Vacuum hose on fuel pump (7.4l mpi)

[rock doc]

I have been trying to resolve a fuel starvation issue with our boat. It is powered by a 2000 7.4l mpi (L29) with the "peanut ports". I have replaced and reclamped the fuel supply line from the tank to the fuel filter/separator, which had a small leak. That seemed to fix the issue until we took an extended cruise and it sputtered after an hour on plane. Once the fuel caught up I could get back up on plane.

I have retraced the fuel lines and noticed the vacuum line was unhooked. Picture and diagram attached. Could this explain why the fuel supply does not keep up while at cruising speed? It looks like it have been unattached for a while. I test drove the boat last night after I re-attached the line (very difficult to reach) and it seems to work fine, but admittedly I may not have run it long enough. This winter I plan to pull the pickup in the tanks to clean it.

 

[Scott Danforth]

the gen VI L29 does not have peanut ports, its an oval port motor with vortec heads

the Gen V motor as found in trucks had the peanut ports


yes, having your vac line to the regulator disconnected is an issue, however that should push your fuel pressure up

so.... that begs the question. what is your fuel pressure

your pump may have issues....... or your fuel system may be clogged......... or your anti-siphon valve may be partially plugged.......or your tank vent may be plugged.....

 

[rock doc]

Tank vent is fine and to be double sure I ran without the gas cap on and it still stuttered during the return trip home.

I wanted to eliminate all the obvious stuff first and I have no tools to check the fuel pressure. Our boating season is wrapping up soon so I'm hoping I can defer this until I do the winter maintenance. This problem started when I unhooked the fuel filter mount to gain access to the raw water pump to change the impeller earlier this summer (which was in good shape - just preventative maintenance). Initially the boat would not even run until I re-torqued the hose clamps on the fuel line.

 

[alldodge]

Have you dump the contents of the fuel filter into a clear container to see if there is anything but clean gas?

Might have a weak pump or antisiphon valve getting clogged

 

[rock doc]

I have dumped the fuel in the separator and now water. Changed the filter too. I plan to check the antisiphon valve next.

 

[wingless]

The image shows corrosion stains running down the engine from the drain onto the hull. The corrosion problem should be identified and resolved.

The 7.4L MPI fuel pressure regulator design sucks, On my 7.4L MPI MAG Horizon engine that vacuum hose connects to the bottom of the intake plenum. Oil mist collecting in that plenum wicks down that hose, eventually blocking the fuel pressure regulator vacuum port.

My solution is to remove / clean / replace my intake plenum annually, including all the vacuum hoses.

The annual cleaning ensures all those systems operate properly.

The FSM includes tests for the fuel pressure regulator. One of those tests is to observe a fuel pressure gauge connected to the fuel rail while the engine is idling and the fuel pressure regulator vacuum port has vacuum manually applied from a hand pump, then no vacuum applied, to ensure the fuel pressure regulator is operating properly.

 

[scoflaw]

The 7.4L MPI fuel pressure regulator design sucks, On my 7.4L MPI MAG Horizon engine that vacuum hose connects to the bottom of the intake plenum. Oil mist collecting in that plenum wicks down that hose, eventually blocking the fuel pressure regulator vacuum port.

You must have ring issues, I've got the same plenum without an oil problem. In your case, if you don't want to fix your engine, I would think it to be a lot easier to relocate the plenum vacuum fitting than to pull it off every year.

 

[wingless]

Good suggestion. Rings have been tested and are fine.

Normal amount of blowby gets sucked into intake, as-designed by factory plumbing layout. Even w/ long, convoluted path, blowby air includes oil mist that collects within plenum interior.

Yes, it is possible to not do an annual cleaning and instead wait for problems, but after discovering this poor design during my initial ownership, then discovering / analyzing / resolving the resultant performance problems I find this maintenance to be easier and less expensive than discarding the damaged parts.

The layout of the 7.4L MPI MAG Horizon engine requires either plenum removal or exhaust removal to clean the flame arrestor. I use this PM to also clean the flame arrestor and the throttle body.

 

[scoflaw]

Interesting, maybe we don't have the same plenum, flame arrestor and throttlebody come right off, nothing in the way on mine. My TB faces stbd.

 

[rock doc]

The image shows corrosion stains running down the engine from the drain onto the hull. The corrosion problem should be identified and resolved.

The 7.4L MPI fuel pressure regulator design sucks, On my 7.4L MPI MAG Horizon engine that vacuum hose connects to the bottom of the intake plenum. Oil mist collecting in that plenum wicks down that hose, eventually blocking the fuel pressure regulator vacuum port.

My solution is to remove / clean / replace my intake plenum annually, including all the vacuum hoses.

The annual cleaning ensures all those systems operate properly.

The FSM includes tests for the fuel pressure regulator. One of those tests is to observe a fuel pressure gauge connected to the fuel rail while the engine is idling and the fuel pressure regulator vacuum port has vacuum manually applied from a hand pump, then no vacuum applied, to ensure the fuel pressure regulator is operating properly.

Good eye. I noticed the corrosion stain too - I believe they did not tighten the drain plug (blue) fully when the winterized my boat (I paid someone for a change). A small amount of was was weeping by the drain.

I'm assuming that if there was oil in that line I would have seen it when I reconnected it? That connection seems like a natural low spot.

 

[wingless]

On my engine / boat, I like to clean / remove / paint stains like those so that if future problem arise, then they are immediately visually evident as a new problem indicator.

When I perform my annual cleaning, I run water, then air through that line to ensure it is clear.

After I got the boat, before I discovered this design problem, I had a performance issue, that I eventually traced to collected oil mist travelling down this hose, blocking the oil pressure regulator vacuum diaphragm, On one engine, I was unable to restore normal operation to that part, requiring part replacement.

That problem was detected by the FSM fuel pressure test w/ the hand vacuum pump.

Yes, the vacuum hose goes downhill, from the plenum to the fuel pressure regulator. On mine, I used nylon wire ties w/ mounting screw holes zipped around various engine parts, so those mounting screw holes would be appropriately-placed for the really small nylon wire ties to secure that vacuum hose. When I remove / replace for cleaning, I just snip those small wire ties and replace so the hose doesn't fall off during usage. It really needs to be secured so gravity and vibration don't permit it to fall off. The fuel pressure vacuum port is too smooth / slippery to count on it for hose retention.

 

[wingless]

FWIW, the existence of rust (shown by the stains) is indicative of a problem requiring resolution.

What is rusting? How bad are the rusted parts? Do the rusted parts require replacement? Do the hoses have accumulation of interior rust? The raw water hoses should feel new when squeezed and should never feel of interior crud.

On my engines, they retained raw water (salt water) in the bottom of my uber expensive cast iron exhaust 24/7/365 with NO way to drain and NO way to flush until I redesigned the exhaust, over three years ago.

Prior to my redesign, I got 3 or 4 years from my OEM Mercruiser manifolds, risers and elbows, before they corroded to an extent that I was unwilling to continue usage. During my latest inspection, after three years of usage, the interior raw water passages still look brand new. My expectation is the redesign will now permit the exhaust parts to last the life of the boat.

My redesign permits draining after usage, followed by fresh water flush, soapy water flush, Salt Away flush, then draining bone dry until the next usage.

 

[rock doc]

Wingless - Sounds like you have made some great mods!

 

[alldodge]

I have dumped the fuel in the separator and now water. Changed the filter too. I plan to check the antisiphon valve next.

Sounds good

 

[wingless]

Yes, the vacuum hose goes downhill, from the plenum to the fuel pressure regulator. On mine, I used nylon wire ties w/ mounting screw holes zipped around various engine parts, so those mounting screw holes would be appropriately-placed for the really small nylon wire ties to secure that vacuum hose. When I remove / replace for cleaning, I just snip those small wire ties and replace so the hose doesn't fall off during usage. It really needs to be secured so gravity and vibration don't permit it to fall off. The fuel pressure vacuum port is too smooth / slippery to count on it for hose retention.

This image shows the fuel pressure regulator vacuum hose attached w/ a small nylon wire tie, to the mounting loop of a larger wire tie attached to an engine hose, identified w/ a red arrow.

This is one of several along the hose run from the plenum to the fuel pressure regulator.

This is the OEM configuration, prior to my exhaust redesign.

Wingless - Sounds like you have made some great mods!

Very astute observation!

Yes, I've owned my boat for 17 years and have upgraded all the systems that have been problematic or didn't fit my boat usage.

 

[alldodge]

This image shows the fuel pressure regulator vacuum hose attached w/ a small nylon wire tie, to the mounting loop of a larger wire tie attached to an engine hose, identified w/ a red arrow.

This is one of several along the hose run from the plenum to the fuel pressure regulator.

This is the OEM configuration, prior to my exhaust redesign.


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Very astute observation!

Yes, I've owned my boat for 17 years and have upgraded all the systems that have been problematic or didn't fit my boat usage.

Holly Molley, the vac hoses on MPI's are at most 1/16 ID,also run down the back side of the motor. How many adapters do you have on the line to shrink it back down on both ends?

Rings have been tested

This one also has me scratching my head some

 

[wingless]

This image shows the fuel pressure regulator vacuum hose attached w/ a small nylon wire tie, to the mounting loop of a larger wire tie attached to an engine hose, identified w/ a red arrow.

This is one of several along the hose run from the plenum to the fuel pressure regulator.

Holly Molley, the vac hoses on MPI's are at most 1/16 ID,also run down the back side of the motor. How many adapters do you have on the line to shrink it back down on both ends?

The factory uses one ID hose attached to the bottom of the plenum, then a reducer to a smaller diameter 9" before the fuel pressure regulator vacuum hose fitting.

That configuration was retained.

Rings have been tested and are fine.

Normal amount of blowby gets sucked into intake, as-designed by factory plumbing layout. Even w/ long, convoluted path, blowby air includes oil mist that collects within plenum interior.

This one also has me scratching my head some

This link shows typical engine ring testing. My engines continue to test fine.

Cylinder / Ring / Valve Testing

 

[alldodge]

Ring testing = compression test without leak down test

Oil in plenum = blow by caused by rings

 

[wingless]

According to many years of experience and every online link located, it is normal for some blowby to to exist, as-shown in this link.

One example of excessive blowby would be increased or excessive oil consumption. That does not exist on these engines, with the oil level being maintained w/o additional oil required between oil changes.

All engines have blowby. The factory designed my engines w/ a PCV valve and with an extended hose path to collect / burn blowby gasses through the intake.


Blowby = normal

Excessive blowby = not normal