I’m going completely insane over this. This is regarding my (new/old) 84 300D with almost 200,000 miles on her. This car has run warm consistently since I bought it 5 weeks ago. But now it is running even hotter.

When I buy a car, one of the first things I do is a thorough cooling system flush and then I replace all the hoses and water pump, which I did. (I believe in preventative overkill, to a certain extent). The system was already clean, as the flush water was not very discolored at all. I used an OEM type water pump (Meyle – is this an actual OEM supplier?)

The radiator was original, and Last Friday it succumbed to the “neck crack” syndrome. Luckily, I caught it immediately, so I didn’t fry the engine. It *did not* overheat. I replaced the radiator with a new Behr OEM type unit. The first replacement thermostat (Wahler) was defective, and I replaced it with a new dealer supplied OE one. It is positioned correctly, with the arrow pointing up. I filled her up with a 40/60 mix of phosphate free antifreeze + Water Wetter. The auxiliary fan functions properly. The viscous fan clutch is O.K. The metal bypass line is clear. There is no air trapped in the cylinder head, since it is self-venting.

The temperature gauge is reading accurately, confirmed multiple times with an infrared non-contact thermometer used on the engine block & cylinder head. The new (2nd) thermostat was checked in a pot of water, and it starts opening at 180 degrees.

After all this, the engine runs at 97- 99 degrees on the gauge, no matter what the load – light load, medium load, A/C on, A/C off, 60 degrees ambient, 90 degrees ambient, etc. You get the picture. Can you spell F-R-U-S-T-R-A-T-I-O-N? I guess that I should be satisfied that it is at least *stable* at 97-99 degrees, but I am ready to try a THIRD thermostat. I can’t think of anything else.

<< UPDATE Since I first wrote this: Today I ran the car on the freeway in hot weather with the A/C on, and also in traffic, and the temp went up to 103-105. >>

IF the head gasket were leaking into the cooling system, I don’t believe that the cooling system would retain pressure in it overnight, which it does. Unless there is such a phenomena as a one-way leak (?!).

To reiterate: The system was flushed thoroughly. I now have a new radiator, new hoses, new OEM pump, new thermostat(s), proper coolant, a functioning visco clutch, a functioning auxiliary fan, a clear bypass line, and this thing still runs HOT. I am at my wits end. Plus, there is a terrible feeling of deja-vu happening here – when I first discovered MercedesShop.com, I was having cooling system problems with another car, hence my chosen name: RunningTooHot.

Anyways, I hope that someone can point me in a direction that perhaps I’ve overlooked. I like to delude myself into thinking that I’m a fairly competent DIY’er, but this is making me feel like a moron.

Thanks in advance for any suggestions (other than selling the car, thank you )

RTH