Okay,
Right after I purchased my 300D I did the right thing and changed all the fluids and a bunch of preventative maintainence. I have been dogged by small coolant leaks ever since then. Every time I think I have got it fixed it pops up again. Well today I went digging around and found the radiator cap that came with the car. It is marked "100" and is really grungy. There is a sticker on the fan shroud that admonishes you to "Only use cap marked 100" So I checked my supply of caps purchased in my quest to track down this issue. Behr from Fastlane marked 120, Motorad from local parts house marked 13psi, and a new cap from local Benz dealer marked 140. Not really sure what the numbers mean. Thinking maybe 100=1bar, 120=1.2bar, etc. If that is so then:
100=14.5psi
120=17.4psi
140=20.3psi
That seems awfully high to me but maybe thats not what the numbers mean??? If MB states its okay to replace a 100 with a 140 I don't know if I agree. Seems increasing the pressure by that much on a 20year old cooling system is an ill-advised idea. Well the leaks started after I changed the coolant and put on the first new cap. Every hose clamp needed to be tightened but I still had a mysterious invisble leak that seems to have come from the thermostat area. The seller installed a new Wahler 87*C thermostat when I purchased the car. I disassembled the thermostat, inspected everything, cleaned the surfaces and reinstalled with the original o-ring. Theres one of my mistakes. When they did the t-stat they didn't use a new o-ring. Well I put a new one in today. Also since I had it apart I figured I would change t-stats to see if it would change the operating temp, the car runs at 95*C. The local indy parts house had cooler French-made 80*C t-stat that is quite a different design than the Wahler but it fits so I figured I would try it. Well the new t-stat o-ring is definitely better than the old one. This one actually compresses when you tighten the housing. There always seemed to be a little leak from the upper rad hose/thermostat housing union and I couldn't get that thin euro-style clamp any tighter. So I cleaned the mating surfaces and used a heavy-duty US style 1/2" marine clamp on it. Reassembled everything and test drove the car. It only runs a hair cooler than before, right at 90*C leading me to conclude that the gauge is not accurate and should only be used as a trend-indicator and not an absolute. Hopefully I have found and fixed this leak issue permanently. I am tempted to put the Motorad 13psi rad cap back on as this is the lowest pressure cap (assuming I did the bar-conversion thing right) and should put less stress on the cooling system. I also suppose that maybe the French thermostat is running a little hotter than it should since it is so close to an American clamp..... You never know. RT

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84' Mercedes Benz 300D Anthracite/black, 171K
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