I just finished rebuilding the engine in bastet44's 87 coupe... and still have been chasing a coolant leak.

Well - I think I have tracked it down!

The following was replaced during the rebuild:

Radiator
Engine
Hoses to and from the radiator
Heater hoses
Water pump (new with engine)
Heater hoses to the heater core

Now, the heater core had not been replaced (I am not into S&M). There is no smell inside the car and no dampness in the carpets, two major signs the heater core is leaking.

Last nite I checked the expansion tank. It was the last place (short of me making some stupid mistake and not torqueing down the heads right) to check.

The tank was ok... BUT! The cap was NOT! The cap has a cork seal. The seal, a nominal 1.25" ID, was missing a chunk, about 3/8" long! And the gasket was wet! Think teakettle. The hot coolant expands into the tank. The steam vapours off through the missing gasket. Over a couple of weeks, enough coolant is vented off to cause a low coolant condition during operation. It also steams off, not boils off. It also compromises the systems pressure, but that's to a lessor extent.

I didn't have a gasket, but I had a nice fat oring, that I then wrapped around the neck of the expansion tank. I will now monitor the situation.

Thoughts?

 

New caps are relatively cheap. I had a similar experience with mine Veert with a small but constant loss of level in the overflow tank and noticed exactly the same issue with the cork gasket missing a section. I bought a GM replacement cap at the local autorparts store and have not had an issues with decreasing overflow levels since.

 

I think that it would be a vented cap anyway. There is no way that it could be a fully closed system. Once the coolant starts going into the overflow the air in the contaier has to go somewhere or it would rupture eventually. I guess it would have to depend on the cap. The venting would stop the flowing spilliage that the gasket stops as well, but I am not sure that it would stop the evaporation or the steaming off. It might slow it down a great deal more though...


RocketSapp

 

The coolant in your overflow tank should not be pressurized. Some of it may "boil off" but if you're running the proper coolant mixture it would have to be EXTREMELY hot for that to happen. You're right the overflow cap does have a "gasket" but the cap should also be vented. If it is not when the radiator tried to suck coolant back into the system it would create a vacuum in the reservoir.

Unfortunately I think you may be prematurely happy.

 

I just got a new pressure cap yesterday. My old one was turning colors and instead of waiting for it to die, I just broke down and bought a new one. $10 from your GM dealer. Since I have an LT1, it is a 15# cap. I checked on them at some other auto parts store and most will try to sell you a 16# cap (apparently it's more common). I just went with the GM part so I knew that it would be right.

Just FWIW.

 

I understand the system has to have a air gap, but, this was more then a pin hole on the lid of the overflow tank. This was a gap... more stuff could get out then was supposed to.

I shall monitor.

 

The cap's vented anyway, Take it off , rinse it off, Wrap your mouth around it & blow.

Loss of coolant at the cap would at most be very incidental IMHO. I have cut a new cork gasket for mine. I get some dribbles of coolant there when I race due to the sloshing around, but it's minor.

Hope you find the leak.