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overheating?

J

joshkbomb

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So a couple weeks ago my truck starts overheating (highway, city, didn't matter) according to the gauge, 260+. I took it to the mechanic and he replaced the thermostat which he said was stuck closed. I drove it around and it was still overheating. He noticed that the computer told him the engine was at 199, but the gauge read 230+, so after replacing the temperature sending unit and seeing that the gauge was still wrong, told me to get a new gauge.

I just got my new gauge in from LMC, and it's doing the same thing. Any kind of accelaration and the temp rises up to max only to slowly drop back down over the next couple minutes to the middle mark. I was even able to get it to do this in my driveway after warming it up by revving it up to 2000 rpm for 5 seconds and letting go. The gauge goes up to max then slowly comes down over a few minutes.

Back in March I had the radiator boiled and the cooling system flushed, so I don't think that's the issue. Oh, and no coolant in the oil or bubbles in the coolant (so most likely not a head gasket). It's an '87. Any ideas? Other than the new thermostat being bad, I'm stumped.
 
It could be a hose sucking closed, air trapped in the cooling system, faulty fan clutch, bad water pump, or even a loose fan belt.
 
4X4HIGH said:
It could be a hose sucking closed, air trapped in the cooling system, faulty fan clutch, bad water pump, or even a loose fan belt.
Click to expand...

I've watched the radiator hoses and didn't see any sucking closed going on. I replaced the fan clutch a week ago as well. The water pump could be bad, but coolant is flowing and it's not weeping.
 
British Slave said:
Did he replace the temp sending unit or the coolant temp sending unit? There are two of them, one for the computer and one for the gauge.

Is it full of coolant?
Click to expand...

The one that was replaced was the one on the driver's side of the block for the gauge. Where's the other one? It's possible the other one could be bad, but I'd think that if my truck was really getting to 260+ there'd be some drama.
 
On mine iv noticed when hooked to a scanner, when the CTS reads 185, which is perfect, my dash reads 220. I doubt there is that much difference between the heads and the thermostat housing, but i could be wrong. Maybe you have a short somewhere relative to rpms for some reason, as it only seems to happen when you accelerate. I dont see a way for it to get to 260 from 210 in 5secs from holding a 2k rpm.
 
454Sub said:
On mine iv noticed when hooked to a scanner, when the CTS reads 185, which is perfect, my dash reads 220. I doubt there is that much difference between the heads and the thermostat housing, but i could be wrong. Maybe you have a short somewhere relative to rpms for some reason, as it only seems to happen when you accelerate. I dont see a way for it to get to 260 from 210 in 5secs from holding a 2k rpm.
Click to expand...

Well the temperature doesn't climb up that high in the 5 sec I'm on the gas, more like the 15 sec after I let go.
 
IIRC you can get a 180* or 220* thermostat.

Maybe you got a 220* and it actually is 240* at the block but the thermostat has not opened yet.

I would use the 180* or test without the thermostat.

Also make sure that water is free flowing thru the engine from the thermostat to the lower end of the pump.

If someone ever used some stop-a-leak there might be a clog of it in or around the pump.
 
British Slave said:
The coolant temp sensor is on the intake manifold next to the water outlet neck. The temp. sending unit is on the drivers side head.

Do a search for 90 ohm diagnosis procedure. You can diagnose the gauge and sending unit by making a simple 90 ohm tester.
Click to expand...

Okay, I made myself a 90 ohm tester with the instructions from here: http://coloradok5.com/forums/showthread.php?p=3015061

If I connect the tester to the gauge wire and ground, the new gauge reads half way and the old one pegs out to 5 o'clock. The problem is, I don't understand step 5. It says "If the gauge responds, but not accurately", but what does "accurately" mean? Both gauges respond, but with different values.
 
joshkbomb said:
Okay, I made myself a 90 ohm tester with the instructions from here: http://coloradok5.com/forums/showthread.php?p=3015061

If I connect the tester to the gauge wire and ground, the new gauge reads half way and the old one pegs out to 5 o'clock. The problem is, I don't understand step 5. It says "If the gauge responds, but not accurately", but what does "accurately" mean? Both gauges respond, but with different values.
Click to expand...

The 90 ohm jumper should make the gauge read full scale. That is "accurately".
 
The nuts on the back of the gauge are a good start, but I thought he replaced it. I'm on my phone and it sucks to go back and proofread. Anyways iv always ran 2 temp gauges in mine, the factory dash one and a Autometer one, the usually read 15 or so degrees diff and they both are on the intake. I just shut it off when they both say hot. Lol I also don't have a mechanical fan, and instead run 2 electric fans, and have no problems even in the wonderful Midwest summers
 
British Slave said:
First question is you are sure the resistors are in parallel.
Next, check the two nuts on the back of the gauge and make sure they are snug. Retest.
Click to expand...


I twisted the leads of the two resistors together so that they were sitting next to each other ||, not --.

The nuts on the old gauge are snug. The new gauge has nuts that are snug too, but its white resistor on the back is just a piece of plastic. I assume that means it doesn't need one.
 
Have you checked your grounds? You should check the one on the engine and the one to the body/frame. If you have a poor ground it can cause all sorts of weird issues. Remember to pull the wire, sand the end and the block/frame/body, and reattach.
 
We need more info to help.

> What year is your truck?
> Is it TBI or carb'd?
>When did you last replace the hoses?
>Have you done a compression test to confirm you don't have a blown head gasket?

I would like to ask something very important: How much have you spent (time & money) trying to fix this by replacing parts and not actually testing them first? You know, pop the thermostat out and drop it in a pot of boiling water to see if it opens? If it did (you can't tell now, of course) then that was not the problem. Then you would know to move on to the next possible problem.

I suspect you can buy the GM shop manual and learn the actual steps to properly diagnose this for less than what you have already spent.

This manual might be what you need, for a 1991, and can save you a lot of money, teach you some great repair techniques, and solve your problem quickly.
 
ChrisPerry said:
Have you checked your grounds? You should check the one on the engine and the one to the body/frame. If you have a poor ground it can cause all sorts of weird issues. Remember to pull the wire, sand the end and the block/frame/body, and reattach.
Click to expand...

I haven't checked the ground yet, but I imagine there would be other things going wrong than just a gauge. Do you know where the ground is for the gauges?
 
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