86 Yoda, Coolant Temp Gauge

[canadianbiz]

I made a couple other posts about my 86 yoda pickup with overheating problems. The temp gauge rises on a general climb.
Anyway, I changed my thermostat. This truck is nuts because sometimes the gauge reads normal and other times it reads "getting too hot".
On more than one occasion, when it reads hot I have stopped the truck for 5 minutes or less. Got in, restarted and drove away. Temp gauge then reads fine and I did not leave it long enough to cool down if it were to be overhot.
I suspect a faulty temp sending unit giving me incorrect readings.
Are these symptoms of a bad temp sending unit? Thanks, Eric

[fourwd1]

My first guess would be no, your sender is OK.

Reasoning - you're at operating temp and you make a quick stop. The coolant stops moving, so it absorbs more heat from the hot block. You jump back in and look at the temp gage and it now reads higher. Start up and drive a few miles, coolant starts flowing and the excess heat is removed, the temp drops back down.

This all could be more pronounced if the cooling system operation was marginal.

I think a bad sender would be more erratic.

[canadianbiz]

thanks for the reply again fourwd1,
This is getting quite mysterious. With the problems I am having over the last few weeks. I changed the thermostat instead of the water pump.

I notice this morning after driving the truck and the temp gauge rising, I shut the truck off for as I said, less than 5 minutes. Restarted and watched as the temp gauge dropped as I sat in the running vehicle for a few moments. Then as I drove away it came down even more.

I did notice when I first stopped it, I looked for any leaks. Of course I did not attempt to open the rad to get a hot spray of coolant all over but I did feel the top rad hose and it wasn't Hot, Just warm.
Would you think that is poor coolant circulation?

[KimMG]

Low coolant level, poor circulation and/or a sticking thermstat. Is there a difference in the temperature reading if you have the heat turned on and the fan on high? Have you checked the coolant level? Low coolant level can cause eratic readings. Have you checked the radiator for cold spots? Clogged or restricted radiator will have a diminished cooling capacity and be noticeable when going uphill.