The car is a 1994 Chrysler New Yorker with the 3.5 L V-6 engine. At 60,000 miles, it has developed a noticable up-and-down vibration at idle I feel through the seat cushion, which is more pronounced with the transmission in either forward or reverse compared to park or neutral. A timing light stroboscopically freezes a mark on the crankshaft pulley at 2000 RPM, but loses sync and doubles up or dances back and forth at idle. This occurs when in turn sensing each of the six spark plug wires.

A chalk mark shows no slipping of the outer relative to the inner ring of the crankshaft pulley.

I tested the coil and both primary and secondary resistance values are normal. I had replaced the spark plug wires with an NGK set, and I have inspected the spark plugs that are Autolites that they have the correct gap and have no unusual deposits. I replaced the upper intake manifold gasket to correct a vacuum leak and a blowby smell in the garage; squirting manifold cleaner reveals no engine stumbles or speed-ups indicating any vacuum leaks and the smell went away. I replaced the crankshaft position sensor, using the stick-on spacer. The car starts and runs fine and has adequate power, but the idle vibration persists.

The next step being recommended to me is to do a timing belt service, but before I do that, does anyone have experience with this condition and what repair did you make?

 

vibrations usually come from a weak motor mount