Mysterious Coolant leak

[Oblithian]

So, my car ('75 280z) had a coolant leak at the watercock. Pulled it apart, replaced the gasket, put it back in, no more dripping. But my car still loses coolant inconsistently and I have no real ideas where it may be leaking from.

I get a whiff of coolant in the cabin when the heater is on, (esp under heavy load) but pulling the carpets there is no evidence of leaking (I am driving without them just incase). I can leave it parked and idling for hours with no spots on the driveway and no visible leaking. No sweet smelling exhaust, no white smoke. Judging by the coolant levels it isn't even leaking... until I drive it. It's not leaking intothe crankcase, the oil is always fine (and the level unchanged).

I am wondering if the thermostat is broken and the higher pressure and heat under load is causing a slow leak somewhere exclusively while driving. If I can find a pressure tester maybe I can force a leak??

Has this happened to yours? Do you have any suggestions? any secret places where large amounts coolant can accumulate inside the car without my noticing?

I ordered a high flow thermostat and some gaskets for the housing, I am going to install them and while the coolant is drained, pull the heatercore and inspect it, it's not putting out a lot of heat either. 

[NewZed]

Check the hoses to the core in the engine bay at the firewall and the return line to the front cover.  Also check the back of the head where it meets the block.  The HG can leak there.

[Oblithian]

So, I ran the coolant system pressure check, and there were some leaks at the hoses (but only under testing pressure). I tightened them up, and the leak moved to another hose that looks like it was replaced. I got them all and it seems to have stopped the leaking, and definitely holds pressure now... though perhaps it will find a new path out.

I pulled the thermostat and it had been sticking, to my knowledge it was fairly new (the previous owner said he had just replaced it, and it was not original). I manually depressed it a few times and it seems to work now but I had already bought a new one... and it seems untrustworthy as it is.

Hopefully I don't have lasting damage. It would suck if my head, deck, or cylinders had warped because of the damn thermostat causing excess pressure/leaks and overheating the engine as a result. On the plus side, I now have a thermostat more suitable for the cold climate (180F vs 160F that was in there), and it's high flow.

Edited September 14, 2020 by Oblithian

[NewZed]

You won't get excess pressure because of a sticking thermostat except maybe at high RPM where the water pump is adding pressure beyond the pressure from coolant expansion.  Warping would come from overheating.

 

Nissan thermostats are best.  For the extra few dollars the quality is much much better.  They're still available.

[Oblithian]

Thanks, yes I am aware warping is caused by overheating, that wasn't the clearest writing. It assumes the reader knows the context by that point (I will amend the prior comment to make it clearer). A thermostat that is stuck closed will cause increased pressure from the pump, amd have increased pressure from the excess heat. coolant that isn't flowing, isn't cooling, but causing rapid overheating (though slower than no coolant).

As an update, I replaced the head gasket (I didn't bother measuring the deck) but the radiator is no longer losing coolant volume and is no longer leaking into cylinder 6.

Edited September 14, 2020 by Oblithian