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2003 4runner v8 Running very warm

My car is running very warm and I have been told that it is probably either a bad head gasket or a warped/cracked cylinder head. How do I find out which one it is if either? I have never let it overheat, but it did lose a lot of coolant once when the top radiator hose came off, and it almost overheated. Now it just bounces back and forth between running at normal operating temperature and a much higher temperature, but not quite overheating. How do I diagnose it?

By that you mean the temp gauge didn’t go high?
That doesn’t mean part of the engine didn’t overheat.
Has the coolant been changed at least every 5 years?
If not it’s possible the radiator is clogged with corrosion.
I suggest you first change the thermostat.
If that doesn’t help it’s time to take off the head.

Removing the head is the only way to tell the difference as the evidence of each is the same. But repair require the head to come off either way. It is at that point you have the head pressure tested by a machine shop. I would have the head reconditioned at the same time if it was a head gasket and replaced if a cracked head.

You can diagnose a cracked head or bad gasket with a kit from the auto parts store to test for combustion products in the coolant. This is it…

As far as the other things that can cause overheating, @circuitsmith’s post tells you what else to look for.

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Remove the radiator cap when the engine is cold.

Start the engine, and as the engine idles, watch the coolant in the radiator.

If bubbles begin to form in the coolant as the engine idles, that’s an indication of a blown head gasket.

Tester

Yeah i changed the coolant about a month ago and I am pretty sure the previous owner maintained it well. Thanks for the advice, I will check it out.

Am I looking for bubbles in the coolant reservoir or under the radiator cap?

OK, will do. Thank you.

Why the jump right to Head Gasket? There are a few other things I’d check first.

Thermostat, Waterpump.