Last weekend I diagnosed the cause of a "Check Engine" light with a P0410 code as a bad PAIR or air check valve. That is the valve on the Motronic 4.4 systems, mounted over the exhaust manifold, which keeps exhaust gases from backing up into the air pump.

After checking all the vacuum lines to be good and also the solenoid I tested the valve by attempting to pull a vacuum on the control port with my little hand pump. All I got was some water droplets. So I ordered a new one after "poolman" on this board concurred that one should be able to pull a vacuum on it.

Sure enough, at the parts counter when I tried to suck on it with my mouth it held a vacuum and I could feel the valve move. While I was there I ordered a new port, "O" ring, and a set of new vacuum line connectors for the connections at the intake manifold.

Saturday morning I put all the new stuff in place and sure enough when I started the car it started making the familiar pumping sound after about 10 seconds and the inlet for the pump now was sucking in a lot of air.

I had reset the code last weekend and the test cycle hadn't been run this week so I didn't have to reset the code again.

I wonder how long it will be before the next repair pops up??????? Seems like the car demands some TLC at least once a year. Last time was a biggy... Head gasket but that had the side benefit of getting the 204 HP cams in the car. Plus it keeps me "intimate" and "bonded" caring for my car.
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'96 965, 16' wheels, Michelin Pilot Sports, rear 18mm bar + Koni, 201 HP cams, 128K. Put 200K on '85 745 TD.