I could really use some help with this one.

The car is a 1986 560SL with 159K. Decent maintenance most of its life, but a lot of sitting the last 5 years - only 10K miles during that time.

When I bought the car two months ago, it had a miss during warm-up. I cleaned and tested the injectors and re-installed with new seals. Problem went away briefly but returned, worse.

Car also had a surging idle timed to the on/off ratio. I replaced oxygen sensor and re-set mixture. On/off ratio at a fluctuating 45%, right where it is supposed to be. Car ran perfectly for a few days, then the miss returned.

Checked for vacuum leaks and found none.

Cleaned cap, replaced rotor, replaced plugs (Bosh W9D copper). No change. In fact, the miss got worse, happening at all temperatures. I returned home from work last week with car running very poorly. Rough idle and miss under load. Car does not smell “gassy.”

I checked the on/off ratio and it was steady. I tweaked the mixture to see if it would change. As soon as I inserted the hex key and pressed down, the idle smoothed out and the on/off ratio started cycling around 45%.

The car started running perfectly, but two days later, the miss returned, as bad as ever.

Ideas?

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe