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?