Thanks, Craig. That's what I think too. I'm really puzzled by the number of folks who say their systems hold pressure when cold - I posted to the MBZ.org diesel email list and got similar responses over there. I'm not sure if they're confusing the "rushing air" noise when opening the cap to be pressure instead of vacuum, or if there are a bunch of other 602/603 engines that are all going to need head gaskets soon as well!
Anyway, follow my logic here if you will:
The cooling system (602/603 engine specifically, but should apply to almost any engine) is a sealed system, with the radiator cap only acting as a pressure relief valve at 1.4bar (~20psi). When it's cold (ambient temp, 50F at my house) if you open the cap, there will be zero pressure in the system. Close the cap - same thing, zero pressure. Drive the car - the coolant gets hot, and expands, which causes pressure in the system. Shut down the engine, it cools, the coolant contracts, it returns to ZERO. If it doesn't there is something entering the system. If there were a leak, it would either lose coolant, and/or not hold pressure in the first place!
To explain how vacuum can exist, picture the pressure relief valve (rad cap) opening when the engine is hot, and releasing some pressure into the overflow tank. When the system cools, there will be less coolant in the system, which will result in slight vacuum. As I said, this is how my car acted for years prior to this month.
I'm going to try the K&W Block Seal route before tearing into the head job. I'm almost afraid to ask the dealer for a quote on the job. Any comments on things to replace while it's apart? I was going to have the timing chain & valve lifters changed, wasn't sure if there's something else I should add to the list...
1987 300D - 228kmi
1987 300D - 235kmi
1984 300D - 206kmi