Originally Posted by William

The vehicle is a 420 SEL, 1986 version. On this car, there is a smog pump located on the passenger side, just under the alternator. The air that discharges from this pump travels through a rubber tube to a "stop valve". From there, the air goes through another rubber tube to a "check valve". The "check valve" is screwed into a connection on the "timing chain cover" which presumably routes the air to a passage in the engine and then it somehow finds its way to the exhaust manifold.

A few months ago, the rubber tube connecting the smog pump and the "stop valve" wouldn't stay connected at the pump. Upon first cranking, the hose would pop off and I couldn't determine if the problem was with the "stop valve" or the ''check valve". I don't know how to run a test on either of these valves and the "Mercedes Service Manual CD" doesn't cover this topic.

If I should decide to dismantle the smog pump system, I have surmised that the "check valve" would have to remain in place at the timing chain cover to prevent exhaust gasses from backing up into the engine compartment.

I would appreciate someone explaining the details of this system. Thanks.