My daughter has an 01 Civic LX with the 1.7, 140k miles, and I am charged with figuring out this overheating problem and fixing it

She can drive it all over town and it stays in the normal range, however when she comes to visit me she must climb from basically sea level to 4500' (California's Sierra Nevada). About 30 minutes into her trip at the 1500' level it begins overheating and will progress to pegging the gauge. Both times I have had to rescue her the recovery tank was full and pressurized with boiling coolant and the radiator was low (obviously).She had started the trips with the radiator full and the recovery tank between the min and max.

This is where I'm at so far....
There are no signs of any leaks in the cooling system.
I changed the thermostat.
The fans are both working normally.
I cleaned the radiator of any debris.
The water pump was changed at 105K with the timing belt and I think it's OK.
I checked the spark plugs for signs losing coolant through the head gasket.They looked OK though one had a little white to it....Hmmmmm
I just did a compression test on a warm engine, throttle wide open. I was hoping that would reviel the problem.........nope....135,133,140,140 and the suspect cylinder was 140.

I'm at my wits end...can anyone help me? Where do I go from here?

Thanks,
Rick

 

Start engine with radiator cap off and rev it, look for any bubbles in the radiator. Small leaks in a head gasket won't affect the compression test.

Overheating when climbing mountains is often because the radiator is clogged up inside. Has "stop leak" ever been used?

 

It's never had "stop leak" in the system. I'm still thinking HG because of the coolant being forced into the recovery tank. Radiator cap was also changed with the T stat. I'm going to test for hydrocarbons in the coolant tomorrow,that should tell the tale on the HG. I would sure like it to be a compromised radiator ....fingers crossed.