M20 disappearing coolant mystery

[lindskrot]

Hi everyone.

I have a m20 with M54B30 internals, Schrick 288 cam, 3mm MLS gasket and quite high compression.

I have had the car for 2 years and it has always used a tiny amount of coolant.
But last 2 weeks it suddenly uses huge amounts of coolant but i have no clue where it goes!

Oil is perfect. No sign of water
No signs of coolant on the ground
no visible white smoke out the tailpipe
running fine
not overheating

It goes from full reservoir to warning for low coolant in less than 10 miles.

Any ideas where it may go?

Heater core? But by now it must have leaked out at least 10 liters that way and i should notice that in the cabin i presume?

head gasket? But should it not be bellowing white smoke with the rate of water disappearing? Or watery oil?

Worst case, cracked head? but that should also give smoke?

I really have no clue where it goes. All ideas helpful!

[lindskrot]

Also, if i change head gasket, should i go for MLS again, or a cutting ring head gasket?

[e30austin]

Pressure test your cooling system, both when cold and warm. That might give you a bit of an indication as to what's going on.

[SethB]

Bad/loose reservoir cap maybe? Pushing coolant out the overflow at the top of radiator, around a bad cap seal, and out the overflow hose? I had a bad cap seal a while back that was letting that happen, couldn't figure out where it was going until I let the car warm up in the driveway and coolant started running out of the fender well.

[digger]

give it some revs in the drievway for a while and see if you can see a leak

[turbski]

Is your floor soaked by the trans tunnel or down by the seats?

[lindskrot]

No water under seats or inside the car.

After some more testruns, im more and more convinced the head gasket is the issue. It started putting out a bit of smoke during last drive.
I changed my faulty expansion tank cap, and this probably increased the pressure in the cooling system, wich also increased the smoke.

If the head is warped, how do i check this? can i use a metal ruler to check for straightness?

[digger]

Straight edge or flat surface with feeler gauge

[zaq123]

pick up the leak down tester.....or cheat and just pressurize each cylinder with the compressor and the compression tester fitting into each spark plug. If any bubbling in the coolant...HG is no good at that cylinder