Losing RPM on STB 350MPI Rinker 342

onislandtime

At cruise speed STB engine RPM fluctuates. When at idle or low speed its fine. After stopped for couple hours and restart, gets on plane runs for 10 minutes and same symptoms occur. 

Willhound

Is the engine actually revving up and down or is it a tach issue?
If actually the engine is maybe suspect a fuel issue. Filter maybe.

Liberty44140

Yep agree I’d so filters first, if you have cool fuel then you have two fuel filters per engine. Make sure to do both. 

onislandtime

Yes I believe its a fuel issue. The tanks are on 1/4 and the issue is becoming worse as the fuel gets lower. Going to change water/fuel separators and fill the tanks with premium gas and add stabilizer. Cheapest way to trouble shot right now.

Willhound

Change the separators and the filters if you have them both. With age the filter material inside the filter cartridge gets soft and as fuel is sucked through they "collapse" which causes a restriction. In my opinion fuel filters should be a once a year thing. Under $20 and not hard to change.

onislandtime

@Willhound where are the filters located?

GMSLITHO

You can add some dry gas it helps if it’s water in the tank 

Willhound

Go here and either enter your engine serial number or look up your model. Someone with more experience can correct me, but I think some just use the fuel/water filter and some might also have a seperate inline fuel filter.
http://www.mercruiserparts.com/

Edit: Don't know if this is your motor or not, but in this page you can clearly see the additional fuel filter:
http://www.mercruiserparts.com/bam/subassembly/31754/15322/60

onislandtime

I believe their is one on the bulkhead in the engine compartment. I just did'nt know if there were more.  

pistole

There are the fuel water separators come filters usually mounted on the "firewall" in front of engines. On the engines themselves would be fuel filters that look exactly like oil filters  near the fuel pump

Liberty44140

Make sure to look up your engine serial number as Will commented. By 05 they stopped mounting the fuel filters to the firewall and stopped the inline and went to a coolfuel module which is mounted to the engine. It acts as a fuel filter and fuel water separator and it has two filters in it per engine. Not sure if this was the case in your 2004. I use marineengine.com to look at my engines diagrams 

onislandtime

Thanks @Liberty44140 I will check that out

Spyderweb

Liberty, 
are there 2 filters inside each module?

reneechris14

21 and 26 are the filters  those are OEM numbers you can look up.

Liberty44140

Yep! As reneechris shows in the diagram parts # 26 and 21 are two separate filters. #21 is often overlooked and can absolutely cause your problem. It is at the very bottom under the large filter and may be so black you don’t notice it. Need to do both at the same time.

onislandtime

UPDATE: Changed water/fuel separator filled up with Premium and loaded up with fuel stabilizer. Ran it for and an hour and no problems. Thanks for all the suggestions and help!!!

Wildboating

Do you want to hear a story about losing rpm on the starboard motor??😂😂😂