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Coolant System Problems

I have a 1988 Toyota pick up, I’m having some problems with the cooling system and was wondering in anyone had any thoughts, I’m stumped. When my truck is warming up the temperature gage reads almost all the way in the red, and then goes down to normal operating temperature. So I put in a new thermostat thinking that it was opening at a hotter temperature than normal, but I’m still having this problem. Any suggestions?

First I Would Follow Proper Procedures In Purging Air From The System
Did you do that after replacing the thermostat? Has the truck been consming any coolant. Have you noticed a lower level in the see-through reservoir?

Should purging air solve the problem only temporarily then you will have to find the source of air entering the system. Hoses, radiator cap, gaskets, including head, are possible suspects. You might have to use a pressure checking tool to see if the system holds pressure and find possible leaks(s).

It doesn’t seem to be losing any fluid, I didn’t even think about purging air from the system and will try it, and then go to the pressure check if the purging doesn’t work.

I also wanted to let you know that I did a test running the engine with the cap off and didn’t see any bubbles coming up. Could I have air getting into the system if it’s not bubbling?

Take A Good Look At The Seals On The Radiator Cap, Too
Is that an original radiator cap? Twenty years is a long time in cap-years. When replacing it with a new one, be sure to get one of the correct size and pressure requirement.

That’s Good! A Bubble Of Air Could Be Trapped Up By The Thermostat (high point of system)

There is a water temperature sender for the engine computer. That shows on the engine computer wiring diagram. But, there may be another water temperature sender just for the dash gauge. A dashboard wiring diagram, which I don’t have, would show that.

Did You Figure Anything Out ?

Let us know what you find, please.

I had this same problem with my 1990 Toyota P/U. A friend showed me a great tip to prevent this from happening. On the mounting plate of the thermostat, drill a small 1/8" hole as an air bleed. This will allow any trapped air under the thermostat to bleed out and prevent this problem from happening. The small hole will also not allow too much coolant from by-passing. I do this now in every thermostat change I do.

The old cap seals were cracked, so I got a new cap, I’ll find out soon if that was the problem and let you know. Thanks for your help.

You had what same problem? Running hot upon startup, or what?

I changed the cap and that didn’t help, then I purged the system and everything seems to be functioning properly. Thanks for the comments and help, it helped greatly!