PeachParts Mercedes-Benz Forum - 1985 300D, Overheating

Turned out it was the radiator (I hope). See photos. The debris clogging the channel tops was fairly soft and most of it easily washed off w/ a hose jet, so strange it wasn't eliminated by a back-flush. In contrast, the bottom side was clean and all channels open.

Early on, I had removed the original radiator and it reverse flowed well (tested w/ garden hose), and I let it sit flat for a week in the hot summer, filled with 1 lb citric acid in water to dissolve rust. I also swapped in a junkyard radiator (in photo, M-B OEM). It also passed a garden hose flow test. I also tried viewing the top channels w/ an endoscope, but hard to get a good view.

I have been fooling with this for about 2 years. I first noticed an issue soon after I added 1 tube of Aluma-Seal (maybe 2, forget). That was because I thought the heater core might be slightly leaking. I had Evan's Waterless Coolant, so wondered if the powder formed a clog in it, but I'm not blaming the Aluma-Seal, since could be coincidental, plus it wasn't there when the radiator in the photo was installed. Initial symptoms were that the temperature would creep up to 100 C when idling at a 5 min stoplight. If I rev'ed in neutral, the temperature quickly dropped. I could still drive on the highway w/ no overheating. It became worse over the years, finally getting where it would push 120 C in a 7 mile city drive.

Besides flushing and swapping radiators, things I tried over the years:

1. Changed to pure water for several weeks in the summer and ran citric acid flush for several days, then cleaned by changing the water many times, then block drained, dried and went back to Evans. At least w/ Evans it wouldn't boil over (can run w/ radiator cap loose). The heater core was bypassed, to avoid damage and to not contaminate the Evans w/ water since it is the hardest part to dry. I installed a wire screen in the top hose to catch any loosened debris before it could clog the radiator.

2. Several thermostats and water pumps. I even inserted a hose to restrict the bypass in case it somehow wasn't getting closed off by the T-stat. I was surprised that no water came out the open top hose when idling, though it would flow when rev'ed. But, I tried the same on my 1984 (has never overheated) and it also didn't flow at idle. That may be OK since with it connected to the radiator, there is no head to overcome if solid water (siphon effect).

3. Swapped temperature sensor in the head w/ my 1984 300D. No difference. Also verified reading w/ IR gun at sensor fitting.

4. Several clutch fans, plus a new thermal clutch w/ coil bi-metal spring (like U.S. cars).

I should mention that on this final radiator swap, I also changed the water pump housing and water pump, just in case there was something funny there. One suspicion being too much clearance between the blades and housing, which is hard to determine by measuring the parts, given the angled interface.

I was motivated to buy a new radiator because I found a hole in the top plastic tube stub. Since I am always careful tightening that hose clamp, I felt all the plastic was now brittle. It was also a worthwhile shot before jumping to "bad head gasket" thought. I was pleasantly surprised to find radiator prices are now affordable. I paid $135 for a Spectra, just a little more than the ones for my common minivans. I recall ~$350 prices in the past. P-P lists a Nissen for $190 which looks so similar it might be from the same factory. It is also an aluminum core so lighter and cools better. Since I use Evans, I have little concern w/ corrosion (aluminum corrodes easier than copper). So far, the temperature hasn't gone above the 82 F T-stat setting, even on a 20 mile drive when 80 F outside, so looking fixed.

After viewing the old radiator core, I am convinced that was the problem all along (for 2 radiators). Apparently it is hard to judge a radiator from a flow test. It may flow well, but only from some channels, with others plugged. Also, citric flush doesn't rid all corrosion. Perhaps this is why in the days of all copper radiators, shops would de-solder the tanks to rod them out, then re-solder. You can't do that w/ these plastic top radiators, plus the rubber seal degrades over time anyway. Many of the aluminum fingers securing the top broke off when I bent them, so don't think you could fix one.