496 Mag Fuel Pump

[Mike250]

New to this board and I'm having a fuel related problem. The engine is a 2005 496 Mag, serial # OWO60256. The problem is rough idle, poor acceleration, and rare but occasional popping sound at 3/4+ throttle.

I measured the fuel rail pressure with the engine off cycling the key and recorded 40 psi.

With the engine running at idle the rail pressure is 38 psi.

Engine running fuel supply pressure is 0 psi.

With fuel supply line crimped closed the fuel pump will pull 11" vacuum.

Running on a remote fuel supply the problem still occures.

The fuel filter has been changed.

The rated fuel pressure is 44 psi for this engine.

My question is would this indicate the low or high stage fuel pump is bad?

 

[JustJason]

Re: 496 Mag Fuel Pump

i'd check your fuel pressure regulator first. Sounds like i may not be closing. Hook up your gauge and with the engine idleing pinch the vac line that goes to it and see if your pressure rises. If it does not rise then it's bad.

 

[newport dave]

Re: 496 Mag Fuel Pump

You should refer to Mercruiser service bulletin 2004-05 "Gen III Cool Fuel Module Diagnostics"

But in my interpetation of that bulletin;

1)Running on an alternate fuel supply should elimnate supply resriction as the problem.

2)If the low pressure pump will only pull 11" of vacuum, that is right on the borderline, but should be ok (there is no way to check the low pressure pump pressure).

3)This leads me to believe that it would be the high pressure pump that is at fault.

 

[Don S]

Re: 496 Mag Fuel Pump

Actually, that Service bulletin is 2004-06

But here is the bulletin so you can download it.
http://www.4shared.com/account/file/75263102/36f65495/Mercruiser_SB_2004-06.html

 

[Mike250]

Re: 496 Mag Fuel Pump

Thanks, I do have the bulletin and service manuals.

I'll check the regulator.

JustJason, can you explain the reasoning behind the regulator trouble shooting as far as crimping the vacuum supply? I'm not sure I understand how the part works.

 

[JustJason]

Re: 496 Mag Fuel Pump

I know the "Cool fuel" system is a little different than a standard VST, but it works essentially the same way.

As with any electrical fuel injections system, the high pressure pump has a fixed output. In your case 40PSI.
When the engine is at idle demand for fuel is low. When the engine is at idle it also creates a high vacuum in the intake manifold. This vacuum is used to hold open a valve in the fuel pressure regulator, which bleeds off excess fuel back into the low pressure side of the system.
When RPMs go up, demand for fuel goes up and intake vacuum goes down. When the vacuum goes down this allows the fuel pressure regulator to close, sending less excess fuel to the low side, increasing the pressure in the fuel rails.

If you pinch shut the vacuum line to the fuel pressure regulator when the engine is at idle, pressure should build on the rails, and you should be able to see this on your gauge. When you release the pinched line pressure should return to normal after a few seconds. If it does'nt then the regulator is not doing its job.

If you have the manual. Start at page 5D-1 and start reading. If i'm reading your case correctly then the manual wants you to replace both the high pressure pump and the regulator. But personally I would check the regulator first.
What the manual doesn't tell you, is to use some kind of foaming spray oil (like a 6 in 1, I use the J/Rude marked cans, but I believe Bel-Ray oil company makes the same oil) Spray all of the fuel fittings and look to see if any of that oil is getting sucked in.
Unfortunately on a high pressure system you can't look for air bubbles on the output side. But if any of the pumps were sucking air you would definitly get air on the output side. Fuel pumps do not pump air.

 

[Mike250]

Re: 496 Mag Fuel Pump

Understood.

Thanks again.

 

[JustJason]

Re: 496 Mag Fuel Pump

sure thing

 

[newport dave]

Re: 496 Mag Fuel Pump

When the engine is at idle demand for fuel is low. When RPMs go up, demand for fuel goes up.

Jason,
To be exact, the reason they adjust fuel pressure with engine vacuum does not have anything to do with fuel demand, it is intended to maintain a consistant differential pressure across the fuel injector(s).

 

[JustJason]

Re: 496 Mag Fuel Pump

Dave,

Correct.

Don't want to confuse anybody here.

Injector duration is low at idle... in other words, they are not spraying alot of gas at idle with a fixed fuel pressure behind the injector.
When the motor rev's up. Injector duration increases... spraying more fuel. Think of an injector as a light switch... it's on or its off. It switches slowly at idle speeds, to quickly or even completely on at high rpm/high demand.

I already said the pumps output was fixed, so I can't get a pump to pressurize the rails to compensate for the pressure drop the injectors cause by being open longer.
But what they (merc) does.... is to use a pump capable of more pressure than they need, and use a regulator to bleed off what the engine doesn't need depending on the rpm.
The engines ECM has no idea what the pressure is in the fuel system. It is programed to switch the injectors on and off based of off an assumed correct fuel pressure. The ECM assumes the rest of the fuel system (pumps and regulators) are doing their job. If a pump or a regulator (or both) malfunctions the ECM does not know that.
Hope that clears it up

 

[Mike250]

Re: 496 Mag Fuel Pump

Update:

Took it to the dealer since it's still under warrenty and showed them the above pressure readings. So far they checked the fuel pressure and did verify it's about 4 psi low according to the specs. The mechanic stated that the slight reduction in fuel pressure is probably not the issue. They are still working on finding the cause.

Based on your experiance could a 4psi reduction in pressure cause rough idle, lack of power and occasional popping?

 

[Don S]

Re: 496 Mag Fuel Pump

Based on your experiance could a 4psi reduction in pressure cause rough idle, lack of power and occasional popping?

Sure it could, you need to find out what is causing the low pressure. Since it's under warrenty and Merc tech should have been conetacted, and they will get it cured without your input. If they don't, then it's probably because the dealer hasn't called Merc tech.

 

[Mike250]

Re: 496 Mag Fuel Pump

According to the mechanic the #5 plug was fouled causing the problem. They installed a new plug and claim it runs fine. I questioned the reason why one plug would be fouled wondering if there might be another issue. I had cleaned and gaped all plugs before winterizing and the problem still occurred running on the muffs. The response was that ?these things happen occasionally but it runs fine now?. Any thoughts? Seem reasonable?

I also asked to have the low fuel pressure problem fixed since the pressure is out of specifications. The mechanic is calling it in to see if it will be covered under warranty even though it?s ?not the problem?. Not having much experience with warranty claims I wonder if this is correct?

Thanks for your help.