F4GIB

The coolant level in my expansion tank seems to like to be 1" below the seam when cool and 3/4"" below when hot. If I add more, it just disappears during the next ride. Thermostat and seals are new and tested. Car is GTS. Comments?

Dave928S

The air void at the top of the tank accommodates the normal expansion from cold, and if overfilled will just expel that excess until there is sufficient volume of that air void to allow the expansion, without expelling through the cap. Mine runs at about 1/2" above the seam cold.

Yours might be expelling a bit more to reach a lower equilibrium, as a consequence of a weak cap, or a lower BP because of incorrect coolant proportion. If it was a continual reduction, instead of just a constant lower equilibrium level, then you'd go looking for a leak.

If it was mine, and I hadn't changed the cap, then that would be the first suspect.

Jerry Feather

After giving this some thought I would suggest that you have a leak in the coolant expansion tank at about the midlevel just above where the hot coolant level is.

Part of my reasoning is that water will expand about 2 to 2.5% in the range of what you are likely calling cool to hot. (I'm not sure about the antifreeze, however.) With 16.8 quarts of coolant in the 928 engine (at least the early ones) that means only about .3 or .4 quart of expansion for the likely temperature range you are operating in. With your level indications from cold to hot, you are expanding only about half that.

I think that in the expansion tank, given its cross section area at the mid point of about 35 square inches and the cubic volume of about 58 cubic inches for a quart, a one inch drop in the level of the coolant at the midlevel is about .6 quart, or just over a pint.

Your difference in level of a quarter inch is only .15 quart, so it is only about half of what I expect your difference should be, hot to cold. On the other hand, I guess you need to know more about what the actual temperatures are you are calling cold and hot.

The real clue to the leak, for me anyway, is the addition of more coolant that goes away when you drive, leaving you back at 1 inch low to 3/4 inch low. That seems to me to be a dead givaway to a crack in the tank. That is especially true since the levels you are dealing with don't seem to suggest coolant going out under pressure at the cap, which is designed to let it out under pressure. I wouldn't expect it to be letting out over half the tank full down to the level you are observing while it is still hot. Also I wouldn't expect the cap to be letting any coolant out if you are not having a significant high temperature indication while you are driving, maybe even if the cap is not holding as much pressure as it should.

Take it for what little it might be worth. 2 cents?

James Bailey

Borrow a pressure tester and pump up the system to assure that it holds pressure and look for any leaks.

Mrmerlin

common leaks are at the heater control valve and short hose ,
and or the passenger side tank at the top where the tank flange has cracked

FredR

Definitely pressure test time.

Rgds

Fred

F4GIB

The cap was pressure tested but not the entire dyatem. How do you do that?

Dave928S

^^^ remove the cap, put adaptor on where cap was, attach tester pump, pump up to normal hot working pressure ... check that it holds pressure, and if not, look and listen for leaks. That method doesn't test the cap (relief pressure), of course.

Jerry Feather

Have you found the leak?