Temp swing 300SD
 

Refribing a 1984 300SD. Replaced the radiator, thermostat and all rubber hoses, etc.. after a full coolant flush with the engine drain plug . Took four passes until clear water came out.

My question -at at idle up to 45-50MPH the temp guage sits steady at 85C. However on the highway at 65-70MPH the temp gauge is at 90C. (No AC or heat on in both cases)

Been two weeks and no change. I had though the faster speed would cool the engine faster

ANy thoughts?

Where did you get the thermostat? I'd maybe pull it out and test it on the stove. See what temp it's opening at.

Also, maybe try a 'point and shoot' temp gauge to see if its really that hot.

HTH, Kat

The reported temperatures do not suggest a problem with the thermostat.

Thermostat from fastlane here.

I have an IR gun & can get a reading when the MB is at idle in my drive way-but I cannot figure out how to get a reading from the IR gun when the 300SD is traveling 60-70MPH ;-)

See if you can get this guy to get the temp from below...

http://www.imdb.com/media/rm1491638528/tt0082971

The engine works quite hard at 70 mph and is running at 3000 rpm. It generates a fair amount of heat. The only way for the engine to increase coolant flow is for the thermostat to open a bit more.

The only way for the thermostat to open a bit more is for the engine temperature to rise a bit.

Many of these engines run all day at 90-95°C. without any ill effects. Chasing this ghost is usually non-productive for most.

My '84 decided to run at about 96°C. a few years ago without any obvious culprit. I did change the 'stat with no benefit. The radiator was also changed a couple of years later due to leaks. No change. I suspect the gauge but I'm not chasing it down.

I hope you are correct. Not an issue now with the cool weather.

However I am in south Florida and have fears of waht will happen to the engine temps when it is 95F outside and the MB is stuck in traffic with AC on high

The fact that the engine runs at 90° on the gauge in cold weather is not firm evidence that it will overheat in hot weather.

The question is simply how much cooling capacity is being utilized when the gauge reads 90°C? The thermostat may only be 1/2 way open under this condition, providing more than ample margin for those hot days.

However, be aware that few older vehicles can manage equilibrium at 95°F. ambients without any airflow and with the a/c on.

The engine provides some overhead in this situation by allowing the temperature to climb toward the 120°C. limit. It will likely do so until some airflow is established.

120C. Wow. that's impressive.