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Matt
Matt, Engineer
Category: GMC
Satisfied Customers: 20615
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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I have a 1992 GMC 1509 carb it over heats I pulled the

Customer Question

I have a 1992 GMC 1509 carb it over heats I pulled the thermostate and still over heats fan work n all else is ok I'm stumoed
JA: Have you checked the coolant? Is there coolant circulation?
Customer: Yes all the obvious is good
JA: Tell me a bit more about what's going on so he can help you best.
Customer: The rad hoses r stiff still with no thermo when she heats up she building pressure with no thermo
JA: Are you hoping to fix this yourself?
Customer: Yes
JA: Great! What have you tried so far?
Customer: Well the water pump is good I took out the thermostate there heater core is circulating but when it gets hot the rad hoses r still unsqieasable
JA: Is there anything else important you think the GMC Mechanic should know about your GMC?
Customer: It doesn't overheat in the city but when on the hiway or hauling it does
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the GMC Mechanic about your situation and then connect you two.
Submitted: 6 months ago.
Category: GMC
Expert:  Matt replied 6 months ago.

Hello

if removing the thermostat doesn't help and the water pump is definitley not spinning on its shaft

Also its possible the belt is slipping so its worth replacing its especially if the rubber ribs are cracked or it looks shiny on flat side of the belt - while its off check that the tensioner and other pulleys all spin freely the exception to this is the AC pump ( if fitted)

I'd also suggest ensuring that you've got all the air out of the system as this sort of episode can leave pockets of air trapped and prevent the coolant from circulating properly.

So check that the small hoses that run from the top of the engine and radiator that run back to the header / expansion tank are free flowing and not blocked

If these are OK then remove the tank pressure cap and run the engine from cold with the heater set to full hot

As the engine starts to warm, squeeze the large hoses running to the radiator to help displace any air - all the time keeping an eye on the level in the header tank, making sure it doesn't drop below the minimum and open any bleed screws built into the top cooling hoses - closing them off as soon as they start to gush water

Once the engine is up to temperature the tank should be quite full. Switch the engine off and leave it to cool

Then top up to the mark once its all cold and replace the cap

Also its possible the belt is slipping so its worth replacing its especially if the rubber ribs are cracked or it looks shiny on flat side of the belt - while its off check that the tensioner and other pulleys all spin freely the exception to this is the AC pump ( if fitted)

I'd also suggest ensuring that you've got all the air out of the system as this sort of episode can leave pockets of air trapped and prevent the coolant from circulating properly.

So check that the small hoses that run from the top of the engine and radiator that run back to the header / expansion tank are free flowing and not blocked

If these are OK then remove the tank pressure cap and run the engine from cold with the heater set to full hot

As the engine starts to warm, squeeze the large hoses running to the radiator to help displace any air - all the time keeping an eye on the level in the header tank, making sure it doesn't drop below the minimum and open any bleed screws built into the top cooling hoses - closing them off as soon as they start to gush water

Once the engine is up to temperature the tank should be quite full. Switch the engine off and leave it to cool

Then top up to the mark once its all cold and replace the cap

Customer: replied 6 months ago.
That's not the issue. No air everything is mechanically good just builds pressure with no thermostat can't squeeze the rad hoses when it's hot it should be free flowing with no thermostat