Have a 78 L82.....seems I have a coolant leak somewhere.....thought it was the thermostat housing....replaced it. When I bought the car it had no expansion tank. I got one from zips and installed it.....seems as if coolant is somehow leaking out of the cap. Any thoughts on this ...

If it is leaking past the cap check the radiator really close..where the cap clicks on may be cracked..or the upper part of the radiator itself..its tough to pressure test for a leak in that area but a very bright flashlight may find some cracks if they are there..don't put a stop leak in the radiator..you will regret it..al lot! It could be the cap gasket also..

Rich

Do you have a wet spot on the floor, engine, rad, cap, water pump, hoses, ...where?

ebo

I think previous owner put some stopleak in heatercore . Going to replace that soon. Seems coolant is somrhow leaking from expansion cap tank....

Not leaking at radiator....seems to be leaking around cap of plastic expansion tank located on inner fenderwell...

tank wasnt full......

Sounds like the engine is overheating, or the radiator cap is not sealing well, and puking the coolant. The plastic reservoir isn't under pressure. The only way it can come out the top is if it is overfull. If the radiator cap is not holding pressure properly, or is the wrong cap, it can do this.  I would be looking at the radiator cap first, as the culprit....