Mercruiser 5.0 MPI Low Fuel Pressure

Trophy062350

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May 16, 2017
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Hi Everyone,

I have an 06 Trophy 2359 with a Gen 3 Mercruiser 5.0 MPI.

1st incident: I was idling around bay when engine started sounding like it was starved of fuel and soon died. Would restart and die again. I started up kicker and went in. Next day boat would not start. A month later I went down it flashed right up and ran great. I pulled it and started working on it. I changed my fuel filters. Both cool fuel filters. Disc looked pretty dirty. Changed the fuel/water seperator on transom that comes directly from tank. Both the main and kicker run off same tank. I changed all plugs, cap and rotor as well just to be up on maintenance. Took boat out for a couple 3 day trips not a hitch.

2nd Incident: Was about 3 miles from dock and at cruise speed and motor bogs down, sputters and dies. Started up kicker and went in. Main would not start back up. I connected a fuel pressure gauge to Shraeder valve on rail and less than 10psi, obviously not enough pressure. Next day I went to pull it again and the main would start and die few seconds or minutes later. Fuel pressure would be 40+ then start bouncing and dropping. I checked cool fuel filters and they were ok but no fuel where the disc sits. I am aware of the cool fuel issue with this boat just want to know if I it is for sure that as the parts are pricey.

Thankd
 

alldodge

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Normally the Gen 3 cool fuel doesn't have the canister filter, but shouldn't create undue restriction with regular maintenance. Your comment

Disc looked pretty dirty

Gives me concern that crud was getting past the canister filter

Gen 3 Cool fuel.jpg

With the pressure fluctuating then being steady I would look into a restriction first. Your tank should have a barbed fitting which the rubber fuel line connects to. This fitting is the anti-siphon valve and may be sticking or causing undue restriction. Check the fuel line to see if it is collapsing on the inside.

The next thing is to see if the fuel tank vent is clear. If it gets clogged the fuel pump will draw a vacuum on the tank. When the motor dies, remove the gas cap and listen for air being sucked in.
 

Trophy062350

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My fuel system is all plumbed in together. The hose from the pick up runs to a filter mounted on the transom. From there one outlet line goes to cool fuel and the other goes to kicker. When this happens the kicker runs perfect. Could it just be that the kicker requires less psi and that is why it works when I am having problems with the main?
 

alldodge

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The kicker requires very little fuel when compared to you motor. The kick also has (in most cases) a diaphragm pump which can suck fuel better the an electric pump. I like electric fuel pumps but they real do not suck (no pun intended) very well. This is why the fuel pump needs to mounted at or below the tank outlet fitting.
 

Trophy062350

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Hi everyone. I have finally had a chance to start tearing down the motor to troubleshoot this problem. I decided to change the cool fuel module as there is a service bulletin out on my serial number and it is a pretty common problem and could be pretty serious. I was possibly thinking the high pressure fuel pump died as I had 0psi at the fuel rail for the last month or so everytime I tried it. So I struggled to get this module off as its on a 2359 Trophy and it was impossible to get at, everything was by feel and looking at the diagrams. So its on the bench. I checked the inside, not a lot of paint coming loose in there, but there was some crud floating around in there. Maybe it was a fuel pump or the regulator, soon to find out I suppose. So I ordered the new upgraded cool fuel module, 8M0047215/16 and forgot to order a few things. I just ordered my gaskets for the riser and elbow I removed. I am thinking about removing the manifold as well to make for a faster install. So this new module still comes with plastic fittings and you are supposed to order a hose kit and it comes with this piece called a 2 barb bib. Why do you have to install this new bib ? Why couldn't I use brass barbed fittings and use the existing larger hoses? Mine is impossible to get at and it would involve tearing the motor down more. Does anyone have any experience with doing this job ? This is my first major job on the boat and would love some advice and help.
 

scoflaw

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Jun 2, 2010
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Done a couple, the plastic merc barb fittings work fine, stuck my existing hoses on with screw clamps. Done.
 

alldodge

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So this new module still comes with plastic fittings and you are supposed to order a hose kit and it comes with this piece called a 2 barb bib. Why do you have to install this new bib ?

Have not installed one yet but I'm of the opinion that if the hose can be cut off and still be able to attach, just use it

Why couldn't I use brass barbed fittings and use the existing larger hoses?

Don't see an issue with brass other then you can apply enough torque to break the pump. Plastic against plastic should seal well enough. The previous issue was the ends would break off. The new barbed fittings should not have this issue

The two barb kit is part number 8M0047216 if you do need it
 

Trophy062350

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Awesome. Well thanks for the advice guys. I think I have enough room to use the existing hoses. The lower hose was a little tight before I cut it so maybe I will have to change it out and use the hose kit. Was just looking for assurance if I could use the old ones it was okay. I wasn't sure if they were trying to reduce the water flow through the cool fuel module for some reason or if it was just the result of changing the barb style fittings.
 

Trophy062350

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Another question. Is there a source to find a service manual downloadable for a 06 5.0 MPI Bravo 3 ? Serial #0W623780 ?
 

alldodge

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Another question. Is there a source to find a service manual downloadable for a 06 5.0 MPI Bravo 3 ? Serial #0W623780 ?

Not that I'm aware of currently. Might be able to get it from Merc, the manual is 31 and part number 90-864260T00
 

scoflaw

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The cf3 model is aluminum not plastic. The previous leaking issue was an aluminum corrosion issue, at the o ring, where the barb fittings pushed into the aluminum module and held in with a bracket.
 

Trophy062350

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May 16, 2017
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The real issue with these modules was paint delamination and clogging up of the pumps and possibly fuel rail and injectors. That is what I thought was wrong with mine but it looked okay but I knew something would go wrong eventually and since I had it out already, what the hell. I will just buy the service manual from Merc.
 
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