Fuel Pressure Problem 7.4 Mercruser MPI Bluewater

[metallab]

I am having a problem with the fuel pressure rising above the 40 to 43 psi normal operating pressure

Turn Key ON : Pump comes on Rail pressure goes to about 60 psi then falls to 41 psi

Start Motor: Motor runs at idle to about 1200 rpm , when holding any rpm for a few min the pressure slowly starts rising form low 40's to 50 to 60 to 70 them max at 80 psi, The fuel presure gage needle is steady and not bouncing. At 80 psi the motor is now running very rough not holding any set rpm, the fuel pressure gage needle is bouncing all over between 60 to 80, the motor is dropping out, throttle is almost unresponsive..

Shut off fuel supply at tank: After a min or so at 2000 rpm, the pressure starts to drop slowly down to the 40's with engine performance increasing all the way there. When the throttle is put back in idle position, the motor runs perfect. When pushing the throttle, the motor is snappy and smooth, then when put at 2000 rpm, it runs great until fuel in the system is gone.

Twin Engine, similar situation on both engines, Thats what I don't get.

Has anyone else had similar issues.

 

[boatguya1]

Re: Fuel Pressure Problem 7.4 Mercruser MPI Bluewater

Bizzar to be happening on both motors at the same time. Fuel pressure regulator is a common culprit for this on one, but both at the same time?

Assuming it's a cool fuel setup I would look at the vacume line to the fuel pressure regulater and also the return line to the water seperating filter where the excess fuel pressure is bled off to.

Did someone just do something to the water seperating filters on both motors? Maybe od brand filters or possibly removed filters brackets to acess water pump impeller then put hoses back wrong?

James

 

[Anchor Management 1]

Re: Fuel Pressure Problem 7.4 Mercruser MPI Bluewater

Are these engines the ones with the cool fuel ,With a cartrage filter and the low pressure and high pressure pump all in one system?

 

[metallab]

Re: Fuel Pressure Problem 7.4 Mercruser MPI Bluewater

Are these engines the ones with the cool fuel ,With a cartrage filter and the low pressure and high pressure pump all in one system?

Yes I assume they are a cool fuel system, Looking at the engine from the front, The fuel pump and presure reg. are located on the right side of the engine below the head at the very bottom of the engine just below the engine mounting bracket. There is what looks to be a copper heat exchanger, the presure reg is bolted to this, and the fuel pump is just below, and all are held in buy a formed plate. The vacuum line goes to some piece that looks something like the presure reg. that is mounted behind the intake manifold on top of the engine.

 

[boatguya1]

Re: Fuel Pressure Problem 7.4 Mercruser MPI Bluewater

Sounds like the one with the seperate water seperating fuel filters. The pressure regulater sends the excess fuel back to the hsg on these filters to keep the fuel pressure where it should be. On many setups you have to move the hsg/bracket out of the way to get t the impellers. If someone removed the lines to get there then didn't put them back correctly it might cause this problem. Also wrong fuel water seperating filters might do it.

Did this problem occur after someone did maint to both motors?

James

 

[metallab]

Re: Fuel Pressure Problem 7.4 Mercruser MPI Bluewater

I purchased the boat 2 years ago out of Maryland, The boat sat from Jan to Aug 2007 it was put in the water for my survay in Aug. Upon starting the engines the fuel was bad from the 10% ETH Fuel in both the main tanks. The filters got filled up with gunk and gel and they were replaced with new ones after the tanks got cleaned out, It took a few days for the marina to fix the Problems,I was not present on the day of the survey, but it must have ran ok, it passed a Marine survey with a sea trial, that I paid 800 dollars from a surveyer that is also a Mercruser tech. The boat was then motored to another covered marina 8 miles away, at just above idle speed by son and myself in shallow water in the bay. I did not try to open her up, it was low tide and I was not comfortable with the area. I backed her in the new slip and she sat there until Jan 2008. Every other week I would go and check on her and start the engines in the slip until Dec 2007. She was then winterizied and when she was moved in Jan 2008 she was never started. the boat arrived in Pa in Jan 2008 and sat winterizied until last week. Thats the history.

Just so you know we are not running her off of the main tanks we installed a new 70 gal clear poly tank where her gen set was, I still dont trust the Main Tanks from the earler fuel problem. She is going to a local lake and we dont need 400 Gal of Fuel, Plus we can see our fuel level just by opening the hatch.

How can I tell what filters are the correct ones for these motors, I have no manual for these motors. Are all the filter brackets the same, should the presure always stay in the low 40's no matter what, Idle, or Full.

The New Filters are RECOR/PARKER S3213 Filter with a clear bowl at the bottom If this helps...... On the filter mount there are arrows stamped in the top. All the lines seem to be in the right places.. Fuel supply in-arrow in, no other lines on this side..... Fuel supply out to pump-arrow out.. return line from the presure reg.-arrow in.. these to lines are next to each other on filter mount.

Thanks John

 

[boatguya1]

Re: Fuel Pressure Problem 7.4 Mercruser MPI Bluewater

I have no clue what the propper Racor filter would be or if that could be the problem. I would be tempted to throw on a couple mercury 35-802893T water seperating filters just to see if it makes a difference. I wouldn't be surprised if your pressure regulators are "gummed up" and not opening up.

The presure will vary some depending on the vacume delivered to the regulator.

James

 

[metallab]

Re: Fuel Pressure Problem 7.4 Mercruser MPI Bluewater

What would be the standard presure swing on a normal motor. 5 pounds + -, just want to know. Can the regs be cleaned or would new ones have to be purchased. Is it possible the filters would have let that gel threw. Can one check the regs with a hand vacuum pump to see if there opening. Trying to understand... at idle less vacuum computer tells fuel pump not to operate , at
let say 2000 rpm more vacuum opens reg ,dumps fuel into return line.. Or do i have it backwards.. What should the vacuum be to the presure reg. if i get a vacuum gage..

Thanks Again John

 

[boatguya1]

Re: Fuel Pressure Problem 7.4 Mercruser MPI Bluewater

The manuals are available for download in the adults only thread at the top of the page. I don't know the specs off the top of my head. You can hook a mightyvac to the pressure reg and vary the pressure at the rail if it's working.
If I remember correctly the vacume change on hard accelleration allows the fuel pressure to rise some to richen things up a little.

With your description of the fuel system fun you've had reviously I'd guess that the pressure regulators are gummed up and not allowing the excess fuel to be relieved from the system. Also make sure the return hoses are not clogged. Can the regs be cleaned out? Your guess is as good as mine. The last one I ordered was for a volvo and it wasn't cheap so I'd probably try. Also be carefull how long you use the fuel shut off diagnosing this, these pumps don't like to be starved.

James

 

[metallab]

Re: Fuel Pressure Problem 7.4 Mercruser MPI Bluewater

Thanks for the pointers, I have some work to do. I will post after trying some of these tips this week. Thanks again for your help.


John