overheating

[MarkM]

Sorry to bring up a topic that has been discussed in several threads, but I still have some questions about normal coolant temps. vs. overheating.

My 85 300D has a brand new radiator, new thermostat, new coolant temp sensor, and climate control works great. Cooling system has always been flushed every two years. Before I put in the new thermostat, coolant temps on the highway were less than 80C (too low). Since I put in the new thermostat, here is what happens:

At outside temp. of 32F, at normal highway speeds, ACC off, coolant temp will be 95-100 deg. C, with some spikes above 100. As soon as I put on the heater (on max.) coolant temps drop to 85 - 90 deg. C. When coolant temps are in 100C range, the top of the radiator feels very hot, but the bottom of radiator and return hose feel cold. Either the radiator is doing its job, or there is lack of circulation. Belts are tight, water pump is good.

I put in a second new thermostat, thinking that I may have a bad one, but conditions are identical. So now I have an extra new one.

I would expect that with cold outside temps, coolant temp would be at steady 85C. However, according to Mercedes, thermostat is fully open at 95C

It seems odd that circulating coolant through the heater core would have such a dramatic effect on coolant temp. while the radiator low end is cold. I would guess that with a properly functioning cooling system, the effect of the small heater core would be noticable, but minor.

According to the Mercedes manual, temps in the 95 - 100 range are normal, and the manual says that under heavy load and hot weather, temps may go above 100. But I feel realy shaky about these temps.

Some previous comments in other threads seem to implicate a bad fan clutch. However, with my car, I cant get the radiator hot enough directly in front of the fan clutch to cause it to engage. Maybe this will only happen during hot weather.

The conditions I describe may be absolutely normal (I suspect this is the case, and commented on previous threads as such). The fact that the radiator is cold during cold weather likely means that I have enough "reserve cooling capacity" to keep coolant at appropriate temps during hot summer ambient temps.

I guess my specific questions is this:

When you folks had put in a new thermostat, what would your temperature guage indicate at normal highway speeds?

Thanks,

Mark