Earlier this week a very well-maintained 1968 euro 280SL driven by an attractive Asian woman limped into the Faded Star Garage. It had a miss at idle and under any sort of load. It did not want to rev, but if you brought it slowly to 2000 rpm, it smoothed out.
A quick test revealed it was sucking air at the WRD. That was solved by a couple of taps with a hammer. No change
Dwell and timing were perfect.
Idle CO was around 11%. A quick linkage check showed that the throttle plate was fully closed, and the pump lever was on its stops, so I brought the CO down to 5% with maybe a dozen clicks on the adjuster. Smelled better, but still missing.
Pulled the plugs (W7DC), slightly sooty, but no sign of fouling, but #1 looked very lean.
Tested for spark with the timing light, showed all cylinders firing.
Loosened each injector connector, noted that #5 made no difference. So I am thinking, probably that fuel injector. Pulled all the injectors out and bench-tested them. To my surprise, #5 was near-perfect. #1 dribbled, but started spraying after a couple of hours with BG44 sitting in it and some exercise with the tester. Put the injectors back in, no change.
Changed the spark plug, no change.
So now I am thinking. It has spark, the injector is good, what's left? Maybe injector line? Maybe something in the plug wire? Since the wire was easier to test, I went there first. The car had what appeared to be an almost new Beru wire set, complete with the vinyl sleeve. I removed #5 wire
and separately tested the distributor end (1K resistance, the wire (no resistance), and the boot (1K). I put it back together and pulled and wiggled the assembly with the meter connected but could not cause a fault. Put it back on the car. No change.
So I scrounged around and found a complete long wire (believe original) that came off a 250SL. Hooked it up, started the car, no more miss. Yay.
So then I tested each component of the original wire. When I put the boot on the 250 wire, the miss returned.
Ordered a new boot PN 000 156 52 10, screwed it on the original wire, problem solved. I am guessing that the original boot was allowing the current to ground against the block instead of firing the plug, or for some other weird reason would not work at idle or under load.
Posted in the hopes that this can help someone else troubleshoot a similar situation. I the course of figuring this out, I learned quite a bit about plug wires and resistance, which I will blather about in a separate topic.
Cheers,
