Mercruiser 350 MAG MPI RPM fluctuations

[thbouro]

Hi everybody,
I have a 2001 Searay 310 Sundancer with twin Mercruiser 350 MAG MPI with Hurth V-Drives, with less than 60hrs on each engine since the long blocks were replaced with new ones by the previous owner, 4 years ago. Last week, being on plane at 22kt with 3800RPM stabilized, the port engine started loosing power and RPM decreased to around 2100 fluctuating (fluctuations around 1500 to 2700 RPM) and the boat went out of plane. Engine temp and oil pressure were normal. I reduced the port engine to about 2000RPM and the fluctuation disappeared but when I was trying to advance the throttle the fluctuation was reappearing. I shifted the trasmission lever to NEUTRAL and advanced the throttle and the engine seemed to respond without any problem (smooth RPM increase, no fluctuations). Nothing wrapped around the prop or shaft.
Could anybody advice me what could be the cause?
Thanks

 

[mainstream]

Sounds like you need a tune up plugs and wires merc says 3 years or 300 hours which ever comes first had to do mine at around 60 hours my boat sat for 3 years as a left over I had same trouble first starboard then port did tune up and all is good

 

[sdleo26]

If it is not your plugs/wires etc on the tune up side, it will most likely be your fuel boost pump. This is the silver pump right next to your water seperator filters. It would be running rough if it was plugs/wires. At that RPM range it will most lilkely not be ignition. . There is also a 2nd pump (fuel pump) which feeds your fuel regulator/fuel rail. I had the same issue and it was the boost pump (pulls fuel from tank). You can change/remove in 15 minutes. I would switch the boost pumps and see if the problem follows. It worked for me.

 

[ardeprint]

Typical signs of a bad cap and rotor. Open your distributor cap and look for corrosion. I would say that is your problem.

 

[thbouro]

Does it also excuses the fact that, with the shift lever to NEUTRAL (engine under no load), the engine seems to function properly?

Typical signs of a bad cap and rotor. Open your distributor cap and look for corrosion. I would say that is your problem.

 

[thbouro]

Do all those faults justify that the engine works well when there is no load (shift lever to NEUTRAL)?

If it is not your plugs/wires etc on the tune up side, it will most likely be your fuel boost pump. This is the silver pump right next to your water seperator filters. It would be running rough if it was plugs/wires. At that RPM range it will most lilkely not be ignition. . There is also a 2nd pump (fuel pump) which feeds your fuel regulator/fuel rail. I had the same issue and it was the boost pump (pulls fuel from tank). You can change/remove in 15 minutes. I would switch the boost pumps and see if the problem follows. It worked for me.

 

[mainstream]

Yes it will run smooth at idle or appear to you won't notice the misfire or loss of rpms until the boat is under load I had my mech in the ER while we brought my boat to 3500 rpms wih his scan tool and he did not catch the misfire until I got the boat to that load your engine computers keep making adjustment to the engine which can mask this condition until the boat is under load conditions at least that's what my merc mechanic said.

 

[thbouro]

Thanks a lot for the feedback...

Yes it will run smooth at idle or appear to you won't notice the misfire or loss of rpms until the boat is under load I had my mech in the ER while we brought my boat to 3500 rpms wih his scan tool and he did not catch the misfire until I got the boat to that load your engine computers keep making adjustment to the engine which can mask this condition until the boat is under load conditions at least that's what my merc mechanic said.

 

[ardeprint]

Does it also excuses the fact that, with the shift lever to NEUTRAL (engine under no load), the engine seems to function properly?

Yes, as stated by another member, a bad distributor cap and rotor will not manifest itself in neutral. These symptoms mostly are felt under load.

 

[thbouro]

Thanks for the details, do you think that the use of a Rinda Mercruiser Scan Tool will give me any fault indications or isolate the fault?

Yes, as stated by another member, a bad distributor cap and rotor will not manifest itself in neutral. These symptoms mostly are felt under load.

 

[ardeprint]

I am not sure, but I don't think it will. I believe other members can give you better info on this question since I have never used one. Changing the cap and rotor is easy and part of routine maintenance. A kit for both is under $100 and changing it won't hurt. I would start there. It that fixes the problem go boating. If it does not, then start looking at fuel delivery, as mentioned by other CSR members.

Good luck!

 

[thbouro]

Has anybody any idea if the Rinda Mercruiser Tool should assist me in troubleshooting?

 

[mainstream]

It will show you engine information but when mine had ur symptoms I had to have my mech in the ER while we put the engines under load then he could see the issues

 

[sdleo26]

Switch the boost pumps before you buy a scan tool. It costs nothing. If you have not given the motors a tune up or do not know when the last time the plugs, cap, rotor and wires were changed, you might as well do it. Total tune up costs for parts for two of these motors is ~450 USD; As a point if reference; I had 10 year old plugs, wires, cap and rotor and the motors ran flawlessly; all be it, they only had 200 hrs on clock. The contacts were corroded behind comprehension; but was not noticeable in terms of running rough or surging as you are experiencing. In my case; as soon as I put the throttle past 3500 RPM or flow scan went past 10 GPH the boat started surging, dropping down in RPM. It turned out to be the boost pump. It could not pull fuel from tank fast enough. Your problem really seems like a fuel issue. Let us know how you make out.

 

[thbouro]

Thanks for the advice, I will let you know when done (hopefully next summer, the season has ended for me, my boat already lays on the trailer...)

Switch the boost pumps before you buy a scan tool. It costs nothing. If you have not given the motors a tune up or do not know when the last time the plugs, cap, rotor and wires were changed, you might as well do it. Total tune up costs for parts for two of these motors is ~450 USD; As a point if reference; I had 10 year old plugs, wires, cap and rotor and the motors ran flawlessly; all be it, they only had 200 hrs on clock. The contacts were corroded behind comprehension; but was not noticeable in terms of running rough or surging as you are experiencing. In my case; as soon as I put the throttle past 3500 RPM or flow scan went past 10 GPH the boat started surging, dropping down in RPM. It turned out to be the boost pump. It could not pull fuel from tank fast enough. Your problem really seems like a fuel issue. Let us know how you make out.