91 Civic Sporadic Overheat Issue

Hi Everyone. New to the site and hoping for some solid advice. Here is the story, I'm working on a 91 Civic and am having a heck of a time chasing down the cause of a very sporadic overheating issue.

The car belongs to my sons room mate so I have never been there when it overheats but have ran a ton of diagnostics and haven't been able to pinpoint the cause so before I tear into it I'm hoping this story will sound familiar to someone. So.....I get a call from my son's room mate who tells me he overheated on the freeway. He pulled over right away and noticed coolant had sprayed all over the engine compartment. He had it towed to a local mechanic who proceeds to tell him that the radiator hose fell off so they reconnected it, filled it full of coolant and sent him on his way. Two weeks later he calls me again and says it started overheating on the freeway again. This time when he pulls over he discovers the overflow cap had come off. When he tells me this I'm thinking the hose and the overflow cap didn't just come off, they must have blown off under pressure. I tell him to have it towed to me at which point I check all the usual suspects. No white smoke, no coolant smell in exhaust, no milky oil, no creamy deposits on the oil fill cap, no external leaks and the fan is working fine so I change out the thermostat, re-fill the system, burp out all the air and replaced the radiator cap. Car runs fine for another couple weeks and then overheats again. Coolant reservoir cap blew off again. So now it's overheated twice on the freeway and once going up a hill. Obviously it's only getting hot under heavier loads. This time he takes it back to the mechanic and of course they can't find anything wrong with it. A week later it does the same thing so.....this time I get a block tester as I'm sure it's a head gasket but the test is negative for exhaust gases in the cooling system (yes I'm sure, I did the test three times). About the only thing that hasn't been tested or replaced at this point is the radiator and the water pump. I would think if it was the water pump it would overheat all the time? Anyway, sorry for the long post but that's the story. Any advice would be welcome..


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Has the thermostat been replaced?

Does the radiator fan run when the engine overheats?

Compression test the cylinders. The head gasket is almost surely blown.
Thanks for the reply. Yes the thermostat has been replaced and it it will run for weeks without any issues. The fan is working. I did several block tests with a negative test result. Also there is no smoke, no antifreeze smell to the exhaust, no excessive water at the tailpipe, no antifreeze in the oil, no external leaks and the heater works fine. Is there anywhere a Head Gasket would leak and not show with any of these tests? I can do a compression test and see what it says but I'm almost sure if it was a head gasket I would have symptoms in one or more of the areas I have checked already. What do you think?
Car was due a timing belt so I went ahead and tore apart the top end. Head gasket was not blown, water pump looked fine. Took the head in and had it cleaned up and surfaced. Machine shop said it was 3 thousands off but everything else was solid. I'll put it all back together, flush the cooling system and see how she does.

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