Having a weird overheating issue with my 80. It runs right at 195 -200* when I have my climate control on the hot setting and have it set to def, heat or vent. It does not seem to affect the temp at all regardless of the fan setting. As soon as I turn the climate control to the cold setting (AC system is not hooked up at all, no belt on the compressor) the car almost immediately jumps up as high as 250 - 260, obviously not good. If I pull over and shut the car off and immediately turn it back on, it will usually read about 220* and go down from there pretty quick. If I turn it back to heat on the climate control it jumps right back down where it should be. I have also had it go from about 250* all the way back down to 200* in a VERY short amount of time by itself (the gauge does not bounce around, it sweeps smoothly).

To add to the mystery.... The Spal fan temp probe is on the right side of the block between the headers, and the coolant temp probe is on the right side of the block between the headers. Oddly, when the temp spikes the fans don't even kick in so there appears to be a major temp difference between the right and left sides of the block. (The fan temp sender work properly)

Radiator is a brand new Dewitts with the stock radiator cap, water pump is moving plenty of water through the cores as was observed by removing the cap. I verified that the thermostat is working correctly by putting it in a pan and warming it.

Any ideas what could be going on and how I might begin to troubleshoot?

 

Weird overheating or weird gauge reading? I kind of doubt one head is hotter than the other. I think your electric gauge goes full scale (hot) when grounded. If when you change the climate control it rubs on the wire and goes to ground it will look hot on the gauge. Is it really hot or is it the gauge? The fans work correctly with their temp switch?

 

Appreciate the input 74modified. There is nothing rubbing when I move the climate controls that would affect the gauge. Would the gauge jump up instantly when grounded out, or would it sweep like a 'normal' temp gauge? The fan temp probe and fans work fine and appear to turn on when they should. That is what's so weird, the Gauge can show 250* and the fans aren't kicking in. That's what makes me think something is weird in the coolant flow path or something. Turning off flow to the heater core should not have that dramatic effect on the engine temp I wouldn't think.

 

I would bypass the heater core and let the engine get to operating temperature. If the car overheats, you probably have a flow problem. If the car doesn't overheat, I would turn the climate control to cool. If the gauge indicates the car is overheating, then you have an electrical problem.

 

Thanks MajD, I will try the bypass and see what I get for temps. Any ideas where to look next if it turns out to be a possible flow problem?

 

Is the cooling system using water? What I mean is, do you have to keep filling the radiator?

 

Nope, not going through coolant.

 

Since your radiator is new and the thermostat and fan check out, I would not know what to tell you if the car started to overheat, especially since it's not losing coolant. I really believe that if you have a flow problem in the engine, the car would overheat regardless of the climate control setting. However, running the car with the heater core bypassed would remove all doubt.

 

You may want to have your coolant checked for ex gasses. It could be a leaking head gasket. Doesn't sound like the typical symptoms, but none the less...it's an easy check.

 

These two sentences contradict each other. if the fans work fine, then they would turn on at 195. My guess is you are not really 250.

 

x2 on something funky happening under the dash when turning the ac on

op said when turning the heater on it wasnt reading hot only when turning the ac on unless I misread

 

grounding issue I had same problem gauge would read 250 I would park car and temp would steadily go down but fans on temp switch wouldn't turn on and once fans did turn on the temp wouldn't rise any more. I just made sure my raddy had enough water and bled it and haven't had any problems so far but barely on day 2. We'll see if problem persist I checked wires to sender and it was fine if it keeps going the cluster is coming out.

 

My white face aftermarket temp gauge is a cap tube into the left cyl head, and it reads a consistent 10F too high, the engine runs fine in 100f air on the freeway into commercial traffic even pushing A/C......

but from IR and other temp probes, I know the truth, so just to know that.....

 

Run the engine with the radiator cap on the first notch and see how it reacts.......