Sometimes before the last service I have noticed the engine water temperature shot over 100C occasionally. Sometimes it was 110C when I came home and parked the car.
Nevertheless I had Pak Lok to flush the radiator but it didn't help much in the following Saturday. It helped a bit but not much. On the positive side the engine still ran smoothly and willingly without a hitch.
I sent the car back to Pak Lok for a check and replaced the thermostat and the water temperature sensor. The water temperature soon dropped below 80C all the time, and reached no more than 85C in in the most stressful condition. But then engine was not responsive, sluggish, and unwilling to rev. And we could smell the unburnt from the exhaust in dead or slow traffic if the window was open. Obviously the engine was not in its optimal running temperature resulting in an incomplete combustion. It was a relief that the radiator and the engine cooling was excellent. The water passages and jackets were not blocked, otherwise the water temperature would remain high even after the part replacement.
It turned out that the Pak Lok used an OEM thermostat of 71C for general replacement. While it worked for all old MBs except for my own cool C36. The lack of on-road performance due to over-cooled water temperature has troubled me. So I reverted to my old "overheating" thermostat while waiting for the factory thermostat 86.5C ordered from Mercedes. Immediately the performance was back and the engine didn't mind it was 90C or 110C. Just like before, it simply pulled like a train smoothly and not complained a bit.
Finally today I have the thermostat replaced. After some hill climbing and slow crawling the water temperature shown was very stable at around 85C to 89C. Not bad in a ambient temperature of 25C to 28C. The engine seemed to enjoy itself in the narrow band of warm water temperature - quiet, smooth and torquey responsive and instant power on demand are how I would choose to describle it.
This hot summer will come the ultimate test. I will report then how it turns out.
This proved that the thermostat was faulty at the outset. And now the problem should be considered solved.
April 5th - After several days of running in different conditions, the engine temperature remains steadily in between 80 and the 100 lines of the water temp gauge. So far so good and the engine itself is longer stressed or labored. It just revs smoothly and effortless across the wide rev range. I am glad that it has gone back to what it supposes to be.
Now the windshield wiper was not working hard enough like it used to be. Time for replacement is not far away, I guess.
