7.4 MPI High Fuel pressure

[marksbudboat]

I have a 2000 Caravelle Interceptor with a Merc 7.4 MPI, cool fuel system.
In the last 3 years, I may be about to replace my 3rd fuel pressure regulator.
When opening the boat for the season, the boat will idle rough.
Standard tune up components verified and don't seem to be an issue.
When I test the fuel pressure, key on engine off, 38psi.
Start the motor, shoots up to near 80psi, the gauge bounces wildly, the engine idles rough.
If I feather the pressure relief on my pressure gauge, down to 38psi, engine smooth as a champ.
Changing the water separator filter changes nothing (always do the basics, I've tried a lot of these, and always prime the filter
before installing it)...
Unplugging the vacuum line to the regulator changes nothing, but pinching the line closed kills the motor.
(this sort of tells me the regulator is working??)
Connecting a clear tube to the gauge relief valve, I get some air coming out initially starting up, but none once running.
Engine off, the pressure holds 38psi indefinitely.
Put a remote camera under the engine, I don't see any problem with the braided supply line to the regulator.
The last 2 times this happened, the regulator was replaced, and no problems until re-opening the following season.
Wouldn't have a problem throwing a new regulator on, but the motor has to be lifted to replace it, which I can't do at home.
(If only Caravelle gave me 6 inches more clearance per side......)
Throwing $700.00 at it every season is getting old, and the tech at the marina can't see why it would fail repeatedly.
I leave roughly 12 gals (1/4 tank) fuel in over the winter, with SeaFoam stabilizer, and add 5 gallons fresh when opening.
Looks like the boat might be driveway dressing this year.....
Any direction would be GREATLY appreciated!!!!!!!

 

[alldodge]

:welcome: to iboats

Going to do some digging but it would help to know your engine serial number?

 

[Swill008]

Off the top of my head, I had a restricted return line from the fuel pressure regulator. Check hoses coming from the regulator to the fuel filter housing assembly. My restriction was the actual little screen filter between the regulator housing and cool fuel cooler housing. Maybe you have something that is continuing to contaminate that little screen filter or kinked / restricted return line.

 

[alldodge]

The serial number would help, but do you have a fuel damper on the fuel rail?
When you pinch off the vacuum line where are you doing this, right at the lower regulator or up next to the plenum?

Manual indicates
Turn key on, no start, fuel pressure should be 34 to 38 PSI
Start engine, fuel pressure should be 3 to 10 PSI
If 10 inHg vacuum is applied to regulator pressure should be 3 to 10 PSI
If applying vacuum does not change pressure, replace regulator

If you have a restriction in the return line you could be exceeding 60 PSI and this will damage the regulator

Edit: looks like our messages crossed paths

 

[marksbudboat]

Thanks for the replies!!!
I don't have the serial number handy, but will post in a few hours.
Not certain the manual you're seeing, but my Merc manual for this motor states 38psi at all times.
This engine won't run on 3-10 psi.
I am pinching the vacuum supply to the cool fuel regulator below, which kills the engine.
What I have not done as yet, is perform vacuum readings. Only visual cue checks.
I am unaware of a fuel damper on the fuel rail - but can take a closer look.
Have been told there is another regulator below the intake plenum, but am uncertain at this point.
If I have a restriction in the screen in the return line, I'm pulling the motor to get to it, unfortunately, unless
someone has an idea how to clear it without removing it.
Which brings me back to having the motor pulled......
Thank you all, again.
Will post serial number shortly.

 

[alldodge]

Manual is No 23 which cover 1998 to 2001 454 and 502 engines. Starting on page 5E-68 it goes into diagnostics of the fuel system.

The fuel regulator on the fuel rail (if it has one) it is actually a fuel damper. This is one of the reasons we need the serial number to determine if your engine is a MEFI 1, 2 or 3. In you previous post you indicated the pressure was bouncing, hence my thought of the extra damper.

If there is a restriction in the line, only way I know of clearing is with removal. Sure wish Merc put those things in a better place. Did one for a buddies twin inboards and we supported the engine with a truck screw jack, then removed the motor mount to gain access. Safer with a screw jack due to no hydraulics to leak.

 

[Fun Times]

Sounds like you may have debris inside your fuel tank that's affecting your fuel cool system. May want to inspect the fuel tank.

The most common item to cause high fuel pressure is the little filter screen under the regulator. Next time your in there, just remove the filter screen all together as that has become a common practice over the years by boat owners with no ill effect. It's item #16, http://www.mercruiserparts.com/Show_...ND+FUEL+COOLER

The fuel damper on top of the fuel rail is not designed to have an effect on fuel pressure. There is no regulator under the intake plenum.

Just a quick thought at trying something different since you can't gain access to the pump area. Maybe you could try removing hose number 26 on top and put some pressure into it to see if you can knock any debris out of the filter area. Might be better then letting the boat sit another year.:noidea:

Good luck with getting her going once again.

 

[marksbudboat]

Thank you all! The engine serial number is L324964. I've got a hunch that screen is the culprit, or even something in the tank. Sorry my manual is a Clymer for Mercruiser, not a manufacturers manual - but it's given me good support to date. The removal of the mount idea, using a screw jack, is a valid one, and I have one, but again, Caravelle left me no room to maneuver - can't even remove the cover plate for the cool fuel system. Knowing it just pops off to remove makes it that much more frustrating. Which hose is #26? The vacuum hose to the regulator? That would be simple enough. Removing the regulator, screen and hose will require pulling the motor. Removing the fuel tank will require pulling the carpet (new - non snap in, arg!) and the flooring. Again, thank you all for the support. This is a great forum, and I intend to help others where I can. Guess it's gonna be a dry summer.... first one in 15 years. Haven't given up quite yet....

 

[alldodge]

Your serial number says you have a MEFI-3 and your fuel pressure should be 41 to 45 PSI. If you click on the link in Fun Times post it will show item 26 is the fuel line going to the fuel rail. Removing the fuel line from the cooler or the rail is done by the book and rotating a cam type lock.

Here is a link to download manual No 23
http://www.blainehanson.com/Manuals/...02-Engines.pdf

Edit: just thought of something. You have a MEFI-3 system and your fuel pump should be putting out 43PSI, have you been buying part number 861126A 1

 

[marksbudboat]

Thanks for the manual links, AllDodge & FunTimes! I wish I could tell you the part number the tech used was the one listed, but my previous receipts don't show a part number. The best I can say is I know he used a Merc part..... I will try the 26 fuel line and let you know what happens.