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Old 05-27-2014, 07:53 PM   #1
Lt. JG
 
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Default Engine misfire Over 2000rpm

Having engine issues with my 2400SC with the 350 MAG Mpi engine. When under load it misfires badly over 2000rpm which then also causes vibration from the engine. In neutral it revs up no problem well over 4000rpm so whatever the issue is it is only happening when under load when engine is under more strain. Something is obviously breaking down at that point. I know there could be numerous reasons for this but was wondering if anyone had the same problem with this engine? My local boat mechanic has already cleaned the distributer with no effect. He is going to check the compression this week to see if there is a problem with any of the valves. If it's not a valve I'm guessing it's an electrical fault.... any advise or help would be appreciated.

Thanks,
Ernie
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Old 05-28-2014, 12:18 AM   #2
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Has the mechanic tried to read codes out of the engine computer? I suspect it is a bad engine sensor.
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Old 05-28-2014, 09:16 AM   #3
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Hi Mike,

Yes he hooked it up to the diagnostic but found no faults. This was when the engine was in neutral not when it was under load. The issue is only apparent when out in water.

I found a thread online last night where a guy had exactly the same problem with this engine - same symptoms.... His mercruiser dealer / mechanic spent the best part of a year trying to find the problem and they replaced just about every part in the engine without success. Eventually it was discovered that there was some bad wiring in the house battery set up - some live wires, etc. and this was what appeared to be causing the problem.... I have recently had a house battery added and on top of that I've also had some new downlighters added to the cockpit and some extra 12V sockets....

If the engine was hooked up to the diagnostic machine under load would this highlight the problem? I'd be happy to take the mechanic out and help him do this if it meant we could get this resolved... I just don't want to end up like that other guy spending a fortune on replacing parts which don't need replaced....
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Old 05-28-2014, 11:41 AM   #4
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The computer will store any faults so any that would be flagged when under load would have been still there when he read them out.

When was the last time the boat ran good? Last year before the electrical mods? Is it easy to isolate them and see if the problem goes away although I am having a hard time understanding what it would do to create the issue?

Try removing the fuel filter and dump it's contents into a glass jar and let it settle, then look for signs of water. How old is the gas and was it treated for the winter?
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Old 05-28-2014, 12:18 PM   #5
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Mike, I bought the boat this January. It had a full service from a mercruiser mechanic in February. he didn't have the boat out but said everything was in good order. The new battery and lights, etc were all fitted several weeks ago and the boat ran fine up until last week (a few weeks after the electrical mods) However, since day one I always felt that there was a small issue between 2000 - 3000rpm but it wasn't a big issue and the it would be fine over 3000rpm and wasn't sure if it was just my imagination. I've got a gut feeling that this was always an issue which has now escalated. Again, my gut feeling is that this is an electrical issue and not a dodgy valve and I'm hoping it is because a valve issue is a major job to fix... I just don't want to end up changing loads of parts which don't need replaced.... The problem with this type of issue is it could literally be 10 different things...

I'll update you when the mechanic has had a look this week...

Thanks for the advice.

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Old 05-28-2014, 01:48 PM   #6
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The valve train does not see any more mechanical strain when the engine is under load than it does when revving in neutral. The only thing that does change is the pressure in the cylinders from the additional fuel/air when under load. It is possible that one or more valves are not sealing completely and when cylinder pressure gets high enough under load the symptom shows up. Equally possible that the ignition system is not producing a hot enough spark to light the mixture when cyl pressure is high.

The mechanic wasted his and your time cleaning the distributor. The only thing to clean would be the mechanical advance - if the MPI engine even has mechanical advance. If it does, and the MA was sticking, it wouldn't show up as a misfire, the engine simply wouldn't rev well and would definitely be down on power.

Given that the mechanic performed a "full service" (what parts, exactly, were replaced?), and under the circumstances, I would try to get down to the boat at night, open the engine cover and start the engine. You're looking for any evidence of electrical sparks (arcing) from the ignition system; distributor cap, wires, plugs. Drape a blanket over the cover and yourself to make it as dark as possible.

I've read that there is a ground connection at the very rear of the engine that, if not clean and tight, has caused people all sorts of grief. Find and check it. Have the mech check the fuel pressure.
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Old 05-28-2014, 02:16 PM   #7
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Thanks again for the advice.

The mechanic who serviced the engine was down in Southampton, England where I bought the boat from. I live up in Scotland a good 600 miles from there. He did email me a full list of everything that was done. The mechanic who is now looking at it is the resident mechanic at my marina at Loch Lomond. He's been there for over 20 years and I've always used him for my last 2 boats. His plan is to firstly check the compression to see if it's an issue with the valves - like you say possibly not sealing properly or sticking. If it's not that, and he says he hopes it's not as that involves a full day's labour to strip the engine down, then it could be several electrical issues. He has told me that he will basically have to do the old trial and error.... try something, take her out and see if it's resolved. This is my worry... we could change half a dozen parts and still not get the problem resolved...

I will try having a look at night as you've suggested. I will also ask him to check the ground connection and the fuel pressure.

Thanks for the help
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Old 05-28-2014, 03:22 PM   #8
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No valves are sticking, Ernie, for if they were they would do so without regard to engine load. The valve train on these engines is pretty robust so even a "burned" valve is unlikely IMO. Additionally, if the misfire is so bad that the engine won't rev past 2K while under load I think there would have to be quite a number of valves not sealing. Also unlikely IMO. Finally, if one or more were not sealing I would think you'd be able to hear the misfire with the engine in neutral while being revved - though it may be less audible. I don't think it's a valve problem, and I'd be very skepticle if that's your mechanics "go to" diagnosis so I would opt to be present when he did the compression check. But that's just me. Remember that a compression test must be done with all the plugs removed and the throttle wide open, and preferrably with the engine warm, to be valid.

If you can get him to run the boat on the water with his diag tool that would be a good step. I'm not sure what engine parameters he is able to monitor, and hopefully log, using that tool but at least some things could be ruled out. Fuel pressure needs to be checked at idle and at RPM. Maybe eliminate the house battery wiring and connect the main cables directly to only 1 battery while sorting this out to remove that variable. Check all ground/Earth connections - and not just a visual. Disconnect/reconnect all ignition system connectors. Verify the connections to the coil, and even measure the coil.

There's a lot to do before you start throwing parts at it.
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Old 05-28-2014, 04:17 PM   #9
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ss and I are in agreement that more investigating needs to happen prior to throwing parts at it.

I still say check the quality of the gas. The do the compression test.

BTY: did you buy the boat from a private owner or a boat store? Any warrenty?
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Old 05-28-2014, 10:23 PM   #10
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Again, thanks for the advice boys. You're both spot on, my concern is we start throwing money at it and replace parts that don't need replaced. I'd also thought about disconnecting the 2nd house battery to see if it had any effect. That's something I can easily do myself. Thanks for highlighting the issue with the valves, i.e. they would be doing this also without being under load too... would this also be the same for valves which were faulty in any other way? If not then why are we even checking that??

I'm going to nip down tomorrow and speak to him in person. As suggested I will offer to go out on the water with him and let him run his diagnostic check while under load....

I will make a note of all your suggestions and ask him to make sure we check them all before we start ordering any new parts...

fingers crossed....
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