I have been reading a lot about burping and bleeding of air from the cooling system. I am curious, how can you be sure there is air in the system? How do you tell how much? How do you bleed it (properly)? and How do you know you got it all out?

Have checked the archives and other posts, but the answers only make me have more questions. Can someone please explain proper proceedure and how to tell if it needs to be done. Have a feeling that others may be asking same question.

Thanks,

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William
76 240D - 550K miles
78 300D - 200K+ miles

The bleeding procedure kind of differs depending on which model you're working on. On some models such as the V8s you can fill the coolant through the upper hose (disconnected at radiator) and hold it as high as possible to take advantage of gravity which will make all the air go to the top. But hold the hose as high as possible when filling. This method can take a while, but is the best way to clear the system of air pockets. In diesels, you can just fill the coolant through the expansion tank until it hits the full mark and run the car (I like to drive it to force out the air pocket) with the heat on so you know the t-stat has opened. If you go driving after filling up the system, be sure to take extra coolant along with you and stop periodically to check the level. Don't worry if the temp gauge on a diesel goes above normal, those engines can certainly handle the heat. On gasoline powered cars, once you have filled the coolant to the specified mark, just idle it (rev it periodically) until you feel heat coming through the vents (turn the defroster on full blast) and top up the level afterwards. Some of the V8s such as the 4.5 were notrious for air pockets and when running them to bleed them, it' normal (on any model) for the auxiliary fan(s) to kick in and run for a while. Hope this helps!


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Rgds,
Aaron Greenberg
MB technician
Precision Motorcars, Cincinnati, Ohio
'67 250SE Cabriolet
'77 450SL
'80 300SD
'85 380SE
'86 420SEL
'89 420SEL
'93 300E 2.8
'74 Jensen Interceptor Mk.III
'81 DeLorean DMC12
'84 BMW 745i Turbo

On my W116, 300SD I always disconnect the electrical wires going to my heating water pump, which is right next to to the climate control servo. I believe it's job is to pump hot coolant through the heater hoses. I make a self made tool out of two 3-4 foot lengths of wire each with an alligator clip on one end and a butt connector on the other ends. I hook the pump up to the battery directly while filling the coolant reservoir.

I do this because my manual say's to. Is this needless or a good idea?