Miss goes away when #4 spark plug wire is removed?!
 

Someone explain this to me: Merc. 502 MPI, ECM 3; I had a miss in my stbd. tach at idle which seemed to go away above 2500 RPM. It was time for plugs cap and rotor anyway so I started with that. I discovered that the # 2 cylinder had not been firing. I removed the plenum and checked the injector wire with a noid light- it was getting spark. I installed a new injector, and #2 is now firing, but I still have a slight miss- hardly noticable on the tach at idle, but obvious between 1200-2000 RPMs, then it seems to go away again (probably just because the tach is unable to pick it up at higher RPMs). I started removing plug wires one at a time with it running at about 2000 RPMs- All the wires made the RPMs dip and the miss worsen except when I removed #3- then the RPMs stayed the same and the miss WENT AWAY! I'm getting spark at the plug end of the #3 wire but that's as far as I got before giving up in confusion- any theories as to what the f#@k is going on here?

your not using a big enough hammer.



maybe another bad injector?

Plug wires crossed?

Do a compression check. If rpm's do not drop when you undue a plug wire it means that cylinder is not working to begin with. If compression shows good or equal to the others then at least you will know you do not have any interior damage.

I would think if one cylinder was completely down you would notice it especially trying to plane the boat i really dought that if hes owned the boat for awhile , thus before getting the miss maybe low compression and as he bulds rpm it will fire here and there just my thoughts ?

i would second the crossed wire, double check your firing order

Check for a vacum leak at the lower manifold to cylinder head. A leak there will lean one cylinder out enough under light throttle to induce a miss fire. Under hard throttle, its rich enough to fire.

If the wires are not crossed the wires may be cross talking .Subject to the age and condition of plug wires, they may require changing . Also ,as the injector was changed that wire was moved....The wire or connection may have been the problem .

Check for consistent spark with a timing light at various rpms. A worn plugwire not unlikely.

Thanks for all the ideas- turns out its another bad injector- I removed the coil lead and cranked the eng. several seconds, pulled plugs from 2 working cylinders & could smell gas. Pulled #3 plug- no smell. Took a wooden dowel and held it to my ear and a couple of working injectors I could get to with the plenum on, eng. running and could hear a definite snap, snap, snap. Put it to the #3 injector- no noise. Must have damaged the electrical connector during the install process when I was trying to line them up, damn it. As far as the miss going out of the tach when I removed the #3 plug wire yesterday- I removed it at the distributor and the post was arching to the plenum. I guess completing that circuit fooled the tach(?)