Heater control valve causing overheating

Hi all. Recently purchased a '96 4 cylinder 2.7 liter automatic 4wd. She runs fine, temperature normal, as long as I keep the cabin temp knob turned all the way hot. It doesn't matter if the cabin air is on or off, if I turn the knob from hot to cold within seconds the engine temp starts to rise. Only takes a minute or two to get in the red.

I've searched all over TW and elsewhere, haven't found a clear connection between the heater valve being closed and the engine immediately overheating. Most posts talk about the heater not working, the AC fan being on, or other conditions that do not apply to my specific situation. When I turn the AC on it blows ice cold. When I turn the heat on it comes out hot. The only clear cause and effect is if I close the heater valve the engine overheats. If I keep the heater valve open, she runs at a normal temperature (driven an hour like that and no temp issues).

I've already flushed the radiator twice and filled with new coolant, and the second time I had a shop burp all the air out in case I was screwing it up. Flushed and recharged the AC coolant. Installed a new water pump. What do y'all think?

 

Searched everywhere? Hehe, the solution is so simple you won't find something like this on the forums. So your heater is acting as a second radiator. Your main radiator is not able to cool the coolant so using the heater supplements heat dissipation. There are several solutions to this and they're mostly cheap.

1, make sure you have sufficient AND proper coolant. Green 50/50 blend of coolant/water.

2, replace the radiator cap. An old cap is so weak it can't keep the coolant from boiling. this would cause overheating.

3, check your thermostat. it might just be bad and not opening up when it's supposed to.

4, make sure your coolant hoses are good. if they expand too much they can't handle pressure causing the coolant to boil.

5, make sure your fan shroud is good. If it doesn't work right, air won't pass through the radiator when stopped/parked.

6, check your belts. a bad fan belt will keep the fan from cooling the radiator.

7, a bad fan blade will keep the fan from cooling the radiator

now here comes the more expensive things, luckily there's only 2.

8, fan clutch. a bad clutch won't spin the fan blade allowing the coolant to overheat when stopped/parked.

9, radiator might just be clogged. you can try flushing the system, it does work sometimes.

Good luck!

 

Thanks for the advice, you're probably right about the simplest solution being correct. I'll work my way down the list and see if anything sticks.

 

Feel for cool spots in the radiator where parts of the core might be plugged, if you feel cooler spots [or use a wire temp probe on the fins in different spots] then your radiator is due for replacement.

 

I can add 1 possibility to that list....I had a vehicle that I seemingly could not stop from overheating no matter what I tried and similar to yours the heater on high seemed to help.....after many weeks of trial and error it turned out to be that someone had installed a new water pump for an older year model and didn't realize the difference was the newer year/engine had been reconfigured to use a serpentine belt which made the impeller inside the water pump turn backwards.....a serpentine belt typically drives a water pump with the backside of the belt, a V-bet uses the inside or side with the v shape....I imagine it's not likely but a possibility with used vehicles......ok my brain is tired now.

 

Ha interesting, thanks for the unique possibility.