Hey there all:
My 84 toyota 2wd fuel injected truck is doing something very strange, and beyond my abilities to diagnose and fix.
Two years ago, I replaced the alternator and the voltage regulator, and while I was at it, all the belts and hoses on the front of the rig. During the test drive, the temperature indicator on the dash pegged itself in the red, stayed there for a minute, then suddenly dropped into the normal operating range, and stayed there where it had historically been comfortable. Since I have only one vehicle, and believed that the motor was not overheating, I continued to drive it. This cycle of indicating an overheated condition and then dropping (suddenly-not slowly) into normal continued to the present.
I replaced the sensor and the thermostat this week hoping that things would clear up. no observable change.
The truck has no electrical problems that I have detected, and is in general, a very, very good runner.
Has anybody else seen this and if so, did you figure out how to make it go away? I have no intention of selling it, but if I were a buyer, this problem could be a show stopper.
thanks in advance for any input!!

 

Mine does that sometimes. Start the truck cold and it pops up to the normal spot, then after a few seconds it drops back down and works normally. I've just been ignoring it.

 

I also have this problem. My old thermostat was stuck open so I changed it about a year ago. This behavior started shortly afterwards. I then replaced the thermostat again but the problem has not gone away so I am quite confident it is not the actual thermostat but some other part of the temperature / sensor mechanism. I would be very interested in seeing what a proper solution would be!

Shane.

 

Sounds like it's your sensor causing the gauge to spike not the vehicle actually overheating.

 

Ok... so if I was going to replace this part what would I ask for exactly? A thermostat sensor???

Thanks Shane.

 

I have had similar symptoms so I have done some reading about it. In some cases the thermostat won't open till the heat gets to the cold side. People have drilled holes in the T-stat to help the heat get across, I think Roger Brown's site mentions it. The sensor is in the hot side so when the hot water builds up at the thermostat the sensor will read high till the heat radiates through it enough to open it. Then it reads normally.

In my case it was the sending unit that was shorting out and making the gauge jump all over. Hit a bump and it would go to the top stop, hit a second bump and it would go back to normal. Eventually the sending unit (Temp sensor) went totally bad and it would jump to the top immediatly at startup on cold mornings. The actual temp was around 0* F and it would pin at the top of the gauge.

 

Sounds like a possible air pocket in the cooling system. Some 22R series engine were difficult to bleed.

On a new thermostat, you notice small valve that will jiggle when you shake it. You can cut this off. Place this hole in the direction of the upper radiator hose. This might help in getting the air pocket out.

Jay