quick test for plugged cat: undo the EGR tube to open the exhaust manifold, yes it will be noisy but you'll quickly find out if the miss is restricted exhaust related

quick test for restricted air: take out the air filter

test for air entering fuel system: use a large plastic pop bottle full of diesel, disconnect fuel lines so suction from fuel pump (side of IP) is drawing straight from bottle, and disconnect return line and stuff it in the bottle too. (clamp and plug off where these lines used to connect to prevent tank from spilling)
After you run it a while to work out air from doing this operation, see if bubbles continually come from the return line. Go for a drive with load, with bottle secured under hood.

test for too-soft return check valve in the IP, by pinching off the return line so it is nearly shut. Go for a short test drive.

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Cheers!
Scott McPhee

1987 300D