Was driving my sons 86 se (base), about 120 miles of mostly hiway some city as well. The temp gauge never got above 1/8 (most time below 1/8.) It was cool (50F and damp) The car also got poor MPG. cruising on freeway at 70mph. Got 15 MPG. The heater was putting out hot air. Could the ECU be keeping the engine on cold fuel map? Is the engine really cold or are the sensors sending bad data? He bought the car in Dec and has gotten bad milage to whole time. Back in Jan. I put a used thermostat out of a spare s4 engine we have and no change in the temp display back than.

 

Sounds like your thermostat is stuck open, or has been replaced by one of a lower temperature. A used thermostat is probably bad...

The S4 gauge is fairly accurate. 1/4 is about where the car should run. Halfway up is about 90C, and top of the gauge is usually around 95-100C.

 

so if I pull the thermostat out and its closed, does that mean its opening at too low a temp and opening all the way at that temp?

 

I know my s5 gauge was horribly off the car would be way over heated before it would read hot on the gauge i just went aftermarket because coolant temp on a rotary is very important

 

S5 Coolant guage:

New Engine

Normal

Cold

Right Aaron?

 

Yep, the S5 gauge is more or less a warning light.

Cold -> Normal -> Rebuild

The S4 gauge is linear and actually useful.


You can take the thermostat out (or the one you removed before), and test it per the FSM. The test involves heating water in a pot w/ a thermometer. Cracking temp is about 180 F, and it should be fully open by 200 F.

 

not a very truthful guage
-cody