Engine breaking up at high RPM. Only after running for a long time

[christoner2002]

Re: Engine breaking up at high RPM. Only after running for a long time

Boat ran great tonight. I ran it for a long time, and never once heard the ignition break up at WOT. I was out there for over 3 hours, and only stopped at the beach for about ten minutes the whole time, just to change props. My only variable besides the coil tonight, was that I didn't have the convertible top for the boat laying back there.(it sits under the engine cover, and almost lays on the top of the engine) The coil I was given to try is a automotive coil. The MerCruiser dealer is the one who gave me it to try. Anyone know if the coil is USCG part? I have never had any kind of inspection by USCG in my life, and I want to know if I should purchase a MerCruiser piece($58) or can I run the automotive one($21). It felt good to have it run good the whole time. I was even able to help out a stranded family, and tow them back to the boat ramps. I will test it more this weekend, and see how it acts. I have my fingers crossed. I will check the plugs again, but I thought they were a little tan. I will check them again to be safe.

 

[ChrisCraftJohnny]

Re: Engine breaking up at high RPM. Only after running for a long time

One other thing...


Did you gap the plugs to your engine sticker's spec?

My NGK's were off by .005" - .008" out of the box.

Always check gap on new plugs.

Too big a gap can cause misfire/loss of power at high RPM's

A small detail, but important.

Johnny

 

[christoner2002]

Re: Engine breaking up at high RPM. Only after running for a long time

Thanks for the idea. I did gap them to MerCruisers specs when I installed them.

 

[Shamus O'toole]

Re: Engine breaking up at high RPM. Only after running for a long time

that plugs tan. It's not lean according to the plug. But with all the additives in fuel these days it's getting harder to read plugs.

 

[akfishin]

Re: Engine breaking up at high RPM. Only after running for a long time

So I have a similiar issue on an older Volvo 5.0 FL.

Changed plugs, issue got better, but not cured. Old plugs were shot.

Added Star Tron tank cleaner to tank, 2nd racor fuel/water seperator. Better but no change.

Interesting part here though. Just got back from talking to my Volvo mech and was describing how it acts up.

Running 3400-3500 rpm, boat not over loaded, running 26 knots for 1+ hours.

The return trip, engine, when almost at harbor, popped 3 to 5 times, I throttle back to 1500 rpm, setup tabs to get back on step, and start rallying again. lasted for about 5 to 6 minutes, happened again. Important note, the engine NEVER dies, just falls off step. Let it run lower rpm for a minute then get going again.

Here is what we think is going on.

We believe the engine itself is getting too hot (inside cowling). The fuel is igniting in the intake manifold. If it were to get too hot, potentially you could vapor lock the motor. (IE, vaporize fuel, has to cool down to turn to liquid again so u can run).

It's interesting because what I didn't tell him was I was idling for about 20-30 minutes after running 3600 rpm while pulling shirmp pots and cleaning shrimp.

His suggestions were to visually inspect the cowling vent lines to make sure they were not plugged, blocked, or crushed. When it happens, run blower for a while to force air to change thru cowling. Also to maybe add additional vents.

Old wives tale, that he has heard from other techs that has worked. Take wood clothes pins and clip them to your fuel line. Evidently it's suppose to somehow draw off heat and cool gas as it makes it's way thru the fuel lines. Of course, my lines are the bent steel or copper, not braided.

I really think he's right for me. Engine is running cooler after swapping in new plugs, thus it's hardly happening to me now. If you all have hot weather, it could be something like that!

I'm going to try it out Sunday and will holler back at results. Anyone else who might think on this rant of mine, please feel free to sound off.

Dan

 

[dalenoman2005]

Re: Engine breaking up at high RPM. Only after running for a long time

Solved (touch wood....)

Took it to my mechanic, who is also the owner of the small marina where i keep the boat. He took it for a spin on the lake and the problem happened for him (thank god!). It was pretty severe - happened even at lower rpm too. go real slow, no problem. increase throttle and it coughs, skips, etc. As if it's running out of gas - exactly the way others in this thread have described it. however, in my case, it never led to problems starting it or keeping it running.

The mechanic listened to it for a couple of minutes and said he'd like to start with the two cheapest and easiest potential causes - fuel filters and coil, both of which he had in stock. I have two fuel filters on my motor. both were clear.

He removed the coil and found it was the factory original (20+ years old.) I didn't know what a coil is/does. he explained it as a type of capacitor that stores power from the battery/alternator and delivers it in short bursts to the plugs. it's not a commonly replaced part, but does suffer from wear and tear over time. Perhaps the right approach would have been to test it under controlled conditions. but considering the low price (under $50) and the fact that he had a replacement on hand, we agreed to just replace it, at least for the weekend.

took 5 minutes to replace. i started it up and began driving. seemed better, but the problem was definitely still there. this time, it even stalled while idling, which was new. oh no, it's worse!

returned to marina and the mechanic told me to drive it that way for the weekend if i wanted to, and he'd continue with more formal tests the following week. i eased out of the harbor and started driving at about 1500 rpm. ran it that way for about 10 minutes, then gently increased. wow - problem seemed to be solved!

my engine's WOT rating is about 4500 rpm. i opened it up all the way and achieved 4500rpm and about 52 km/h. not bad. equal to what i had achieved in the past prior to these problems mind you, but much better than the last few weeks!

ran it WOT for about 5 minutes. no problems. reduced to 4000 rpm and ran at 45 km/h for over 3 hours, stopping several times for: gas, giant slushy, let the dog pee, etc. after 3 hours, i reduced to trolling speed and ran the downrigger for some lakers (quite unsuccessfully mind you). I wanted to see if running slow for an extended period would re-create the issue. It didn't!

took the boat out again the following day with the family and it ran great.

i consider the problem solved, and learned the following - but be forewarned - i'm no mechanic! this is my layman's opinion:

An aging coil can cause these problems. It will lead to an insufficient spark at mid-high rpm. my theory for why it didn't solve the problem immediately is that the old coil had caused fouling on the plugs from poor ignition. Replacing the coil didn't clear the fouling off the plugs, so the problem continued to exist for a little while. running it at low rpm, then gradually increasing all the way up to wide open cleared the crap off the plugs and returned things to a-ok.

I learned to treat problems like this as potentially solved even though the issue may still appear to be present. if you're having issues like this, i encourage you to do your best to run the boat for a while after each potential fix and see if the issue clears itself up after 10+ minutes of running.

i understand that not everyone can do this - for example, if your boat tends to stall and not be start-able, this could be a very costly and time consuming approach.

anyways, thought i'd share my experience.

good luck to you sorry s-o-b's who've already replaced your coil plus a dozen other things and still have a problem like this! i don't envy you one bit!

 

[whitney]

Re: Engine breaking up at high RPM. Only after running for a long time

Good to hear! I'm going through similar issues and just found your thread. Here is my story which is looking like it's going to end the same as yours (hopefully): http://forums.iboats.com/showthread.php?t=430118

 

[christoner2002]

Re: Engine breaking up at high RPM. Only after running for a long time

Ran the boat all day yesterday with the new coil, and it ran perfect.
Hope this says this way. I have my fingers crossed.