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Discussion Starter #1
I have a 99 suburban overheating, i have changed the thermostat,fan clutch, flushed the radiator, still the same thing, it's a 7.4 with 150,000 miles, any ideas ?
 

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Did you flush the entire cooling system, engine and all?

If not you need to. But from what you said you did. So we go one to the next thing, thermostat. It can still be bad, try running it without the stat. you could have a bad one.

Then there is always the dreaded head gasket. Is it loosing fluid?

Unless you have owned this from new someone may have put some type of sealant in the system trying to fix a leak and if not done properly that can plug cooling passages.

Just guessing here.
 

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I need a little more info to help you out.
Is it overheating all the time, or just in stop and go traffic?
What does it do while driving at 55+?

Does the temp creep up or does it just peg the guage after driving it for a few minutes?

And like 84Fiero said, are you loosing water?
 

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Another thought..... Is the radiator the original or has it been replaced. The cooling system may not be flowing completely through the radiator, thus returning still hot.

Another thought.....the transmission could be running hot and overheating the cooling system, however the 7.4 2500's should have a tranny cooler///


Just a thought
 

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Ya, normally overheating at low/no speeds is an airflow issue. And overheating at 40+ is a volume/capacity issue. That's assuming everything else (pump, stat, ect) is in working order.
 

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Discussion Starter #6
I didn't flush the engine, it will start running hot just sitting still or in traffic, if i stay out of traffic & driving, it's fine, any ideas ? How do you flush the engine ?
 

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Easy to do

The DIY network has a great article on flushing the cooling system. Complete with a video. See it at DIY.com
 

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Discussion Starter #9
I finally broke down & carried it to the shop, come to find out, the tanks on the radiator were stopped by some big rust.
 

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So the shop fixed it for you? What did they have to do? (remove tanks or just flush rad)
 
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