Temp gauge at 120C-but no clue! Help

[Gary Ganaway]

No problem with temp gauge or overheating previously since I bought car in May. 1995 SL500, 65K. Went on drive tonight, stopped to fill up with gas, then after about 5 minutes happen to look at temp gauge and it was at 120!!. No coolant smell or leaks as I stopped and checked under hood. Immediately drove home, about 5 miles, and checked under hood again. No leaks but I do hear a hum like sound coming from, what sounds like under engine below exhaust manifold on passenger side. Engine has been off about 15 -20 minutes and I still hear this humming sound, like a very small motor noise. Aux fans were not on before or now with engine off.

Any ideas what to look for here? This is first benz I have had where I have seen temp gauge go up like this. Gaug does work as when I turn off ignition the needle drops down to bottom of gauge as it should. Thanks

[elau]

Cannot tell you what caused the humming noise.

Did you check if you aux fan is operational? One way to check is to have the car idled and turn on the A/C. The aux fan should come on after a short while. In addition, I believe you should be able to turn the fan by hand. If the fan does not move, than it is frozen.

Did you change the coolant? Since you mentioned you just purchased the car. You may want to check if air is trapped in the radiator hose as well.

[iwrock]

My 400E hit 120 when that AUX. fans quit on me. I quickly pulled over, and replaced the fuse, and it immediately brought the temp down.

I would check to be sure that your aux. fans are working and not frozen... This could be your overheating problem.

[Gary Ganaway]

Coolant is ok. I don't believe the aux fans were even on since it was only about 80F outside , no air cond on, top down and only had been on the road for about 10 minutes. But I will check fuse, etc.

[Gary Ganaway]

update: Just let car idle with air on, also drove around the block. Temp gauge went up to 120 and almost into red. Cannot see any leaks. Fuses ok.

Would a defective coolant tank cap be suspect even if it does not appear to be leaking pressure?

Would a faulty water temp sensor cause temp gauge to go high like this?

What temp would auxilliary fans to come on. They did not come on at 120 or slightly above that but they do turn freely.

What about t stat causing temp increase?

thanks

[engatwork]

I had a simliar issue recently on my '81 300D and it turned out to be a bad radiator cap. Replaced the cap and everything is back to normal.

[Gary Ganaway]

Update: Put in new t stat and water temp sender plus new expansion tank cap. Now flushing system and putting in new coolant. I really now am a believer in Murphy's Law. Why do straightforward jobs turn out to be nightmares? First, could not loosen bolt that holds dipstick tube to engine. Had to pound allen wrench into hole in bolt and then pound wrench with hammer and it finally broke loose. This took about 1 hour. Could not get to back bolt on t stat housing so had to take out dipstick tube. It would not budge but finally after about another hour finally came out. Put in new t stat and then had to re insert dipstick tube. Again, after a couple of hours I had to take off dist. cap on passenger side, push up plug wires and insert tube behind wires to line up with hole going to oil pan, then get wires in front of tube. Tube would not go all the way down until I kept pushing and twisting for about another hour and finally got it down to where I could re attach it to bracket and bolt back to engine.

Appears that while I am flushing and let engine idle the temp is ok but won't know for sure until I take it out on a drive.

Another simple job that took about 5-6 hours in a 90 degree garage with sweat running in my eyes. Why do I do these things? Oh yeah, getting the hose elbow back onto the t stat hosuing was a real pleasure also..about another 1/2 hour.

[Gary Ganaway]

Everything ok...temp gauge steady at about 85C. Not sure if problem was t stat or water temp sending unit but problem solved by replacing both.