Overheating Issues.. Flushed System, Fresh Coolant, New Pump, Thermostat

[300SD81]

I was driving today and noticed that my temp gauge was in the red above 120 after about 20 minutes of cruising at ~4500rpm. I recently changed the thermostat to a 75degC unit, and the temp still stays slightly over the 80 mark idling in the summer. My other car always stayed at 80, occasionally rising to 90 after long periods of high speed cruising, but this one usually sits around 90 while driving. I have changed the water pump, and have recently done the citrus flush on the system. This is really starting to bother me, since its about to get hotter, and I'm not sure if my system can keep her cool. What else is there that I should check? Is it time for a new radiator? I doubt it can be the fan, since there should be plenty of airflow at speed...

[tankowner]

How are the fins on the radiator? Do they need to be cleaned out or straightened out? Seal on the cap good and holding pressure? Just a couple ideas.

[leathermang]

It would not hurt to be sure the fins are straight and the corrosion is cleaned off the AC condenser and the radiator.....not unusual if examined closely to find 10-20 percent of the fins bent...
Since you recently did some cooling system work... have you checked to see if you have any air bubbles in the system ? Are you running the recommended 50 percent mixture of the proper antifreeze ?
Have you checked the condition of on your radiator cap ?

[300SD81]

I'll have a look at the radiator tomorrow morning before class, have to move it anyway since I parked in a restricted lot tonight.. I doubt there are any air bubbles, since the cooling system work was done about 3 weeks ago, shouldn't they have worked their way to the expansion tank by now? I'm running closer to a 60/40 mix, 60% water. Should make it cool better if anything, since water can absorb more heat than coolant.

I'm considering putting on a smaller water pump pulley off a gasser to see if that'll help if nothing else works. I saw several at the yards today, but they looked like a pain to pull, since it looked like I either had to pull the pump or the crank pulley to get it off.

[pop & blow]

from what you have said changed thermostat, flushed system, the radiator is your next source to check. I have worked on many cars over the years to do the water pump, thermostat, flush system to find out the radiator is almost stoped up and was not circulating properly and causing the over heating problem look for and all aluminum radiator to replace it with cools much better then the old style,,,,,,,

[kerry]

I assume that prior to the water pump change, thermostat replacement and citric flush, that the engine was operating at normal temperature? If so, I would look for the problem to be associated with the recent work that you did. I would first consider a bad thermostat to be causing the problem. I'd remove thermostat or install the old one and see if the problem went away. If it was running fine prior to the work, I would not consider the radiator to be the source of the problem for an initial diagnosis.

[leathermang]

Originally Posted by 300SD81

I'm running closer to a 60/40 mix, 60% water. Should make it cool better if anything, since water can absorb more heat than coolant.

For long term MB suggests the 50/50 or 45/55 ( water/AF ) partly due to the anticorrosion features..

They warn against any higher percentage of Antifreeze ( than 55 percent) in the mix because it decreases the cooling capacity of the system.

So you are on the correct side of the percentage at this time with respect to heating and cooling... once you get the overheating figured out you probably should get back to near 50/50.

[300SD81]

It was running hot before I replaced the thermostat, and has since I purchased the car. The water pump blew on my way home after buying it, so I had to replace it on the road.

[kerry]

How long have you had it and what have the temperatures been doing since you owned it? When you say the water pump blew, what did it do exactly? Does your system retain pressure after you shut it down?

[LUVMBDiesels]

It is most likely the radiator. I had the same issue in my 81 300SD. In fact JohnHef has my 1 year old replacement radiator out of that car. Try PM'ing him and see if he still has it and what he wants to ship it to you...

A sign that you have air pockets is fluctuating temps. If yours are hot but steady, you probably don't have trapped air.
To burp the system, point the car uphill (the steeper the grade the better) take of the radiator cap (cold system of course) and rev the engine up to about 3000~3500 a few times. This will get the air out of the heater core and the rest of the system.

[leathermang]

Although I am in the ' check the radiator' at this point group...
You might want to check your lower radiator hose...
this is a sucker... thus vacuum is created by the pump.. and if the hose is not supported inside with wire or stiff enough on it's own... can collapse ... but only when above idle... thus people typically do not SEE it happen when looking around...
feel of it... if you can squeeze it together replace it...

[JonL]

I'll add my .02 to the radiator. I would not fool with changing the pulley size. It'll be a lot of work (you'll have to find the right belt length also) and it might even make the problem worse. At highway speed, the faster pump might start to cavitate, and that will reduce the coolant flow and increase the temperature.

[kerry]

How fast were you driving at 4500rpm's??

[sd300td]

Originally Posted by kerry

How fast were you driving at 4500rpm's??

+1, I'd like to know too...that'd be pretty much WOT in my wagon which'll heat things up a bit though I wouldn't go as far as saying its the cause of the overheating. Certainly couldn't help.

[dieseldiehard]

I'd place my bet on the radiator. If its over say 10 years old, just replace it. I also saw a thermostat problem that caused overheating. even a new thermostat is to be suspect. A sticking thermostat will mimic a bad cooling system.