1980 300SD Cooling System No Pressure but overflow?
 

Hello,

This has baffled me since the day I bought the car. Occasionally coolant will exit the system via the overflow hose. Now the coolant amount can be at any level near empty or filled to capacity. It will exist the system via the overflow hose. Now it gets weirder. There is no pressure in the cooling system. I have removed the cap directly after running the car and shutting it off and there was zero pressure. Not even a slight his. Where should I proceed? The temp gauge is indicating that the thermostat is stuck open (although I have hot heat). I have verified it is exiting the expansion tank via the overflow hose because I have observed the hose while it was leaking out of the system and saw watched the hose wiggle and slightly bulge as it passed the coolant through.

I have replaced the cap with a brand new cap(worked flawlessly on m other 1980 300SD, now a parts car)
I have also replaced the hose that connects to the top of the radiator and goes back into the expansion tank.

Thank you!

I do not believe we have a thermostat design which would allow you to come to that conclusion...... based on the temp gauge...

If the system is not under pressure.... for whatever reason.... then the coolant is going to boil at approx 212 degrees F... or 100 C...

You may need to check to see if you have exhaust gases present in the coolant .

You need to check a few things here.

The system needs to hold pressure. It also needs to get hot enough to make pressure in the first place.

So, what is the temp gauge doing that makes you think the thermostat is stuck open? If you run the engine for a good while and get out, the upper radiator hose should be too hot to hold in your hand for more than a second or two. The lower hose will be much cooler. The top of the radiator will be very hot like the upper hose. If the upper hose IS like that, proceed. If if isn't too hot to hold, the thermostat is stuck open. Replace it.

If the tstat is working normally, the tank and cap need to be examined. The cap has to have NO ridge in the rubber seal. The tank should have no corrosion where the cap clamps down and seals. If the system is getting up to operating temp and the cap isn't holding pressure, get a new cap. They are cheap. It really isn't worth troubleshooting with a used cap from another car.

When I got both of my 190Ds, they had cooling system issues. The 85 was running way too cold all the time, and the 84 was overheating. The 85 got a new thermostat, new expansion tank, cap, coolant, and water pump. The 84 got a new radiator, both hoses, 80* thermostat from a 2.5 Turbo, expansion tank, cap, coolant, both temp sensors, manual fan over rides for the clutch fan and high speed aux fan, and a new heater core.

I'm no stranger to these cars and cooling system problems, but my 84 sure as hell has NO issues any longer and won't as long as I still own it. ;)

It was a problem like this that led me to replace my header tank. The pressure was escaping via a poor bond between the brass filler neck and the plastic tank.

80 has a tank? Go to auto parts store and borrow a radiator pump tester... pressurize the system replace what leaks...

Who would of guessed. The expansion tank was leaking air under pressure. While pumping air into the tank, I watched the coolant make waves inside as the gauge never even built up pressure. Swapped the tank from my parts car and presto! Leaky expansion tank.

I also tested the radiator caps, bot the new and the old original. Surprisingly the OEM cap held better pressure @ 13psi than the new Chinese Autozone cap @ 10 psi.

The car does run cold, and this weekend I plan to flip in a re-cored radiator (not that anything is wrong with the radiator already in the car) and a thermostat.