Mercruiser Mag MPI Fuel Pressure Problem

Engine ? 300 Hp Mercruiser Mag MPI 350 Cubic Inch
Electronic Ignition / Fuel Injected
Serial Number ? 0L684218
Year ? 2001 (I believe)
Hours ? 150 Since totally rebuilt in January 2007

Prior to having the engine removed to replace the oil pan because it was rusted and leaking oil the boat performed as follows;

RPM at WOT ? 4300 to 4600 RPM
32 MPH
Fuel Pressure ? Perfect as required by Mercruiser I believe 43 PSI

Upon removing the engine from the boat I decided it would be advantages to go ahead and replace the spark plugs, wires, starter, rotor cap, rotor and gimbal bearing on the Alpha One outdrive.

Upon taking possession of the boat back from the mechanic the boat performed as follows;

RPM at WOT ? 3500
15 MPH
Fuel Pressure ? Does not reach Mercruiser requirement I believe it?s at 25 PSI

The fuel pressure at this point does not register what the Mercruiser manual indicates which is around 43 PSI and the engine is starved for fuel. At this point the engine was removed from the boat again and a new Mercruiser fuel pump and fuel regulator was installed and the fuel PSI still does not register what is indicated in the Mercruiser manual. It is registering as follows; idle 15PSI and at load 25PSI.

I decided to take the engine to a Mercruiser mechanic is Miami and he has yet to figure out what the problem could be. Last week the injectors were inspected to make sure that one or more of them were not staying open thus not allowing the fuel pressure to build up. I paid $203.00 for testing and cleaning the injectors only to be told that the injectors are fine and it?s not what is causing the problem.

Everyone is totally baffled and don?t know what else to do to resolve this issue.

Again, before the engine was removed the boat was running perfect.

Hope you can help!

Thanks,

Max Barnes ? (305) 788-8572 Cellular

Re: Mercruiser Mag MPI Fuel Pressure Problem

Injectors won't make the fuel pump pressure low, but you could have a problem with the pressure regulator. Perhaps the vacuum hose got pulled off the regulator while it was being removed or installed. You could also have a kinked fuel line, or an air leak on the fuel intake side. Since it worked ok before, I doubt the pump just suddenly went bad, while possible, I would still look for other problems before going for a new pump.

Re: Mercruiser Mag MPI Fuel Pressure Problem

Just a Guess, was the engine out of the boat for a while? What is the fuel flow from the tank like? is there a strainer at the tank /in the tank? fuel filter condition?/
These are the first things that I would look at before I start replacing parts.
I know that in the US you folk seem to get a lot of crud in the fuel which we seem not to here..

Re: Mercruiser Mag MPI Fuel Pressure Problem

Hello Gentlemen,

Thanks for your quick replies.

Still after changing the fuel pump, fuel regulator and fuel hoses the problem continues. I tend to agree that it appears that there is an air vacuum somewhere affecting the fuel system that hasn't been identified. I do recall that the engine was placed upside down on a table to replace the oil pan and I suspect that this may have caused something to break or come loose.

This situation is totally baffling even to the certified Mercruiser mechanic as well as the Mercruiser technician who are called for assistance.

Re: Mercruiser Mag MPI Fuel Pressure Problem

Run the tests in this thread and see if it locates the problem.

http://forums.iboats.com/showthread.php?t=283269

Re: Mercruiser Mag MPI Fuel Pressure Problem

Hi Don,

Thanks for the information for testing for air and vaccum in the fuel system; I printed it and faxed it to the Mercruiser mechanic in Miami.

Unfortunately, the boat is in Ft. Lauderdale and the engine is in Miami so load testing is not possible at this time.

Again, many thanks and if you think of other things I can try, please share them with me. This situation has been going on now for 3 months.

All the best,

Max

Re: Mercruiser Mag MPI Fuel Pressure Problem

I would sure try the vacuum tests and clear hose test, out of the water too. You never know what you might find.
If nothing else, hook a hose up from an outboard tank to the engine, just to see if the pressure from the pump comes up. It also quickly eliminates the boats fuel system. If the pressure comes up, the problem is in the boats fuel system, if it still stays low, then it's in the engines fuel system somewhere.
Would sure narrow down the search for the cause.

Re: Mercruiser Mag MPI Fuel Pressure Problem

Hi Don,

Prior to removing the engine from the boat the second time I performed a remote fuel tank test using a 5 gallon fuel can in order not to use the main fuel tank and the results are the same. Unless we can get the fuel pressure to 43 PSI the engine is not going to perform.

Does the new spark plugs have anything to do with the fuel pressure; just guessing?

Thanks,

Max

Re: Mercruiser Mag MPI Fuel Pressure Problem

Does the new spark plugs have anything to do with the fuel pressure; just guessing?

Click to expand...

Not at all.

Re: Mercruiser Mag MPI Fuel Pressure Problem

This is almost an impossible scenario. You aren't working on the engine yourself, you are on a forum, A Merc dealer is working on it but can't figure out why it won't wrok, So you really don't know what they have done or haven't done. I can't hear it, see it, feel it look it over for clues or anything else, the motor is not in the boat. Geeeezzzz, what else could possibly complicate this.
There is a way to test the pump to see if it's capable of creating pressure, it's in the OEM manual (#24) Section 5G Chart A4 starting on page 5G-32.
Have those procedures been followed?

If they did follow that and the pressure is still low after changing the pressure regulator and pump, then it may be as simple as a corroded, rusty connection.
The Cool fuel cooler, pump, regulator assemby is down under the port engine mount and you had to pull the engine to change the oil pan ............ Because it was rusty ???????, ON a 2001 Engine ????? That thing must be a mass of rust and corrosion. It may be the fuel pump relay or it's connections causing low power or poor ground to the fuel pump or relay because of rust and corrosion. Has that ever been looked at?

ALso, I know for a fact that Merc dealers have a number to contact Merc Tech for problems like this. Did they???? Why Not????

Re: Mercruiser Mag MPI Fuel Pressure Problem

Have you verified power supply to the pump while running?

Faulty wire connection or 'pinched' causing a partial short, poor ground?

Any of these and the pump would fail to provide rated pressure.

These pumps are capable of pressures in excess of 100 psi, failing to make only 43 could be poor power supply or that regulator is blown thru.

Re: Mercruiser Mag MPI Fuel Pressure Problem

Though the engine is a 2001 model, it has not been in use since then. The engine came out of a Donzi that caught fire and burned after only 50 hours. It was in storage until it was totally rebuilt (machine shop and all) and conditioned in January 2007. That being said, there is no rust or corrosion to speak of. Only the bottom of the oil pan showed signs of rust and was leaking oil a drop at a time. In a two week period it would leak around 1 quart of oil.

Thanks,

Max

Re: Mercruiser Mag MPI Fuel Pressure Problem

Also check for possible fuel line obstruction before the pump.

Either old hose deteriorated inside, split liner folding over blocking flow, or an object left from 'capping' or plugging' a line while disconnected in attempt to keep dust and dirt out.

Disconnect supply line from fuel pump and from supply tank, connect an outboard primer bulb and see if you can pump fuel thru freely.

Re: Mercruiser Mag MPI Fuel Pressure Problem

Since everything that has been recommended thus far has been done with no positive results; though not likely that a new regulator may be faulty, yet another new one will be installed. Will let you know what the results are.

Thanks,

Max

Re: Mercruiser Mag MPI Fuel Pressure Problem

Would it be possible that the engine computer may be faulty?

Re: Mercruiser Mag MPI Fuel Pressure Problem

Blue Seas said:

Would it be possible that the engine computer may be faulty?

Click to expand...

Nope, the ECM turns the pump on or off only. Has nothing to do with pressure.

Re: Mercruiser Mag MPI Fuel Pressure Problem

Problem has been identified and FIXED!!!!!

What I was told is that apparently their are several fuel pressure regulators for this engine. Something about a blue line and a pink line drawn accross the regular that identifies the PSI it's designed for. In this case the fuel regulator is the one with the pink line drawn accross it; does that make any sense? Also, the coil needed to be replaced as well.

Thanks,

Max