Merc 350 MAG mpi wont revv under load

[richardkc29]

Hi Guy's,
I need some help from you all about a problem ive got, Ive got a 2002 Maxum 2700SCR with a Merc 350MAG mpi, we recently had a service by Southern Marine Services and they also replaced the manifolds, risers & riser blocks. The engine is perfect but when i try to accelerate onto plane the boat will not get up and the engine will not rev past 3,000rpm, I have also cleaned the flame arrestor and checked the fuel tank vent but still have the problem. The prop is the original so i dont think it could be under propped. The fuel in the tank is old but there are no signs of that being the cause ie misfiring, lumpy running, The engine has been plugged in and shows no faults.
Any ideas?? any help you can offer will be greatly appreciated.
Cheers
Richard

 

[moonraker 36]

was it ok before the new manifolds+riser were fitted.
are they the wrong parts and resticting the engine some how?

 

[richardkc29]

was it ok before the new manifolds+riser were fitted.
are they the wrong parts and resticting the engine some how?

Hi Thanks for the reply,

Its certainly a possibility although the company i got to do it is a reputable one, I will put the question to them.

The marine engineer did come out on the boat with me and we plugged in into his diag whilst berthed and revving freely but we did not do a live diagnostics check whilst out on the water.

Cheers
Richard

 

[[2068]]

I'd start with the obvious stuff.
As in the really obvious stuff.

Think about what might have been disturbed, and check it out.

Are all the plug leads connected correctly and nice'n'tight?
Plugs seated firmly?
Distributor cap okay (although I'd expect a misfire if not).
Check fuel filters for crud.

 

[jimmy_the_builder]

I'd start with the obvious stuff.
As in the really obvious stuff.

Think about what might have been disturbed, and check it out.

Are all the plug leads connected correctly and nice'n'tight?
Plugs seated firmly?
Distributor cap okay (although I'd expect a misfire if not).
Check fuel filters for crud.

Dodgy king lead from the coil to the cap praps? A incipient failure in this lead can present as no problems when just revving the engine, but restricted power when under load.

Cheers
Jimmy

 

[[2068]]

There are also some simple things that will cause the ECU to go in to a "lo power" mode, but I'd expect that to show up on the technician's laptop plus some sort of beep.
e.g. Low outdrive oil level, knock sensors u/s, something overheating,

 

[Peanuts]

I noticed you state it is revving freely, do I take it from that you mean it revs to max rpm when not in gear?

 

[richardkc29]

I noticed you state it is revving freely, do I take it from that you mean it revs to max rpm when not in gear?

Hi Sorry yes it'll rev happily whilst not in gear, but as soon as you engage it wont rev past 3k.

Thanks to all for the ideas so far, makes sense to start with all the little things and eliminate them (even though it was fully serviced i guess i can't rule out leads etc)

I was thinking a fuel delivery problem (under load) maybe a faulty fuel pump but most comments have pointed towards electrical so worth looking into.

Keep the ideas comming

Thanks again
Richard

 

[moonraker 36]

would have it checked underload.just incase something electrical is braking down..
but if everything was ok before the work was started. probley something simple gone wrong somewere!!
i run a 5.7l mercrusier but on a carb and thunder bolt ignition pritty basic compared to yours.. hope you get it fixed soon as.

 

[zt260]

Hi Richard i had the same problem last yr with a 6.2mpi just before i sold my boat.
symptons were would rev to the limit in neutral but would only rev to 4200. under load,
i bought a fuel pressure testing gauge and found that it was running below pressure but would frre rev to max when in neutral, so we changed the fuel pump thinking this was the problem however with new pump we still had the same problem,so we looked at the fuel pressure regulator thinking this was sticking,it turned out to be the pressure regulator has a vacuum pipe running to the throttle body just under the flash eliminator this pipe was partially block causing the regulator to allow fuel to return so we werent getting full fuel pressure,i would try cleaning this before spending £ on a new fuel pump,
If you disconnect the rubber tube under the flash eliminator/throttle body (it just pulls off)
then give the boat a run under load see if you get full revs,if you do your pipe is blocked

hope the above makes sense
regds ashley

 

[richardkc29]

Hi Richard i had the same problem last yr with a 6.2mpi just before i sold my boat.
symptons were would rev to the limit in neutral but would only rev to 4200. under load,
i bought a fuel pressure testing gauge and found that it was running below pressure but would frre rev to max when in neutral, so we changed the fuel pump thinking this was the problem however with new pump we still had the same problem,so we looked at the fuel pressure regulator thinking this was sticking,it turned out to be the pressure regulator has a vacuum pipe running to the throttle body just under the flash eliminator this pipe was partially block causing the regulator to allow fuel to return so we werent getting full fuel pressure,i would try cleaning this before spending £ on a new fuel pump,
If you disconnect the rubber tube under the flash eliminator/throttle body (it just pulls off)
then give the boat a run under load see if you get full revs,if you do your pipe is blocked

hope the above makes sense
regds ashley

Cheers Ashley, that makes perfect sense, i will try that at the weekend. if ive understood that correct the fuel regulator sits on the fuel rail and will have a small diameter vacumn rubber hose going to the throttle body?

I must say im impressed with the amount of help offered and the speed that youve all left your ideas, Thank you all for that

If all else fails i do have the engineer booked for next week to turn up with a box of new parts to try until it is fixed, This time he will put it on a live diagnostic whilst on seatrial !

I WILL leave the harbour on bank holiday weekend !!!!!!!!!!!!!!!!!!