1978 300SD and Water Pump

About a month ago I realized that it was time to deal with my 300SD's near overheating problem on the highway - especially with the AC on. Normal driving she always ran at 175-190 degrees F, but on the highway she would go right up to 235 degrees F. Then I shut the AC down and hit the heat to bring her back to 190 degrees F.

I felt that the root of my problem was the radiator - so I took it out to be cleaned. Unfortunately, the fins started to fall off when it was being cleaned out - just too many years of being driven in snowy, salty roads.

So, I had the shop recore the radiator as aftermarkets and OE's are not as readily available and as cheap as the 123 body.

Since I had the radiator out of the vehicle, I did a complete job - new fan, new fan clutch, new belts, new hoses, new t/stat - everything just short of the water pump.

I put everything together, filled up the oil, tranny, and antifreeze - and drove off. The car now ran at 200 degrees F - w/o AC. I was fit to be tied! I would be 1 mile from my house and she would be at 205 degrees F already! But she did not go any higher than the 210 degrees as my auxillary fan (which works) never came on.

I pulled my t/stat out and through the old one back in with the new seal. I started her up and everything was perfect.....She ran at 175-180 degrees. Running through the city of Denver on a hot Saturday afternoon netted nothing more than 200 degrees with the AC on - great for heavy stop and go driving. The new t/stat went back to the Benz dealer and I got a new one.

I have been running her for almost four weeks with the same results - she's been a pleasure to drive.

Now, I start her up on this past Saturday and she heats right up to 200 degrees - even when the outside temp is only 55 degrees! So I drive her a little bit to she she is running real hot for the outside conditions, but she is not overheating. Every now and then she kicks the 212 switch for the aux fan.

This is not the way she has behaved for the past 12 years....

I put in a new t/stat only to have the same results. Is it the water pump? When I press on the top radiator hose I feel heat but not much pressure.

The car can be running with the gauge at 210 degrees but both top and bottom radiator hoses feel cold for a long time while the engine feels like it's gaining heat. Two t/stats can not be wrong...can they?

The temp gauge in the vehicle has always been reliable and I have no reason to doubt it. I did check and recheck it's connection however.

Did I not go far enough with my cooling system overhaul?

This is the first time that my lady has let me down....




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Bisric