'91 W126 500 Rough Idle and Miss

[Eddie Gilmartin]

I have changed the spark plugs and occasional rough idle remains. Also the revs are hunting at idle. Also intermittant misfire at all speeds. Can I plug this car into a diagnostic ? Can it be "chipped" in any way ?

Hi Eddie,

Check the vacuum line to the transmission modulator at the right rear of the intake manifold. On US versions this is a white , vinyl hose. They are prone to pop off when the black rubber connecting hose deteriorates,and the engine has a lean backfire. Also there is a rather large plugged port on the top at #2 cylinder intake riser that may have become dislodged , and yet another smaller one at # 5 (or 6)again at the riser. If the roughness is subtle in nature, the problem could be in the EGR vacuum plumbing. Try disconnection from the EGR whilst it's running and cover the hose w/ you're finger. The surge in the idle sounds like a large intake leak.

Just thought of something else. We see bad plug wire ends (the sparkplug push on connector) on the M117s alot.But you'll need an ignition scope to isolate the high spiker.They are available individually,and screw off counterclockwise. However, sometimes ,on higher mileage wires the threaded brass wire adapter twists off. Spray w/ solvent/penetrant first. Failure means buying a wire ( also available separately)

[Eddie Gilmartin]

Thanks. will work on that. It is the 117 engine euro-spec. BTW, she passed emissions (pre)test today. Full state test Wednesday.