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Engine will not warm up at all

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I have a 1986 GMC 3500 with the 454. I've been driving the truck a lot and it hasn't been giving me any issues until recently. I put some cherry bomb glasspacks on it a few days ago, and thing sounds great, but it seemed like right after I did that, the temp gauge suddenly will not get above 100F. I can let it sit at idle for a while, and it might reach 120. I replaced the thermostat because I assumed it was stuck opened. That didn't fix the issue.

 

Either my temp gauge is going out, or the engine is not reaching operating temp. I assume it's the engine because when I try to rev it up, it will sometimes hesitate like it's cold. I touched the radiator cap, and I was able to keep it there about 3 seconds before it got too hot.  I'm not sure how hot it would normally be, but it seems it should get hotter.

 

Like I said above, I already replaced the thermostat, but that didn't fix the issue.  I'm lost at what to do.

4 Answers
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https://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-test-the-coolant-temp-sensor-1

Not sure how this is possible on a 1986. It doesn't have computers in it.

but it does have a temperature sensor/sender

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@mmj has the answer.  Your temperature sending unit is malfunctioning or your gauge is kaput.

https://shop.advanceautoparts.com/find/1986-gmc-k3500-coolant-temperature-sensor.c8355

Where would this part be located on the engine?

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Look for the picture I sent and look on the intake manifold near the radiator hose outlet from the engine to the radiator.

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Woah, typically you hear big blocks having overheating issues, not 'under heating' problems, haha. It could be the gauge/sensor, the things just old. But based on what you said it may be the engine is actually running cooler than usual. (I'm no expert on older cars, but I know with a new car that's all computer run that if a sensor were bad and gave the computer a false reading, that could make the car think its cold, and change how it runs and what the gauge displays)

How's your MPG been recently? When you start it up cold and let it idle for a bit, do the rpms drop as if it were fully warmed up or does it still at a high idle almost like a cold start?

I'm assuming you checked the coolant level. 

The idle doesn't drop like a modern car because it's carbureted. It's either high or not.

I can't really tell any MPG difference.

Yes, the coolant level was the first thing I checked. I opened the radiator cap with the engine cold, and let it run 15 minutes then topped it off. That still didn't help anything.

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