Ideas On My Engine Problem?

[Smokinal]

I have a '96 Nitro with a 75hp, 3 cyl Mercury on it; owned it for around 12 years now; always winterized and Stabil Marine used in every fill up; always ran great until this year when an issue started. The issue is that when I go to get on plane and start to give it throttle, it cuts back to 2 cyl (starts missing a cyl).

Some details:

---it only does it after its warmed up and been running a while

---I replaced plugs and wires=no change

---I then replaced fuel filter and rebuilt the fuel pump=no change

---It's such a sharp cut-out that it really feels like spark to me, not a fuel issue

---today when it did it I checked the primer bulb and it was full and hard

 

So, recap, after its hot and I go to get on pad, I get around 3/4 throttle and it starts missing a cylinder. I can back off the gas and it will pick back up and run on all 3 cyl until I goose it again, then it loses a cylinder again. I know this makes it sound like a fuel delivery problem but I really think it's spark. And if I just run it at around 1/2 throttle, it stays fine, only does it when it starts to rev.

For what its worth, the tach has never worked correctly on this boat since I got it. It always gives screwy readings. But it's done this since I got it and it's always run fine; probably just coincidental; not sure where the tach reading is taken from.

Any and all ideas are welcome.

[sako85]

a longshot but how is your battery.I had a 2stroke motorcycle  that would cut out one cylinder  intermittently.After I tried everything I asked the dealer to find the problem,without hesitation he said you need a new battery.I did and that was that.

Put a load test on it. 

Good luck.

[Way2slow]

First thing you need to do is determine which cylinder it is. The next time you are using it, drive it a couple of minutes while it's skipping. Then while still holding it at WOT, turn the ignition switch off (they call this a plug dump). If it comes and goes, be sure to do this while it's skipping. Remove the spark plugs and see which one is wet and darker than the others. That one will be the cylinder causing your problem. (be sure to take tools to do this). Also trimming the motor all the way up makes it a lot easier to do from inside the boat on the lake. It's wise to take a spare spark plug and an extra of each tool also. It's very easy for one to end up in the bottom of the lake. You can head back to the ramp at WOT, shut if off and use the TM to dock and load it on the trailer and do it on dry land.

Once you have identified the bad cylinder, it makes it a lot easier to chase that rabbit.

Now is where my problem comes in, I know very little about your motor. If it has individual coils, switch the one for the bad cylinder with another, and see if the problem move to the other cylinder by doing the plug dump again. If it moves, you probably have a bad coil, if it doesn't, then you have other problems.

It always pays to do a compression check. Should be around 100psi with not more than 5% variance on a warm motor.

If this is a carbureted motor, and a carb for each cylinder, switch two of the carbs the same as you did the coils and try it. Again, if the problem moves, it's the carb. Before doing this, have some sort of manual or directions on how to do a synchronization on them, that is a must. That's also something you might want to check before doing them. If it's off, that can cause problems.

From there, make sure all connections a clean and dry, I would even get me a tube of Permatex Dielectric grease (most auto parts stores have it) and put some of every connector, including plug wires. CRC makes it in a spray can. Put in on, and then plug and unplug them a couple of times to clean the terminals. I put this stuff on all electrical connectors, even in my automobiles. Can't count how many times a problem disappeared after doing it.

From this point, you are pretty much down to a bad switch box (power pack), trigger module, or stator, and since I don't know the Mercury system, I can't tell you how to troubleshoot those except for swapping them out. I would guess the switch box, but that's an expensive guess if wrong.

That's about the most help I can give you with a black motor.

[Smokinal]

a longshot but how is your battery.I had a 2stroke motorcycle  that would cut out one cylinder  intermittently.After I tried everything I asked the dealer to find the problem,without hesitation he said you need a new battery.I did and that was that.

Put a load test on it. 

Good luck.

Ty Sako, I will test it to make sure.

 

First thing you need to do is determine which cylinder it is. The next time you are using it, drive it a couple of minutes while it's skipping. Then while still holding it at WOT, turn the ignition switch off (they call this a plug dump). If it comes and goes, be sure to do this while it's skipping. Remove the spark plugs and see which one is wet and darker than the others. That one will be the cylinder causing your problem. (be sure to take tools to do this). Also trimming the motor all the way up makes it a lot easier to do from inside the boat on the lake. It's wise to take a spare spark plug and an extra of each tool also. It's very easy for one to end up in the bottom of the lake. You can head back to the ramp at WOT, shut if off and use the TM to dock and load it on the trailer and do it on dry land.

Once you have identified the bad cylinder, it makes it a lot easier to chase that rabbit.

Now is where my problem comes in, I know very little about your motor. If it has individual coils, switch the one for the bad cylinder with another, and see if the problem move to the other cylinder by doing the plug dump again. If it moves, you probably have a bad coil, if it doesn't, then you have other problems.

It always pays to do a compression check. Should be around 100psi with not more than 5% variance on a warm motor.

If this is a carbureted motor, and a carb for each cylinder, switch two of the carbs the same as you did the coils and try it. Again, if the problem moves, it's the carb. Before doing this, have some sort of manual or directions on how to do a synchronization on them, that is a must. That's also something you might want to check before doing them. If it's off, that can cause problems.

From there, make sure all connections a clean and dry, I would even get me a tube of Permatex Dielectric grease (most auto parts stores have it) and put some of every connector, including plug wires. CRC makes it in a spray can. Put in on, and then plug and unplug them a couple of times to clean the terminals. I put this stuff on all electrical connectors, even in my automobiles. Can't count how many times a problem disappeared after doing it.

From this point, you are pretty much down to a bad switch box (power pack), trigger module, or stator, and since I don't know the Mercury system, I can't tell you how to troubleshoot those except for swapping them out. I would guess the switch box, but that's an expensive guess if wrong.

That's about the most help I can give you with a black motor.

And TY Way2. Great idea on swapping the coil. I manage an auto center and that's how we diag a car too; just never thought of doing that with my outboard. This thing just seems foreign to me for some reason.

I'll keep you posted.

[Fisher-O-men]

Coil was my guess too.  Johnsons have something called a power pack that can act like this when bad.

[Smokinal]

So I took it out today and ran it until it dropped a cylinder. I held it at WOT for a minute or 2 then did the plug dump; took the cowling off and removed the plugs and found 2 wet plugs...??? So I swapped the coil from the dry plug cylinder to one of the wet ones, put it back together and ran it but couldn't make it drop a cylinder again. So i fished a good looking shoreline and caught 4 bass instead.

My take on it is that I was having a contact/ground issue that got resolved when I disturbed the coils. I think I'll go back in, make sure everything is clean and snug and call it good until it happens again...if it happens again.

I thank you for your help and suggestions!

[Way2slow]

Sounds more like a trigger module, or power pack to me.  Keep the stuff with you so next time you will have it and if it does it, check it again, see if the same plug it still dry.

 

When you replaced the plug wires, what kind did you use?  The graphite automobile wires usually don't work.  You need the high performance, wire wound type wires if using automobile wires.

 

Don't forget the dielectric grease on everything.

[Smokinal]

Ya, I'll definitely keep the tools with me and check it again if it does it. And I used steel core wires not graphite; my local NAPA made them up for me.

Thanks again

[Smokinal]

Odd update: Took the boat out yesterday, it ran great in the morning until early afternoon, then started running awful. Took it out today, ran horrible again right from the start. We fished a few hours, then I said I wanted to try to diag it again. I was going to pull off one wire at a time and run it so I shut it down, took the cowling off, started it back up to run it until it skipped but it ran flawlessly immediatetly after I took the cowling off. I ran it all over the place with the cowl off and it was perfect; put the hood back on and it was still perfect. ??? Which was similar to the first day I tried to diag it.

2 thoughts=air blockage in the cowl. How and where does the air get into the cowl on a black motor? Other thought is what I thought before, something electrical heating up. taking the cowl off gave it enough cool air to stay working properly?? But even after I put it back on it was still perfect up and down the lake.

No clue anymore...

[Napierk]

Had the same problem on my 250 Merc and found my fuel line was being pinched. It would run fine until I would go past half throttle. The engine was starving for fuel. I just replaced the hose and its been fine.