Yet another 300SE/300SEL/M103 overheating/coolant change thread

[ThePhoenix]

Originally Posted by JamesDean

I would only use one of two brands when replacing a radiator on these cars: Behr and Nissens.

Nissens is usually a bit cheaper than Behr. Behr is original equipment manufacturer. I wouldn't mess around with cheapy brands

Yeah, I will be buying the Behr.

One other thing I noticed on my next drive, if I keep the revs in the 3k-3.5k range, it runs cooler on average. Babying it makes it heat up more. I'm going to check out the thermostatic fan tomorrow. If I find that it's bad it still doesn't explain the higher highway temps though.

[JamesDean]

Originally Posted by ThePhoenix

Yeah, I will be buying the Behr.

One other thing I noticed on my next drive, if I keep the revs in the 3k-3.5k range, it runs cooler on average. Babying it makes it heat up more. I'm going to check out the thermostatic fan tomorrow. If I find that it's bad it still doesn't explain the higher highway temps though.

Typically running on the highway will cool better as you have much more airflow through the radiator.

You might have a bad fan clutch if you experience higher coolant temps at lower speeds/idle.

[Ferdman]

ThePhoenix, recommend replacing the thermostat. Be sure to orient it properly ... with the vent hole at the high point.

Check the tension on the fan belt because if it is too tight the fan bearing bracket will wear out prematurely. I had that happen on our 1991 300CE when a tech over-tightened the belt. The newer style tensioners eliminate that possibility.

[ThePhoenix]

Originally Posted by Ferdman

ThePhoenix, recommend replacing the thermostat. Be sure to orient it properly ... with the vent hole at the high point.

Check the tension on the fan belt because if it is too tight the fan bearing bracket will wear out prematurely. I had that happen on our 1991 300CE when a tech over-tightened the belt. The newer style tensioners eliminate that possibility.

Re belt tension, from what I gather you tension it until the arrow lines up with the line on the tensioner body, which is what I did.

The thermostat was holding at a perfect 80-ish before the fan bearing work was performed, doesn't seem that it could suddenly be bad now. May replace it anyhow as a process of elimination.

[ThePhoenix]

Originally Posted by Ferdman

ThePhoenix, recommend replacing the thermostat. Be sure to orient it properly ... with the vent hole at the high point.

Check the tension on the fan belt because if it is too tight the fan bearing bracket will wear out prematurely. I had that happen on our 1991 300CE when a tech over-tightened the belt. The newer style tensioners eliminate that possibility.

UPDATE - PROBLEM(S) FOUND:

I ended up finding a good used Nissen radiator and dropped that in, but she was still overheating. It turns out the thermostat was the problem. As a test I removed it and she now runs a little less than 80°C (unless I get stuck in heavy traffic, in which case the electric fans keep her at about 85°C). So a new t-stat and a functioning fan clutch should have things back to normal.

Lesson learned: A perfectly functioning t-stat can go bad if stressed by other factors (in my case, apparently by the fan bearing going bad, causing belt slippage and then moderate overheating).

Thanks for all the input, I'm glad to have her back on the road!

[ThePhoenix]

Originally Posted by JamesDean

Typically running on the highway will cool better as you have much more airflow through the radiator.

You might have a bad fan clutch if you experience higher coolant temps at lower speeds/idle.

I agree with that, but I'm seeing slightly lower temps around town if I rev her a bit more, which seems to run counter to what I'd expect from a bad fan clutch. In any case I will check it out.

Still wondering why this is happening when the only thing I changed was some coolant. It wasn't even running this hot when the fan housing bearing was going bad and the belt was likely slipping. Weird.

[JamesDean]

Originally Posted by ThePhoenix

I agree with that, but I'm seeing slightly lower temps around town if I rev her a bit more, which seems to run counter to what I'd expect from a bad fan clutch. In any case I will check it out.

Still wondering why this is happening when the only thing I changed was some coolant. It wasn't even running this hot when the fan housing bearing was going bad and the belt was likely slipping. Weird.

When you had everything apart, did you check the water pump? Did it spin freely? No signs of leaks?

I would probably replace the radiator and then see how things go from there.

[ThePhoenix]

Originally Posted by JamesDean

When you had everything apart, did you check the water pump? Did it spin freely? No signs of leaks?

I would probably replace the radiator and then see how things go from there.

Yes, I checked everything that is spun by the serpentine belt, including the WP, all good. No signs of leaks at the WP or anywhere else other than the small radiator weepage.

I just read on another thread that the fan clutch should always be stored vertically, and mine sat laying flat for some time while I changed out the bearing housing. I'm leaning toward that being the main issue now and will check into it further.

[Ferdman]

ThePhoenix, interesting to hear the fan clutch should be stored vertically. I would call your local MB dealer's Parts Department/Service Department and confirm that is the case. I have a difficult time believing during transport and stocking of a fan clutch that it's always vertical. That requirement doesn't make sense to me. When I replaced the fan bearing bracket on our 1991 300CE the fan clutch was horizontal for weeks, and I haven't experienced any problem with it since reinstallation 2 years ago.

[ThePhoenix]

Originally Posted by Ferdman

ThePhoenix, interesting to hear the fan clutch should be stored vertically. I would call your local MB dealer's Parts Department/Service Department and confirm that is the case. I have a difficult time believing during transport and stocking of a fan clutch that it's always vertical. That requirement doesn't make sense to me. When I replaced the fan bearing bracket on our 1991 300CE the fan clutch was horizontal for weeks, and I haven't experienced any problem with it since reinstallation 2 years ago.

Yeah, I tend to agree. One would think if it were true that there would bright red stickers all over the unit, the packaging, etc. to that effect. Considering my sequence of events (and how I had let it lay flat for an hour or two whilst doing the fan bearing swap), it was the only thing that made sense at the time.

On another note, if your radiator is weeping where the aluminum core is crimped over one of the plastic tanks, and you think you can improve things a bit by crimping the aluminum tabs down a bit tighter, don't do it! It will only make things worse, I speak from experience! LOL (New radiator on its way, stay tuned...)