barnswallow

So a couple months back I was experiencing a weird problem where my coolant temperature gauge on the combometer would randomly climb way up above red. Sometimes right at start up, sometimes in the middle of a trip, sometimes for a little while, sometimes only for a few seconds. I got a little freaked out so I took it to a mechanic who sprayed WD-40 on the coolant temp sending unit and all of a sudden no more issue with the gauge. The temperature would at normal, with the needle running a little to the left of middle after I got it warmed up.
Then I was driving to Tahoe this weekend and the problem happened again. The temperature gauge will randomly climb way up all the way, and then just as randomly go back down to what seems like the normal operating temperature.
Couple questions - based on my reading of the manual and some other posts on this forum, it seems like my coolant temp sending unit is malfunctioning. Correct? Tests I can do to make sure that's the only problem?
Also - where the heck is the coolant temperature sending unit? The picture in the Haynes manual has a little arrow pointing to what looks like a bolt.

xxxtreme22r

search for the term temperature overshoot this is most likely your cause.

SoCal4Running

Dont thinks its overshoot because it does it sometimes right at start up.

My 86 does the same thing. You could put the key in and even before you start the engine after sitting all night and it would read like its at normal operating temperature. Or it would be at normal temp and then all of a sudden sky rocket to the top.

I put in a aftermarket gauge on the dash and it reads from its own sensor at the thermostat and I can tell you that in my case the cluster gauge is WACKED OUT!

My guess is that your temperature is actually ok reguardless of what the needle says and the problem is either the sender, the harness or the cluster.

barnswallow

I guess to clarify I should say that it would do it right after start-up, but when the engine was 'warm', maybe having rested for 30 minutes or so. Usually it doesn't do it until after running for a while, and it only stays in the scary red zone for a few minutes before it crawls steadily back down to a normal temp.
However, the idea that the cluster gauge is wacked out has tons of merit because the speedo is definitely wacked out - it wobbles all over the place until you get up around 45mph and then it steadies out. At 10-35 mph it goes up and down rapidly and loudly over about a 10-15 mph range. Then when you get going faster it only wiggles over a 3-5 mph range. And the loud clicking noise goes away, or becomes unnoticeable compared to the sound of the road

rpij137g

Hey barnswallow, i had the same issue on my 1993 pickup. The temperature gauge would climb up to red line, but then drop to normal. I thought it was a sensor problem too. However, the problem was the 7$ thermostat. It was not opening at the right temperature, sticking closed, but would heat up and then open. The rush of cooler water over the sensor would cause the cluster to register normal temperature. It ended up overheating the engine on 91 in Vermont and I had to do a head gasket job. I would go replace your thermostat for the 7$ and see how it functions.

SoCal4Running

Well the spedo is a mechanical connection and the jumping is most likely the shaft lightly binding somewhere. The faster it spins the less it sticks so it smooths out the faster you go.

The temp gauge is electronic though and there could be quite a few reasons it doesnt work.

rpij137g

The temperature gauge may be off. However, thermostat will cause the temperature gauge to look like it's off, even though it's right.

SoCal4Running

Yea, my first step before the aftermarket gauge was to get a BBQ meat thermometer and push it into the radiator fins. I could watch the thermostat open and close as the temp went up and down but never higher than 190 even though my cluster gauge would be pegged in the red for eternity.

barnswallow

To be safe I'm gonna replace the thermostat and install an after-market temp gauge and keep an eye on it. Should I try to get the two-stage from a Toyota dealer or just get an OEM from a parts store?

SoCal4Running

For the little bit that they cost for how long they last I would get the Toyota one.

rpij137g

I looked at the 18$ toyota one, but went with the cheaper one. Not the really cheap one from the parts store, but the next level up.