350 Mag MPI - Fuel Pressure Problems

[Partycove2]

Hello,, Sorry for the long thread but I am having a problem that I hope someone can shed some light on.
My set up is twin 2008 350 Mag MPI (SN 1A066746), last year the Stbd engine died on me going at no wake idle, it restarted for a moment then died again then would not restart. Troubleshooting I found I could make it run off the left tank but not the right tank. Cleaned the right tank anti-siphon valve and all was good for the last month of the season.

Now this year, launched the boat was idling out and again same engine died. This time I replaced the anti-siphon valve but it did not solve the problem. I again could get it to run on the left tank (after allot of bleeding at the rail) but then switched back to right tank and in about 30 secs I could see the pressure dropping and engine died. I thought I had it narrowed down to the right tank again but the strange thing is now I cannot get any pressure using either tank. Today I am going to run the port engine off the right tank just to make sure it does not die.

So here is what the pressures read initially with the gauge hooked up on the fuel rail. (now I can't get any pressure from either tank)
- key on (not started) is intermittent, sometimes 0-10 and sometimes it comes right up to approx 40
- when the key on pressure is 40, it will start and the pressure drops down to around 35, revving it goes back up to around 40.
- I checked this against the other engine and it seems the same.

So now that I seem to be getting no fuel to that engine from either tank, I suspect a fuel pump problem which I where I need help.
I assume the low pressure pulls fuel from the tank and then the high pressure sends it to the ejectors
- When does the low press pump run and when does the high pressure run?
- How to I determine if it is the low not pulling fuel in or high not sending it to the rail?
- Where is the fuel pump relay located
- Is there a fuse for the pumps?
- This has the older gen 3 cool fuel assy, is it possible to replace only the pumps in this or does the whole assy have to be replaced?

Today I plan on pulling the filter out and check it and the disc filter in the bottom but looking for other ideas
I pulled the top regulator last year to check for paint and it was clean, did not check the regulator on the cool fuel assy

Thanks, Jeff

 

[alldodge]

Howdy

I don't think its a tank problem, more then likely a pump issue, and that is assuming you have changed the fuel filter?
http://www.mercruiserparts.com/bam/subassembly/31790/15322/60

It is possible to replace each pump, and prices below are just from one vendor
Complete pump - 8M0047215 - 1108.38

Or parts
Low pressure pump - 866170A01 - 247.84
High pressure pump - 866169T01 - 315.45
Filter - 35-8M0093688 - 55.57
Seal - 26-892683 - 12.39

That said: using the above part numbers, you can find after market ones and possible Airtex pumps which make them for Merc

The pump relay sits on top the motor next to the ECM and there is also fuses next to them with rubber covers

 

[Bt Doctur]

the cool fuel mounted on the lower right side has only the hi pressure pump, the cool fuel III has 2 pumps mounter low left side

 

[Partycove2]

so spent the better part of the day troubleshooting this, and long story short it is without a doubt a delivery problem from the right tank. I made the left engine run off the right tank and sure enough it quit too. After bleeding the rail on both engines they will both run off the left tank but neither will run off the right.

So it seems to be from the tank to the shutoff valve (manifold) where you can change valve positions for what ever tank you want.
Strange thing is I can disconnect the line from the shut off valve going to the tank and use a manual pump and pull fuel from that tank.

A little baffled right now, any idea let me know.
Also I'm quite certain that I have cool fuel 3 but it is mounted on the lower front right (beneath the alternator)

 

[alldodge]

Your correct and according to your serial number it confirms Gen 3 cool fuel (both pumps in same housing).

Still no sure its just the right (starboard) tank, but will follow your lead. {one time neither would run off it} So the only thing between the motors and the tank is the fittings, anti-siphon valve, and tank dip tube. If you cannot remove the dip tube, blow air through it

 

[Partycove2]

Yes the reason I said originally that I could not get the right engine to run off the left tank anymore is because the damn bleeder on my pressure gauge was not working, so I;m pushing it thinking it is bleeding the system and nothing is coming out......once I fixed the tool then I was able to bleed successfully. And yes I am certain it is a right tank delivery problem now

Thanks for the input