Hi All,

I have a weird leak on my 603...sometimes in the morning I come out and see a little stain of coolant under the car. I jacked it up, crawled under, and it looks like the leak is coming from the coolant thermostat where the thermostat attaches to the housing (not the housing to the block, that's dry). My only guess is that the gasket is toast...is this common? I have never read about it.

The weird thing is that it does not leak when running (assuming the hot coolant expands the gasket) and it does not leak all the time...sometimes there is a coolant stain on the driveway and on the thermostat, sometimes not. What really threw me for a loop was when I pressure tested the system I found no leaks...I have pressure tested it a few times and no leaks, not even at the thermostat (though when I do see a stain the thermostat is wet with coolant, and there is a clear leak where the thermostat meets the thermostat housing)...

Any thoughts? I figured it is the gasket, I have no record of the thermostat being changed, I have an extra thermostat, so I am just going to replace it. Thanks for the help!

Rent, borrow or steal a cooling system pressure tester.

Sixto
83 300SD
98 E320 wagon

I did, and used the pressure tester, got it up to the correct pressure and nothing leaked! I held it there for 15 minutes, still nothing...I did this a few times...still every few days in the morning, a little coolant on the driveway and the thermostat is wet

Weird, right?

Check the sensors located just above the thermostat housing.

Ill check it...though it is clearly wet right where the thermostat meets the thermostat housing, right in the crease that separates the two, and on the lower bolt hold the thermostat on...judging by the sensor position, doesn't look like it could come from there and be wet where it is..but ill still check!

Weirdest thing is it doesnt leak all the time and have yet to find the leak while using a pressure tester!

Give that lower bolt the slightest CW turn you can apply.

Sixto
83 300SD
98 E320 wagon

I tightened it, as much as I was comfortable with, still get a slight drop, very slight, from the thermostat when system is pressurized.

Let its like I'll be replacing the thermostat when I replace the auxiliary water pump, as I discovered that was leaking as well