90 HP four stroke misfiring

[kimbrell224]

2008 90 HP Mercury four stroke (1B707931)

The #1 cylinder, if thats the top one, is not firing on this engine. I've swapped coils and spark plugs with the cylinder below it and the problem did not follow. I can unplug any of the other three coils with it running and the engine dies. The engine does not change when I unplug the top coil. Any help or direction is appreciated.

[vicsservice]

Did you do a compression check? what do the plugs look like?

[kimbrell224]

Did you do a compression check? what do the plugs look like?

No I haven’t done a compression check and the plugs look fine.

[vicsservice]

I would actually want a cylinder leak down if it was at our shop, then I would be looking at the air/fuel/spark for the cylinder that doesn't respond, would need to verify fuel is being delivered through the injector, spark is strong, then intake and exhaust valves function.

[kimbrell224]

The compression is 185-195 psi on all four. The top plug or cylinder that isn’t responding is the only one that doesn’t smell like gas. I’m assuming that I have a fuel problem since swapping the coils and plugs didn’t move the problem. Any suggestions on troubleshooting further?

[vicsservice]

Sorry for the delay I would unplug the fuel injector if it doesnt make any difference then you have a fouled injector or short in the harness to the injector

[kimbrell224]

Sorry for the delay I would unplug the fuel injector if it doesnt make any difference then you have a fouled injector or short in the harness to the injector

thanks I’ll try that soon

[EuropeanAM]

Likely is a fouled or failing injector.

[kimbrell224]

I’m still working on this problem as it is very intermittent. I pulled the fuel rail off yesterday and the bottom injector was discolored and had oil on it. What does that mean? It’s not even the problem.

[EuropeanAM]

Several possibilities there- starting with something as simple as caused by misfiring.

[kimbrell224]

Update: after swapping the top two injectors suspecting the the top injector was the problem, it started misfiring again today. I was able to get back there and unplug the coils and the problem cylinder is still the top one. So far I’ve swapped the plugs, coils and injectors between the top two cylinders and it has stayed the same every time. The top cylinder is still the one not responding after every change I’ve made. I can speed up and it usually smooths out and goes back to normal. Most of the time this engine is somewhere between 2000 and 3000 RPMs since it’s on our pontoon.

Any other suggestions?

[EuropeanAM]

Likely a wiring harness problem, then. Will have to determine which SIGNAL is being lost: Injector or Coil (ie: does it have spark when the cylinder drops out).

That can be a bit interesting with Pencil-Coils...

And even more so since things are being unplugged to diagnose (can't rely on fault history info as some of it is "diagnostic attempt induced").