My 1980 242DL with B21 and auto trans had developed an odd spark knock. Knocked only at highway speed (3500 rpm+) intermittently while cruising and also while accelerating at highway speed. Would NOT knock under load at lower speeds. About 6 months ago, as part of general clean-up, I replaced injectors, control pressure regulator, thermal time switch, engine wiring harness, intake manifold gasket, plugs, rotor, cap, ignition wires, and all vacuum hoses. Also had mixture/idle set. Improved, but still a bit of a loping idle and the damned spark knock -- although not as frequently. Knocking took place even with premium fuel.

This past weekend I replaced the vaccum hoses with high quality silicone hoses. Previous hoses (replaced 6 months ago) were Volvo OEM, my buddy in the Volvo parts dept. gave me. VOILA ! No more spark knock and nice even idle. Obviously there had been some air leakage around the hoses, but who would have thought that would happen with brand new, OEM hoses ? The old ones fit snugly, but the new silcone ones were maybe a bit more snug, and more pliable for a better seal. Sometimes ya gets lucky.