Hey- Just as the title says. I can drive around town just fine when the engine is at low RPMs and it stays around 175-190 F. But when I'm on the highway for even just a minute with high RPMs, it gets dangerously close to the 250 F redline. I know when an engine is overheating it could be a million things.

My coolant is at the proper level, though it has a cloudy-murky look to it. The radiator cap looks like it could be the original from the crappy rusted look to it, but it looks like the spring and seal on the cap still seems to be functional. What do you guys suppose is the likely problem? Either of these? Belt on the fan too loose? Thermostat not working? Pump petering out? Thanks for any advice.

 

Do you know if your fans are engaging? What exactly happens when it overheats?

It may just need flushed

 

Yea I'm going to be flushing it sometime soon, hopefully tomorrow. There is one fan whose belt is kind of loose, but it is spinning sure enough.

The fan is a goofy setup, where it is not directly connected to the pully that turns it, so it can spin freely of the pully. I assume this is some sort of oldschool fan clutch. I actually ended up taking off the fan a couple weeks ago in hopes of sliding my alternator out past where it was connected, and found a really strangely shaped peice of metal that attached inside the fan near the fan's pully. Maybe it's somehow heat-activated to control how well the fan grips the pulley. I know that I reattached them in the correct order, but I'm not sure if there was any particular orientation in relation to each other they were supposed to follow in order to balance correctly? Anyone know what that's is all about? Im nearly sure that it has nothing to do with my heat troubles either way; The fan spins when the car is running, albeit the belt on it is loose.

Nothing happens when it overheats as far as I can tell. I haven't actually hit the 250F redline but I've come dangerously close twice now. I think I might have turned my transmission fluid into varnish though by overheating because the shifting is a little bit rough now.

Keep that help comin. BTW my car is a 1975 300D

 

Running warmer as speed increases is usually restricted flow. Partially plugged radiator is the most likely suspect. Lower radiator hose could be collapsing at high speed restricting flow. The fan clutch is primarily there for low speed where lack of air flow would be present. In rare cases thermostats are missing and that would cause coolant to not remain in the radiator long enough to cool it down. Especially at higher speeds. I've also seen on older cars bugs and debris plugging up the a/c condenser in front of the radiator which would restrict air flow at higher speeds. What I've given you are the most likely causes.

 

At highway speeds the fans under the hood have no effect. At best the fans can move air at about 25 to 30 mph so as soon as you're over that speed forward motion provides more than enough air to cool the engine without the fans.

I agree with AMG the radiator is most likely the problem. For the temp to raise so quickly at highway speeds there almost has to be a major blockage somewhere and a flush may not get it.

Don't overlook the cap because if the system can't pressurize then it will over heat rather quickly. You also had a good guess about the thermostat and I would go ahead and change it and the cap even though I don't think that is your problem. I consider both of those items "maintenance items". They are cheap and easy to replace and they will catch you at a bad time and strand you somewhere --- or worse.

Look hard at the radiator and make sure there are no obstructions in front of it and then plan to send it to a radiator shop and have it cleaned or replaced.

Good luck,
Tony T

 

This is the same car that had loose belts causing a charging problem, correct? Do you still have other belts loose causing slippage and maybe a water pump not spinning fast enough? Just a thought.

 

Hey guys-

I appreciate the help. Thats solid advice. The coolant really is pretty murky so I wouldn't be surprised if it is clogged with gunk or something.

Yes it's the same car with the loose belt on the alternator. In fact, it's the same exact belt that drives the fan. I can't see what drives the water pump, and I'm nearly sure that no belt has direct contact with it. My guess is that the same pully that spins the fan somehow leads back and powers the water pump. Tightening the belt may have fixed it; I just haven't had a chance to get on the highway since I tightened the belt. I'll take what advice I can get in the meantime incase that isn't the deal.

The coolant cap is a good idea. The mercedes maintenance book I have says you should change the cap every time you flush the coolant, and like I say, the one that's on there looks like it came with the car from the factory 31 years ago. Needless to say, I bought a new one just in case that's a contributor.

I'll keep you guys updated.

 

You know, another thought that crossed my mind is, is there any chance that it could have to do with the oil cooler? Whenever I have been driving and shut the car off, the oil cooler is by no means hot. And when I touch pipe leading from the top of the oil cooler, the pipe is cool to the touch. I don't know at all how the oil flow through it should be, but maybe it's supposed to be extremely passive and there's nothing wrong with my oil cooler?

 

How does one flush the radiator?

 

The last time I did a flush I bought the "prestone" coolant flush kit. It comes with a chemical that goes in the radiator and a Tee that installs in the heater line that allows you to connect a waterhose to it to "backflush" the system. It worked ok, I guess, it looked clean when it was finished. I didn't cut my hose like the instructions said. I used two loops of regular heater hose and replaced it when I was finished.

I've been told that there are several good flush products on the market now but I have not used any of them.

Good luck,
Tony T