300CD runs hot on highway - Page 2 - PeachParts Mercedes-Benz Forum

#16 04-17-2007, 10:51 AM

Quote:

Originally Posted by wbrian63

In VERY cold temps, sitting at idle, I suspect that most, if not all diesel MB's will cycle just like this. At speed, the water probably gets hot enough to not cause the thermostat to shut, or if it does, it's for so brief a period that it's not visible on the gauge.

Possible I suppose, but I've never seen it happen and I've driven mine in -10F temperatures.

JimSmith · #17 04-17-2007, 02:03 PM

Quote:

Originally Posted by mespe

wbrian hit it on the head with his suggestions. I saw about a 10C drop after burping the system. Incredible how easy it is to burp,,, disconnect the small line to the overflow tank, raise the end up high. squeeze the top radiator hose, just before you release the top radiator hose, block the end of the small hose, this will force coolant in from the expansion tank. keep doing this until coolant flows out of the elevated small hose.

I have had difficulties "burping" all my 240Ds, from the 1975 to the 1983. Air seems to be able to be trapped in various cavities in the head and the block. I developed a procedure where I drive the car up a hill (nose up) in my yard that is pretty steep, and that will put the radiator at the high point. Older MBs didn't have expansion tanks, so the radiator cap was on the radiator. With the radiator cap the system highpoint I can fill the system and burp most of it by squeezing the radiator hoses. Then I run the car up to near normal operating temp with the cap off and fill it up as it self burps. This gets enough out that the rest is small enough in volume to not really cause any problems - at pressure and temperature most of the remaining air can either be dragged around by water flow at highway rpm to the radiator, or, the surface area it affects is trivial and it does nothing to hinder coolant flow. Remember to always have the heat on "full" while you do this so you don't trap air in the heater circuit. Jim

patbob · #18 04-17-2007, 02:58 PM

Quote:

Originally Posted by chetwesley

hmmm... uh oh, I didn't realize it wasn't supposed to run that hot. My 300TD runs 100 (but never goes above) as the normal operating temp. 95 if it is cold out and I am pumping the heat.

Mine behaved exactly like this except it would go slightly above 100C on a long uphill pull.. until a few weeks ago when I decided to look into it. Burped the system of about 2 quarts of air, temps dropped by ~3C. Replaced thermostat last weekend, and temps dropped ~10C.

The thermo replacement also included drilling a 1/8" hole in the thermostat housing where the arrow is (it's so much easier to tell you got the right end up when there's an easy-to-see hole there

), another 1/16" in the back plate, then replacing the ~1.25Gal coolant I removed with ~1.5Gal of 50/50 mix, and lastly, doing a best-possible job of burping (fill very slowly, burp, fill, burp, jumppered monovalve & aux coolant pump to fill heater core, several miles of 3000RPM driving). Now it seems to hold a pretty steady 90C and I no longer worry its going to boil over when the ambient goes up.

I think the new thermo & diluted coolant might have had the biggest effect. The stuff I got out seemed more syrupy than what I replaced it with. I'm hoping it goes to ~80C after I flush the system this summer and replace with the proper coolant.