Short version.
I'm losing coolant somewhere.
I've replaced a bunch of stuff.
It's been block tested.
Oil looks fine.
Need ideas on next steps.
Full details below.

I have a 99 lx that I've been chasing overheating issues for a while now. I've done a ton of stuff and now I'm stumped so hoping for some help. I'm at 267k and it's stock. It's been into a mechanic a few times with this. It's never actually overheated on me. As soon as I see the needle start moving I've stopped so I've never had that empty radiator steam everywhere issue.

So it started where it would only overheat under high load on hot days. Like driving up the canyon when it was 98 out. Realized the overflow tank wasn't holding fluid but radiator itself was never low. Pressure test system and replace overflow tank and radiator cap. Pressure test it and it holds. Few months later driving a canyon and crack shows up on plastic part of radiator. Pretty broke so I patch it. Everything's holding and fluids not disappearing. Few weeks later water pump blows out. Get it home let it sit six months.

Get water pump replaced by mechanic. Pressure tests everything and all good. Get maybe 5k miles no issues. Starts overheating in a drive thru with the ac on. Check fluids. Overflow is fine but radiator is low. Refill and talk to mechanic but before getting to take it back upper radiator hose explodes. Replace upper and lower hose. Bleed system. Runs fine and take it to mechanic.

Mechanic friend is doing this as side work. Has it for two weeks. Multiple different driving days, 3 block tests, highway driving. Can't get it to overheat. Says it pressurizes faster than he'd expect but he can't find a leak, pressure leak, or any indication of blown head gaskets. Says he doesn't trust it but isn't site what to do. Take it back and fan stops running. Call him and he says fan ran fine when he had it and never broke 205 degrees. I trust the mechanic, he's a personal friend and I work with his wife but can't keep torturing the guy.

I check the fan. Works when jumpered. Replace ect (on front of block) and thermostat switch (where fan plugs in at thermostat.) Fan works now. Drive 700 miles from slc to bend Oregon. No issues. Coolant levels fine. Drive back, coolant is pushing into overflow tank and radiator is a little low. Overflow tank doesn't pull back into radiator. Replace radiator that's pretty old with lots of bent fins. Same issues still but now I need to refill radiator every 300 miles.

So in the last 3000 miles I've got:
New water pump (professional install not side work but the same mechanic)
New radiator
New upper and lower radiator hose
New ect sensor
New fan switch
Properly bled maybe 5 times now

Can verify fan works, at various points in this the thermostat has opened and fluids seem to flow so I think that's fine. Heater core isn't leaking. No visible leaks. Oil is not milky or showing visible abnormalities. AFAIK no white smoke. Some possible misfires but I think it needs a new distributor cap.

I'm stumped. What next. I don't really want to do the head gasket if I don't need to. I have the capabilities to pull the whole engine but I've already got an outback xt in the middle of that

 

If you press your mechanic buddy for an opinion, he probably says the HG is blow. I would agree with him.

Blown head gaskets are among the most difficult to diagnose.

Do the block test yourself. It's a free rental at AZ. Just pay for the indicator fluid.

Can I assume the cooling system has been thoroughly bled? Have you read the service manual procedure?

 

Yea heater on radiator cap off. Even with the front jacked up. I got pretty good at it after the third time.

That's also my thought. He said he did it three times because he was convinced it must be the head gasket. I'd rather not tear it down that far while trying to rebuild another engine. Trying to figure out if it's worth it as the car really only need to go for a few more months and I'm not sure what's gonna happen after that.

As long as the radiator is full it runs fine. Took it up the canyon yesterday at 95 degrees with a bike on the roof in third gear. Needle didn't move.

 

Be highly vigilant with the coolant level, and you will survive a few months. And since you have not severely overheated the engine, it CAN be fixed by a really GOOD mechanic.

Best of luck, man.