jmac7795

Hey guys I'm new here and I hope I'm posting this the right way. I have a 96 C280 2.8L that is having an intermittent overheat issue. It started running hotter than normal a few months ago and I recently just acquired this car in October of 2015 it now has 320K miles on it and it's still running like a dream but I can't figure this out. So a few months ago I replaced the thermostat from Mercedes $77, radiator and a few hoses and for some reason since then it will overheat if I'm not running the heater. I've tried to purge the system of air multiple times using various methods and I find that the cooling system isn't holding pressure for some reason. What's curious to me is that I rented a cooling system pressure tester and compression tester to find that using my testers the cooling system and all 6 cylinders held pressure. I don't understand why the cooling system isn't pressurizing on its own, I ran the engine with the pressure tester on it at roughly 13-15 psi and it held for a good 10 minutes before I shut the car off, all 6 cylinders held consistent pressure so I've ruled out head gasket failure because I'm not losing coolant nor am I seeing white smoke out of my tail pipe. The only thing I have left to think of is the water pump, I will say when the car is running I can see the pulley on the pump jiggling around unlike the other pulleys. It isn't leaking though so I'm confused. I thought the system would still pressurize from the heat even if the water pump was toast. Hopefully someone can help me with this as I'm really starting to get frustrated. I will say one more thing, the cars temperature gauge seems to rise awfully quick after starting the car up after sitting overnight. I'll start it and 2-5 minutes later it's at above operating temp. Sorry for the long post, just really stumped here.

Thanks everyone

cgshawaii

Coolant as I understand it does not only circulate through the block and head. Is your oil clean? Is your trans fluid clean? Auto trans cools in the radiator, so a leak into the trans cooler could vent through the dipstick.

A pulley "jiggling" is a pulley (or bearing) that has gone bad and should be replaced.

jmac7795

Both coolant and oil are clean. Trans fluid is clean as well, I suspected this before but my radiator is new so there shouldn't be any leaks within it I wouldn't think. I think I may have figured it out, I believe the problem to be coming from the coolant temp sensor. The connections on it look a little corroded and after cleaning the connections on it a little, the overheating seems to have stopped and after replacing my expansion tank cap the system seems to be holding pressure now. Thanks for the reply

cgshawaii

Good on ya. Bet it feels great to have solved the issue yourself, not to mention the savings in labor $$ and possible parts replacement.

Next!