that it gets to know when the water is hot.
Is that hole plugged with a flake of something?
GC
Also, make sure the TStat isn't in backwards.
GC
That little hole is in the thermostat so that air can pass by, this prevents an air bubble from forming around the thermostat. The water gets plenty hot even without moving.
Sounds like either a bad sending unit or thermostat. You mentioned that you replaced the "engine temp sensor". Being fuel injected, that engine has a coolant temp sensor for the ECM and a temperature gauge sending unit that operates the gauge. Are you sure you replaced the right one ? Have you checked the coolant temp with a thermometer to see if is actually getting that hot ?
Are your heater vents wide open during this? It happen to me, but I was told it was normal. I had just finish rebuilding the timing assembly and head gasket. Try to make sure you have no bubbles in the system.
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