350 Mag fuel pressure

[Jpettinat]

Having an issue with fuel pressure. Its not consistent but pressure goes from 39psi to zero. Then i turn the key off and on and get pressure back. Ive changed the filters and both pumps. Boat will run fine for 15 min and then i notice the pressure gauge moving slightly and then it drops off. It happens at all rpms and Ive tried adding ground wires to the motor to see if that effects it but no change.
Going to check the tank vent next and fuel lines. The pump relay may be causing this not sure. Any thoughts?

 

[alldodge]

First thought is regulator but what's your serial number?

 

[Jpettinat]

1A628103 its a 2010

 

[Jpettinat]

What regulator?

 

[Scott Danforth]

Fuel pressure regulator

 

[alldodge]

1A628103 its a 2010

Thanks, you mentioned replacing both pumps so you should have a bravo drive
Here is your motor

https://www.mercruiserparts.com/350-...vo-ec-1a623185

The fuel pressure reg is item 12

https://www.mercruiserparts.com/bam/...31758/15323/90

Take it apart (note clip item 14) and see if something is clogging it up. If not probably need to replace it

 

[Jpettinat]

I removed the regulator, didn't see any debris . So I ordered a new one. Sound be in next week, i’ll let you know if it solves this issue.

 

[Jpettinat]

Im told a malfunctioning regulator would not lower pressure to zero. What are your thoughts?

 

[alldodge]

If pump is running and it drops to zero, its either the reg or its spraying gas somewhere

Is the pump running?

 

[alldodge]

Put a voltmeter on the lead feeding the fuel pump, watch to see if it goes to zero

 

[Jpettinat]

Ill install the regulator and if issue persists, ill try wiring in a volt meter and watch it while in operation.

 

[Randy Dive]

Hey guys, Im having the same issue but the pressure drops to around 20 psi. Regulator is new, and the mechanic said its the fuel pump. Ideas? Sorry if I highjacked the thread. Thx...

 

[alldodge]

Hey guys, Im having the same issue but the pressure drops to around 20 psi. Regulator is new, and the mechanic said its the fuel pump. Ideas? Sorry if I highjacked the thread. Thx...

Start another thread, tx

 

[Jpettinat]

Hello Randy,
I haven’t taken mine out since replacing the regulator some im not sure if my issue is resolved. Since i just changed my fuel pumps, I may be able to help with your issue. There is two fuel pumps a high pressure and a low pressure pump side by side. Since your issue is intermittent like mine its hard to resolve by testing the function of either pump. If it isn’t working correctly at start up then you can try using a primer bulb on the inlet to see if you get pressure. Other wise you probable have to just change the high pressure pump and probably the low as well while your in there. I found it eadies to remove the serpentine belt and unbold the altenator leaving wires attached off on the side. Then you can get to the top of the pump housing fairly easy. If it hasn’t been apart for awhile you may break a bolt, if so then you want to remove the complete housing and remove the bolt and clean up all the threads. I bought sierra pumps on Amazon or ebay. Good luck!

 

[Jpettinat]

I tried switching relays no change, tried opening up fuel cap to see if it was a vacuum issue no change. Guess ill try checking fuel line from tank and pump voltage next

 

[Jpettinat]

Plan is to disconnect fuel line at pump and see if i can siphon fuel out of the tank freely. Then next Ill try voltage gauge on pump and pressure gauge on rail and see what happens.

 

[Jpettinat]

Doesnt appear to fail at idle by the way.

 

[alldodge]

Doesnt appear to fail at idle by the way.

Meaning it will continue to run without problems at idle?

It only happens under load at higher rpms?

You should not be able to siphon fuel from the tank, it should have an anti-siphon valve connected. If valve gets stuck or clogged this will cause a restricted fuel flow

 

[Jpettinat]

I just dismantled the check valve, seems to be clean and functioning properly. I did blow air back into tank and that seems fine. I hooked up a primer and pulled about s as gallon out of tank into a container. Going to hook up the rail pressure gauge again and rig up a volt meter as well. Ill also bring an independent fuel tank with me when i go out just to eliminate the tank once and for all.

 

[Jpettinat]

Yes only fails over 1000rpms, when it dies usually have to try and start it about three or four times before it will stay running.