On the 617 diesel engines, there is a water bypass line from the cylinder head to the water pump (the small crescent shaped metal tubular one). It is listed in the CD as a “vent line” which should actually answer my own question, but it appears from the cooling system diagram that the venting of air occurs elsewhere. Which leads back to the question – what is the purpose of this line?

Has anyone ever had this line get plugged up on them, and what were the symptoms or consequences?

Thanks Everyone!

Its purpose is to vent or bleed unwanted air from the engine to the top of the coolant tank. IF it is plugged up, you will have air pockets forming inside the cylinder head and this could lead to running warm.

The other vent hose that is near the cap is for over pressure relief.

__________________
Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS

Thank you BenzMac. Sorry that I haven’t replied until now. I’ve been busy replacing my radiator amongst other things lately. Unfortunately, my car has been living up to my pseudonym here, Running Too Hot.

On my (new, old) 84 300D with almost 200,000 on her; the radiator was original and succumbed to the “neck crack” syndrome. Luckily, I caught it the instant it happened, so I didn’t fry the engine.

The (metal) bypass line was not plugged when I checked it. I now have a new radiator, new hoses, OEM pump, a dealer-supplied thermostat (plus a Wahler that was defective out of the box), a 40/60 mix of phosphate free antifreeze + Water Wetter, a perfectly functioning visco clutch, a fully functional auxiliary fan (which I discovered via the wiring diagram is NOT turned on by *any* engine temperature switch, ONLY the A/C receiver dryer switch!! W.T.H.?).

There is no air trapped in the cylinder head, since it is self-venting. The temperature gauge is reading accurately, checked multiple times with an infrared non-contact thermometer used on the engine block & cylinder head. The new (2nd) thermostat was checked in a pot of water, and it starts opening at 180 degrees. The system was flushed thoroughly – and it was clean to begin with; nothing dirty came out, the water was barely discolored at all.

After all this, the engine runs at 97- 99 degrees on the gauge, no matter what the load – light load, medium load, A/C on, A/C off, 60 degrees ambient, 90 degrees ambient, etc. You get the picture. Can you spell F-R-U-S-T-R-A-T-I-O-N? I guess that I should be satisfied that it is at least *stable* at 97-99 degrees, but I am going to try a THIRD thermostat. I can’t think of anything else. If the head gasket were leaking into the cooling system, I don’t believe that the cooling system would continue to hold pressure in overnight, which it does. Unless there is such a phenomena as a one-way leak (?!).

Sorry for the ranting. If you have any ideas, I would GREATLY appreciate them. I may post this as a separate question for the group if you think that would be wise.

THANKS AGAIN!
RTH

RTH - does the temp gauge move around based on how "loaded" the car is. My '85 300D does. When I get in traffic it will get closer to the 100dC mark than the 80dC. Once on the road it drops closer to the 80 mark. In addition, I have not run it since yesterday afternoon and I am sure that there would be some pressure in the reservoir. The thing is, as long as it runs good - don't worry about it.
In the E320 I run "water-wetter" with approximately 40% MB coolant. I have yet to see it touch the 100 mark since going with this set up about a month ago. I can tell when the aux fans come on and go off by watching the temp gauge and considering the engine load.

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Jim

Well Engatwork, that’s the problem – it doesn’t move much at all. When started from dead cold in the morning, it will rise during warm-up in a normal manner to about 85 and then it stays there for maybe 1 or 2 miles. Then as heat soak becomes more pronounced, it rises to between 97 & 100 and stays there no matter what the load or speed is. The only exception is if I boot it going up a long hill - then it will go up to just above 100.

I suppose that many will say not to worry about it, since it is stable within a narrow range & is not at the upper end of the scale. BUT this is not normal, and after effectively replacing every conceivable part of the cooling system that could influence this, I am stumped. And I like to delude myself into thinking that I’m a fairly competent DIY’er.

The only thing that I can think of is the thermostat is bad, but like I said, I did test it in a pan of water, and it started opening at about 180 (on my F cooking thermometer) – right on spec. I also raised the water temp to boiling, and it does open the aperture significantly more, although I have nothing to really compare it to, as far as knowing how far it is supposed to open. What are the chances of receiving 2 bad thermostats in a row from different suppliers & manufacturers? My original is gone (trashed it) the first replacement was a Wahler which did not open until it was hotter than 180, the second one is from the MBZ dealer, and is an Original Equipment unit.

BTW, I boiled up both the Wahler & the Dealer unit when I brought the dealer one home; this gave me a view of their comparative function, or in this case, dysfunction.

Thanks for the help!
RTH

Hey guys, I just wanted to thank you for your help. Unfortunately I am still having problems. This is driving me absolutely NUTS.