A few weeks ago I noticed that my engine was running hot... at least that's what the temperature gauge was showing me. I'd do a highway run for just 10 minutes at 70mph right after the engine has sat for hours and all of a sudden the temperature would be at 100C, sometimes even 115C. This would happen either with the A/C on or off.
So I decided to do a complete cooling system service. I throroughly flushed the system of the old coolant, removed the thermostat, drained the engine block of coolant and then put in a new OEM thermostat and new engine coolant (MB coolant) in the system. I diluted to coolant to 50/50 with water.
All these I completed yesterday. Some 4 hours later I took the car on the highway, doing 65 - 70 mph with the A/c off and with the outside temperature at 60F and the temperature gauge went from the bottom to 90C within five minutes. Then after that, the temperature gauge kept on shaking between 110C and 90C constantly and after a while settled at 115C. Not wanting to risk damage to the engine, I turned on the heater and right away I saw the gauge go straight down to 80C and then go back up to 90C. It seemed like the speed of the car didn't really matter, as I exited from the highway and drove constantly at 35mph but the gauge stayed at 110C.
I know my water pump works, because prior to my locating and draining the engine block coolant plug, I had flushed and put water in the radiator and when I finally drained the engine block, all that came out was water not coolant. Or is this an incorrect assumption? Anyway at this point I'm really confused. This gauge seems to be behaving badly. I'm suspecting that it is faulty, could be slipping or something similar. Or the coolant temperature sending unit is defective. Or both. Though I think it must be the gauge due to the way the temperature fluctuates.
I've read several pages on this through the search function. The only thread I could find in which a problem similar to mine was discussed was this one:
300SD nearly overheats at high speeds.
However that thread doesn't help me much because I have a radiator that's only a few years old (still looks brand new as a matter of fact). Also found this one:
How many diesel owners have had problems with overheating????? in which the fan switch was mentioned as a possible cause, but the fan came on while I left the car idling at around 85C and then went off a little after it had stayed on.
Any suggestions on what I should do would very much be appreciated.