overheating problem

I own a 1987 crx with the d15a2 motor and a month ago it started overheating. I'm puzzled as to why its overheating. Ive replaced the water pump, thermostat, radiator, radiator cap, and the fan works fine. I just had my head gasket tested at the local repair shop and i'ts fine. If anyone knows of anything that could be causing my car to overheat, please reply.

what did they do to test the head gasket? it could be a cracked head or block, do you lose a lot of coolant?

they did a combustion gas test on the radiator. It's not leaking any coolant and i have good compression.

was the engine warm when they did the test? head and block cracks don't really open up till the engine is warm. how long does it take before your car overheats?

no they made sure it was cold when they did the test. it takes about 15 minutes at idle to peg the temp gauge. the mechanic is stumped and so am i. Ive been told that if i had cracks, i wouldnt have very good compression, but i have 150 pounds at all four cylinders. is it true that cracks will cause you to lose compression?

cracks will cause low compression but cracks usually aren't even visible so you won't lose much. a leaky head gasket will be a much bigger hole which is why you lose more compression when it fails but cracks open up once the engine is warm and that can cause overheating. this is just a possibility, there may be other reasons that i have not thought of.