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Discussion Starter #1
Have an '89 pickup and an interesting problem.

After driving 20 minutes in city traffic I turn off engine. After 10 minutes, I restart and notice the temp gauge is almost in the red. Engine is 3 litre V-6.

After engine is running a few minutes, the gauge returns to mid-range which is normal.

Questions: Can the sending unit be replaced without replacing the (original) gauge?

Is it a good idea to replace both together (373 K miles on the truck)?

Thanks in advance for advice.
 

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The temperature sending unit is located in the back of the engine, near the firewall. There are 4 sending units back there, the one you want to replace has a green 2 wire electrical plug attached to it. Replacement's can be sourced for as little as $14.

Many have complained that the stock gauge doesn't read above it's "normal" setting. Since yours does register in the red for a short period of time you're one of the lucky ones.
 

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Cressida nut
91 Pickup (Hilux)2wd
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it's perfectly normal for the temp gauge to read extremely hot for a while after shutting off the engine. keep in mind that the engine is not a uniform temperature, the metal (and coolant) closest to the cylinders is going to be significantly hotter than the metal and coolant near the outer surface of the engine. when the engine is running, the water pump circulates coolant to cool the engine so the hot spots don't get TOO hot. after you shut the engine off and the water pump is no longer spinning, convection is allowed to occur. convection is the event of hotter parts of a substance being less dense and rising to the top of a container, as such the hot coolant that was closest to the cylinders will rise to the top of the engine where the sensor is. Then when you restart the engine hot, the gauge is suddenly powered and reads hot (Accurately but only locally). as the water pump mixes the coolant again, it reads normal.

I've seen this happen in every one of my vehicles. my S10 I could watch it happen since it had a mechanical gauge. almost immediately after shutting the engine off it would start to rise. It would get over 250 before falling back down.
 
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Discussion Starter #5
Thanks for the detailed info.

After the Jasper re-manufactured engine was installed, I never noticed the temp gauge going anywhere near the red zone. The only times I have seen that happen (before this year) was when the water pump seized in '98 and when the engine was rebuilt in '07.

After the rebuild, the gauge read HOT after a short drive. I limped back to the mechanic and he discovered the sending unit being bad and it was replaced.

At present, the same thing seems to be happening but not as bad. The radiator was flushed and refilled and thermostat replaced. That may seem like overkill, but if the engine is damaged by overheating, my Jasper warranty will be void.

I will check the sending unit and maybe clean the contacts. Maybe that will help.
 
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