Engine misfire Over 2000rpm

[Ernie]

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

[mmwjr]

Has the mechanic tried to read codes out of the engine computer? I suspect it is a bad engine sensor.

[Ernie]

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....

[mmwjr]

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?

[Ernie]

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.

Cheers

[ss3964spd]

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.

[Ernie]

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

[ss3964spd]

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.

[mmwjr]

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?

[Ernie]

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....

[mmwjr]

valves either seal when seated or not. Reasons can be worn valve, valve seat in the head, weak valve spring, or colapsed lifters. The last 2 may only show up at higher rpms but are very low probability for engines of this type of use. They would also result in clattering noise.

I had an engine that had a bad exhaust valve seat that result in 0 PSI for that cylinder, other than loss of total power the engine reved fine under no load or full load.

Bad gas may not be noticed on an engine with no load but under load yes.

[Ernie]

A friend of mine who has a boat next to me in the marina and who has a much better understanding of engines and electrics is going to have a look over the basics on Saturday. The mechanic still hasn't had a chance to look at it and I'm getting impatient waiting for him.

We'll check all the items you have both suggested - earth, house battery set up, etc. Hopefully we'll have a bit of luck...

If there's anything else you both think we should check and which can be done fairly easily, please let me know..

Thanks again

[ss3964spd]

The only thing I can think to add, Ernie, is to describe the symptom(s) as accurately as possible. For instance; when you start the engine and get it to full operating temp, then idle out of the slip and marina, it runs just fine at its 600 PRM-ish idle speed. As soon as you are clear of the no wake zone you hit the throttle hard. Does it run fine from 600 to 2k under hard throttle? How about if you just ease into the throttle a little bit at a time instead? Does the symptom occur each and every time, under any condition? How about if you run the engine up to an RPM just below where the symptom occurs, and then operate a high-load electrical component, like trim tabs or trimming the drive up a bit? Is it always at 2000 RPM or does it vary; sometimes 2K but sometimes 3K? When the symptom occurs what is the temp gauge reading? Do you notice any dark-ish smoke, or a heavy smell, from the exhaust? That sort of detail.

Good luck with it, let us know what you find.

[Ernie]

HI ss, the engine starts fine, runs up to normal temp after idle for a few minutes. In neutral it will rev right up to and past 4000rpm with no issues at all. When I take her out she will rev up to 2000rpm with no issues. I can run her at under 2000rpm and she runs great. I've tried bringing her slowly (and hard) over 2000rpm but as soon as she gets over that the engine starts to misfire, power comes and goes. there is no smoke or smell from the exhaust, the temp remains normal. there is however, a very loud vibration from the engine. As soon as I drop her under 2000rpm the loud noise and vibration stop. The best way to describe the vibration is to compare it to running with a damaged prop... the whole boat vibrates but especially at the back. One thing I will add is the very first time that it happened, I reduced the rpm and brought her to idle. I then revved her up in neutral and the same thing started to happen but in a higher rev range - approx. 3000-4000rpm. Since then, however, she will rev way up there in neutral, even after idling for a long time, without the issue happening. I took her out for a run last weekend for about an hour. I kept her at about 1500rpm the whole time. (the mechanic told me I could do this without causing further damage) She ran great but again anytime I tried going over the 2000rpm the same issue appeared. Even after an hour she would still rev way up there in neutral with no problem. Whatever it is, it seems that something is failing over 2000rpm but only under load...

[CountryME]

Ernie, when was the last time the fuel filters were replaced? This sounds like a fuel problem to me. I would question a slug of water in the fuel tank that travels back to teh pick-up under heavy load or a clogged fuel filter that starts to starve the engine as the rpms increase under load. Good luck.

-JP

[ss3964spd]

Very intertesting, Ernie.

On two separate occasions if have experienced a very odd symptom. Twice now, after an extended period of idling at no wake speed with the drive all the way down I went to throttle up and experienced vibration and lack of power - though I can't say it was missing. I can't say at what RPM it occured because I backed off immediately. The first time it happened I tried it a couple of times with the same results. I then raised the drive to check the props for damage and/or lines wrapped around them. Found nothing. Dropped the drive and tried again. Same. We idled along for another 20 minutes. For what ever reason, we stopped, put the drive in reverse for a moment, then back in forward. The symptom disappeared.

Same scenario the 2nd time, but months later. Had been idling along for a long time. Came to the end of the no wake zone, throttled up, and there it was again, the very same symptom. Backed out of it, into reverse for a moment, then back into forward and off we went. Very strange. I have no idea what it is.

[leeADAB]

Hi Ernie. Does your engine have ignition coils? A failing coil would show up under load and cause misfire and engine running rough. Just a thought.
Keep us informed.
Cheers, Lee

[Ernie]

OK gents so I was at the boat this weekend. The mechanic hadn't had time last week to look at it but he promised me he would defo get to it this week.
I took her out again on Sat for a trip up the loch. I kept her under 2000rpm and as usual she ran fine. I tried taking her up onto the plane but the same problem happened over 2000rpm again. However, on the way home I tried one more time and she ran absolutely fine! No misfire at all!
Now I may go back and she's got the problem again but I just think its strange... The only thing that I did on Sat which may have affected it was I gave the engine a bit of a hose down. She used to be kept in salt water and I was advised that it is always a good idea to run the engine up to temp then hose the engine down to get ride of any salt which is obviously corrosive. I gave the transom a good wash as it has quote a lot a gunk on it. Could this possibly be the reason why she ran OK? bear in mind though that the problem was still there after I did this and it was only on the way home that she ran ok...

Any thoughts?

[ss3964spd]

Explain "gave the engine a bit of a hose down", Ernie. Does mean you hooked up muffs to the outdrive, attached a hose, and flushed the engine? Or does that mean you simply hosed off the outdrive to remove some of the salt and deposits? Or does that mean you opened the engine compartment and hosed down the actual engine - top to bottom?

[Ernie]

Sorry, I gave the engine a blast with the jet wash, I made sure to stay away from the electrical components. I hosed down the actual engine not the leg, inside the engine compartment. It was running and warmed up while I did it...