Blown head gasket? (Decent compression numbers but cooling system doesn't hold pressure, no external leak)

Question

I have a Mazda MX-5 and it was overheated due to the water pump belt failure. I replaced the belt afterwards and the car drove fine. I brought the car to the shop and they told me it has head gasket problem because cooling system does not hold pressure and no external coolant leak. They told me the compression numbers don't mean anything. Here is the detail about the car:

The good:

-Compression number across all cylinders are decent (175 psi)

-No loss of coolant when I was driving the car daily after the overheat issue (I monitor the fluid level at the overflow tank)

-No external coolant leak was found

The bad:

-Cooling system does not hold pressure

-I needed to top up the coolant overflow tank after the car sat for 3 weeks. I only drove 300 miles since the car was overheating. I drove 150 miles, then the car sat for 3 weeks, and I drove 150 miles more. No loss of coolant was observed when I was driving the car around town during that 150 miles period.

I don't understand why the cooling system doesn't hold pressure. If the coolant is leaking into the cylinder, I should have bad compression numbers, right? Do I really have a blown head gasket?

Answer 1

Low compression can be timing, head gasket, coolant pressure, piston rings and some other issues. But overall check the radiator cap that's usually the culprit for no coolant pressure. Do a coolant leak test to check for leaks, you can add some dye to the coolant and then take a black light to the car to check for leaks.

If you have compression chances are head gasket is fine. However top your radiator off and start the engine, if you see million of little bubbles forming up top you have a head gasket problem. Also make sure your thermostat isn't stuck, less times than more that can cause an overheating issue