Twice in the past month I have had occasion to drain the cooling system on my 77 300d. Once to replace the water pump and then again today to replace the ACC servo. In both instance it took forever to get the air out of the cooling system. I filled the block through the top radiator hose, filled the radiator, removed the vent bolt on top of the thermostat housing, turned the heater on full blast and ran the engine. Both times it took a long time to get the air out and get coolant in the heater core. It took about a hour of fooling around to get the air out. I'd run it, shut it off, drive it around etc. in an attempt to get it full. I'm still not sure why the coolant finally began circulating through the heater.
I figured out in the process that if the temperature goes up when you run the heater, it means there is air locked in the heating loop of the system.
Anybody have any hints on how to speed up the filling of the system? Would loosening one of the hoses high in the heating loop help get air out of it? It looks like there is a small heater line high up on the firewall. I've never had this problem with other cars I have owned. The fact that both radiator hoses exit the block at the top seems like poor engineering to me since it means you can't really fill the engine by pouring coolant in to the radiator.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

kerry edwards.

I usually find a steep hill, and drive the car up the hill so the front is higher than the rear. I am lucky because I have such a bit of geology in my yard, right where I park my cars so it is easy for me. I fill the car as much as possible where I drained it (not on my lawn on the hill), then drive it over and onto the hill and let the system warm up while I top it off. Make sure the thermostat has the little vent hole at the high point too. Once filled that way, I have not had issues with trapping air in the system.

Hope this helps, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

The old diesels are notorious for air pockets. The best way to purge the system is to drive the car and force the air out. The temp needle may start to climb alarmingly fast, but that cast iron engine can handle it. This method was told to me back when I was apprenticing by a long time MB mechanic who spent 28 years working at the factory.

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Regards,
Aaron

Kerry,

My '77 300D engine has a small bolt screwed into the top of the thermostat housing that you remove when filling the water system. You just keep filling until water starts running out of the bolt hole then replace the bolt.

Check to see if yours has the bolt.

Ben
www.reproduce100s.com

Kerry, in addition to Aaron's advice I also use the baby burping trick when filling (put some in, jiggle, put some in, jiggle, etc.). And you don't need a cloth over the shoulder or diapers a few hours later.

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daBenz - 1970 220D

One suggestion is to fill the radiator with water and then loosen the bleed bolt on top of the T-stat housing (or, if you don't have a bleed bolt then loosen the hose that happens to be the highest one in relationship to the ground). Then take a cooling system pressure tester and attach it to the radiator fill neck and pump up some pressure. The pressure will push out a lot of the air that is in the system and expedite the bleeding process.

mike