Port engine fluctuates rpm while cruising

[eauty]

I have 8.1's with 500 hrs on motors and they have run nearly flawlessly over the years. Had some oil consumption problems in past but that seems cleared up. Today while cruising, the port engine would suddenly lose 150 rpm and the the smart craft gauge was fluctuating all over the place. Then it would run perfect for awhile then all of a sudden lose rpm's again. I've changed the plugs at the beginning of the season, also have changed fuel, oil and trans filters this season. It's ever done this before and it Ickes perfectly. The port trans does seem a little more noisy but I can't say for certain. Any ideas? I have original plug wires.

 

[scoflaw]

Might want to hook up a fuel pressure gauge, and see what
its doing when that happens. Big blocks wear plug wires out quickly. Owned several and that has been my experience. Check resistence with an ohm meter.
Scanning the engine might give you a clue as well.

 

[mainstream]

Hello
I had the same problem, when the mechanics had it on the scan tool at the dock everything checked out ,but when we put him in the engine room when under load we found a missfiring situation.He then replaced wires and plugs and all was good until the starboaurd engine started doing it about 15 hrs later. Did the same thing and now its been over 100 hrs no problems and better gas milage not that .8 is that great but better then .6 .7

 

[Gunn]

I had a similar issue with one of my 4.3l's earlier this year. Turned out to be the fuel pump. Because once I got to my destination, I could not get it to start again! Diagnosis was the fuel pump. Changed it and now engine is rock solid. And now thinking back, that engine was giving me subtle rpm issues for the last year or more, just never put it together.

 

[eauty]

I changed the fuel filter today in the port engine and it only had 50 hours on that filter and it showed quite a bit of water intrusion. I took it out for a short run and there was no fluctuation in that port engine. I take the utmost care in trying to get the best gas from reputable marinas but our trip to to the North Channel may have caused me to pick up some bad gas. It is interesting that you and scoflaw both mentioned changing plug wires as mine our original on both motors and they are in their 6th season. Did you replace with Mercury product? The complete set of wires for those motors are close to $500 set. Thanks for your ideas as it is perplexing to be running along and suddenly lose 150 RPM's and then start running normal again.

 

[mainstream]

Hello and yes everything was replaced with mercury parts it was all under warranty since there was only 75 hours on my boat at the time