Mercruiser V6 4.3ltr - Cutting Out at High Speed

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Paul1962

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Hi,
I Would love some advice please.

My Mercruiser V6 4.3LX (2 barrel MerCarb) is fine all day at the lower RPMs ie 3000 ish or lower - no problems at all.

Ask it for full noise - circa 4200 in my case and it will go for about 2 to 3 minutes and then it starts missing or just cuts straight out.

Sometimes it will restart immediately, sometimes it will keep going if I reduce rpm to around 3000 and sometimes it will cut straight out and refuse to go again for about 20 minutes.

I have changed fuel line, changed the fuel filter (it is a genuine Quicksilver), checked the carb inline filter, changed and proved the fuel tank vent and finally today fitted a replacement ignition coil (yes, the +ve & -ve are on the correct terminals and I checked it out with my multimeter first).

The only thing that I think may be linked is that the Tacho sometimes works and sometimes doesn't - no correlation to any of the engine failures though.

I am now stumped - anybody out there come across this before or know what it may be??

Any advice gratefully received.

Thanks in Advance



Paul
 
Obviously, difficult to say without being there and being able to investigate but the symptoms sound like an electrical component / connection issue associated with heat. As the heat increases slight expansion causes an open circuit. (eg. Crankshaft position sensors fail in this way giving the symptoms you describe). If you are able and can get wiring diagrams for your engine then you could trace the engine circuits otherwise you have to get a man in. Let us know how you get on.
 
have you checked the fuel pump?
and as trying say's crankshaft sensors give these kind of symptoms.
hope you get it sorted out!
 
Hi Chaps,
Thanks for the swift replies - I am thrown gently by "Crankshaft Sensor". I do not see one of those on my wiring diagram.
I do see an ignition module inside the distributor - is that the same thing? If so, could that be the cause?
Regarding the fuel pump, do you know how I can check it?

Thanks again

Paul
 
sorry without checking specs / diagrams i do not know specifics on your engine.

some engines use a crankshaft sensor as a trigger for ignition / fuel injection systems.

Sounds like your engine will have a similar sensor in the distributor which could be a cause like i say you'd have to test in that window after the engine has cut out to see whats happening.

As for fuel pump, you can test pressure and delivery rate and check against specs.
 
How old is the fuel tank. It could be debris in the tank blocking the pick up pipe in the tank.
When the engine stops the rubbish falls back into the tank, enabling the engine to restart.

Try using a temporary fuel supply from a 20ltr can or similar to prove that the tank is not at fault.
 
shetland said:
How old is the fuel tank. It could be debris in the tank blocking the pick up pipe in the tank.
When the engine stops the rubbish falls back into the tank, enabling the engine to restart.

Try using a temporary fuel supply from a 20ltr can or similar to prove that the tank is not at fault.
Click to expand...

I agree with the above, I'd be looking at fuel supply issues first.
 
The fact that the tacho works intermittently is a big clue, this sounds like an ignition fault. There is no crank sensor on a carb engine, the tacho gets its signal from the neg side of the coil (grey wire), I would first disconnect the tacho to make sure that isn't causing a problem, if it contunues then its either the induction coil under the rotor in the distributor, or the Hall effect module (ignition module). There is a test sequence for these, but you need an multimeter and a few tools as you have to dismantle parts of the distributor.
Let me know if you need more info.
 
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why not try and see if you can get a home tune guy out that dose's car's and such.
who can test the ignition side out for you.(i'm guessing you keep the boat at home and can run it off a water supply).your engine is basicly a car engine.
it's just guess work other wise.
spannerman advise is good advise. usually if ignition module faulty engine wont start at all usually.
surging usually fuel prob,missing usually ignition.
 
Hi Chaps,
Thanks for the advice - changed out the Ignition module inside the distributor for another secondhand one I had in stock, fitted a new rotor arm and distributor cap. Finally took the beastie out last night and the fault persists, cruises happy as Larry under 3000 RPMs but ask for more and it goes for a bit then coughs and will die unless immediately throttled back. I forgot to disconnect the Tacho as per Spannermans advice but will try that next time.
Thanks again for the advice.

Regards

Paul
 
You've presumably got an Alpha 1 outdrive, so its worth checking/replacing the shift interruptor switch. This is the cheapo switch that cuts the ignition when taking the boat out of gear under load - if it is faulty (and they regularly fail) it will cause ignition problems.
 
Hi Chaps,
Tha saga continues - tacho disconnected as per advice and unfortunately still decides to "die" when asked to perform.
To summarise - ignition coil changed, ignition leads changed, electronic module in distributor changed, distributor cap renewed, rotor arm renewed.
In view of advice regarding the shift interrupt switch - Does anyone know if it can be bypassed just to test it?

Thanks

Paul
 
I had something similar on a previous boat
turned out the water separator in the fuel line (on the engine before the fuel pump) was full with water

Low revs were fine but as soon as the engine wanted more fuel (high revs) it would struggle and die
restart is OK as it would run on little fuel
 
Paul1962 said:
Hi Chaps,
Tha saga continues - tacho disconnected as per advice and unfortunately still decides to "die" when asked to perform.
To summarise - ignition coil changed, ignition leads changed, electronic module in distributor changed, distributor cap renewed, rotor arm renewed.
In view of advice regarding the shift interrupt switch - Does anyone know if it can be bypassed just to test it?

Thanks

Paul
Click to expand...

You can just disconnect the green/white wire going from the switch to the ign module, there is a bullet connector in the wire. Although I don't think this will help as the switch only works when coming out of geat and will stop the engine completely if there is a fault on it.
 
Thank You all for your continued advice.
Will change fuel filter - it is not that old - honest, disconnect shift wire once going and will change the electric fuel pump for a secondhand one I have just got hold of.
Thanks again - will keep you updated.

Best Regards

Paul
 
4.3 Mercruiser cutting out at high speed

I had this problem twice with 4.3s on 2 different boats! It eventually turned out to be the float setting in the float chamber. It seemed that the constant vibration had caused the float arm to gradually bend until it couldn't let enough fuel through. At high speeds you are using lots of fuel and this is where it first showed up.
 
Good Morning,
Happy to report that all is now well and we have full RPM - reliably!
Fuel filter removed and inspected - nothing untoward. Picking up on earlier advice that it may be fuel starvation, decided to remove the tank sender and have a look inside the tank - in particular the fuel pick up tube.
A dirty great bit of old rubber gasket was hanging out of the bottom of the tube and the bottom of the tank was littered with grit, snot and mundungus.
The old rubber gasket limiting fuel flow hence the high speed cut out etc.
A full defuel, a tank rag out and the feed tube removed and cleaned and now all is well.
What I did not appreciate was that the feed tube has its own mesh filter, whilst this is a good idea, it stops the spin on filter from doing its job - ie filtering.
Thank you very much to all who took time to reply with advice and their own experiences.
Happy boating to all and thanks again.

My very best Regards and Thanks


Paul
 
When it cuts out and won't start have you checked if there is a spark, if there is then its pointing to a fuel problem, is there fuel be delivered in the secondary barrel, you can see fuel spitting out of the jet on the primary or it wouldn't run, but when the second throttle plate opens is there fuel spraying in that barrel, if not its going to fade out as its just delivering more air for the same amount of fuel, you will have to run it without the flame arrestor so its a bit noisy.
 
Paul1962 said:
Good Morning,
Happy to report that all is now well and we have full RPM - reliably!
Fuel filter removed and inspected - nothing untoward. Picking up on earlier advice that it may be fuel starvation, decided to remove the tank sender and have a look inside the tank - in particular the fuel pick up tube.
A dirty great bit of old rubber gasket was hanging out of the bottom of the tube and the bottom of the tank was littered with grit, snot and mundungus.
The old rubber gasket limiting fuel flow hence the high speed cut out etc.
A full defuel, a tank rag out and the feed tube removed and cleaned and now all is well.
What I did not appreciate was that the feed tube has its own mesh filter, whilst this is a good idea, it stops the spin on filter from doing its job - ie filtering.
Thank you very much to all who took time to reply with advice and their own experiences.
Happy boating to all and thanks again.

My very best Regards and Thanks


Paul
Click to expand...

glad to hear you now have the problem sorted!!!!

like before surging usally fuel... missing electrical..

hope you can now enjoy the rest of the season..
 
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