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

[ThePhoenix]

Here's my story, sad but true...

Recently purchased a pretty decent 1990 300SEL for a ridiculously low price (story for another time). She (The Black Panther) ran fine for the first couple months I owned her, then that ever so slight bearing noise I thought I had been hearing turned out to be the fan housing bearing. By the time the bearing got bad enough to recognize the issue for sure, the engine was starting to fluctuate between 80-90°C, likely due to belt slippage. A little research on the bearing issue told me that this and other belt driven components were common repairs due to low gearing and high revs. (Fine, I will do 65-70mph on the highway now instead of 75-80.)

No problem, changed out the housing/bearing combo, installed a new belt, and everything seemed to be fine. Had to drain about half the coolant due to disconnecting the heater hose tube that connects to the t-stat housing. The coolant came out very clean with no noticeable debris. Replaced it with fresh coolant with the intent to top it off with distilled water after the system found its level.

(Sidebar - I'm thinking of a mod to the fan pulley housing one of these days (what a lame setup!) - adding a grease fitting, sealing the back side against the block, and ditching the rear seal on the bearing. Anyone ever done this? Seems like the way to go.)

I should mention that up until the bearing issue cropped up, the temp gauge never got over 80°C, even hauling booty up long hills. Now after the repair she is regularly fluctuating between 90-100°C (even at highway speeds), and the electric fan has been coming on very early, like at 60°C or so (weird). Needless to say I'm not a happy camper.

Well it turns out I wasn't privy to the exact procedure needed for refilling as detailed by Duke2.6 in this thread. So this morning I started following his advice. First I checked the 14mm plugs on the front of the head. Coolant was right up top, and loosening the filler cap made coolant start coming out of the plug holes. No air, just solid fluid. ('solid fluid', lol)

So next I started her up without the filler cap, and with the heater running full blast. Kept the revs between 1500-2000 and let her warm up to 80-ish as suggested. At this point the coolant in the catch tank had not moved from its half full level, and it was still stone cold.

Disconnected the overflow hose from the top of the radiator and blew through it, no blockage evident. Shut the engine off, reconnected that end and disconnected it from the catch tank and blew. Seemed like a more little pressure needed in this direction, but can't be sure if that's just from the normal amount of pressure in the system at operating temperature.

Ran her some more, starting feeling around various hoses to gauge their temperature/pressure, watching the catch tank all the time, and the level in the tank never moved off half (other than slight surges when squeezing the larger hoses).

Put the cap back on and kept running her, hoping to see the level in the catch tank drop any moment. The temperature started to climb toward 90°C, and around this time I noticed the top tank of the radiator weeping right near vent hose outlet. Darn it, time for a new radiator.

So if I'm reading into this chain of events correctly, either one of two things is going on (or both?):

1. There is still a lot of air trapped somewhere
2. I disturbed some debris which clogged the vent hose outlet on the radiator (as well as the core itself, also causing a hotspot in the core where the electric fan sensor is located?)

Then any combo of 1 and/or 2 caused the overheating, the excess pressure then causing the radiator leak.

I never touched anything else in the cooling system other than disconnecting that cross tube from the t-stat to the heater hose. Welcome to the world of M103's I guess!

Thoughts?

[ThePhoenix]

Looking around for a radiator I found this.

And this for only $99 (POS?) Anyone have any experience with Everide? Seems pretty iffy considering they don't even show the brand.

[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

[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.

[oldsinner111]

I bought a replacement brand radiator,I'am happy with it.But I will not be keeping car another 31 years

[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 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.

[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.

[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...)

[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!

[ps2cho]

The nissens will be better than the "replacement" brand radiator.

Last thing you want is for that cheap crap to internally break and seep coolant into your transmission fluid. Don't go cheap on critical components.