Low Fuel Pressure 2005 Mercruiser 4.3L MPI

[Tristan1985]

Hang on, just checked your serial number and it seems that you do not have a LP pump, sorry for the mistake. Most of these do so I blindly assumed you have one. I stand corrected. But still do the clear hose test on the return from the HP pump and the clear hose test from the tank to check for air. Also to check for leaks you could pressurize the entire system and do a sniff test. Also, if your motor got destroyed earlier due to a lean run I would consider adding a boost pump, the type from newer serial numbers.

Hi, thanks I will try the clear hose test.
When disconnecting the return line and doing the clear hose test there, do I need to put a closed hose on the filter connection where the hose was?

 

[alldodge]

But still do the clear hose test on the return from the HP pump and the clear hose test from the tank to check for air.

Doing this is not going to prove anything

 

[vroom ZOOM]

Hi, thanks I will try the clear hose test.
When disconnecting the return line and doing the clear hose test there, do I need to put a closed hose on the filter connection where the hose was?

Yes. I think you can also use a 3/8 NPT plug. let the pump run a few cycles before looking for air in order for the air that got in while messing with it to get purged. also MAKE SURE to check for water in fuel.

 

[Tristan1985]

Hi All,

So, I ran some tests today.

Firstly changed the fuel filter to OEM 25 micron. No change in fuel pressure.
Secondly I disconnected the FP relay and ran a jumper over the terminals, this runs the pump without the key being on. Whilst pump running and key on (engine not running) I tested the voltage at the relay and got 12 volts with pump running and static pressure of around 40psi.
Thirdly I think I found the issue with the relay terminals which caused FP to shut off the other day. When the relay was connected the terminals weren’t touching properly so I slightly the bent the prongs on the relay so that when inserted it gets better connection. However this made no change to fuel pressure and still sits at 40psi when static and around 33 when running.
Fourthly when running I purged a bit of fuel out of the pressure gauge connected to the fuel rail, and from what I can see it appears there’s lots of air in the fuel?
I have taken a video of this and seen i can attach below.

The end result is that the fuel pressure issue is still not resolved. If it is air getting in, I don’t know where it would be entering because i pressure tested everything upstream of the fuel filter and there is no leak. And the mechanic said he also put a portable tank and connected it to the fuel filter which made no change in fuel pressure, so that leads me to believe there is no leak between fuel filter and boat tank.

Any further advice is appreciated.

 

[Tristan1985]

I couldn’t upload the video so uploaded some stills. You can just make out the air in the line.

 

[Tristan1985]

Also on another note, with regards to the voltage test. I am wondering because I took the voltage at the relay, what if there was a bad connection down near the pump, could the voltage be lower down at the pump?

 

[alldodge]

pump running and static pressure of around 40psi.

pressure gauge connected to the fuel rail, and from what I can see it appears there’s lots of air in the fuel?

With motor off and pump running you get 40 psi, but when motor is running you get 30 psi, correct?

Any time fuel is released from gauge there is a lot of bubbles, correct?

I don't see an issue with lower voltage at the pump, your good there

 

[Tristan1985]

With motor off and pump running you get 40 psi, but when motor is running you get 30 psi, correct?

Any time fuel is released from gauge there is a lot of bubbles, correct?

I don't see an issue with lower voltage at the pump, your good there

Yes pressure will climb to 40psi when motor off and pump running. When motor & pump running and drawing fuel, the pressure Gauge will sit at around 33.

I bled the gauge in three short bursts whilst engine running. First time lots of bubbles, second time lots of bubbles, third time it didn’t appear there were bubbles but could have been.

 

[alldodge]

Things just don't add up with pressure

You can pinch off the return hose and pressure goes up close to 60. Can keep it slightly closed to get 40. With pump jumped and motor off you get 40, but with it running you get 33.

The regulator should be set at 40, so pinching off line more should not cause pressure to go up to 40, it should only cause it to above 40. If injectors were leaking then there should be a lot of fuel coming out the exhaust and pinching off line should have no effect. If return line was removed from filter and Reg was actually set at 40, and rail was at 33, there should be no fuel in the return

If pump is having a hard time sucking fuel from tank there should be a vacuum in the line. Remove one end of the rubber fuel line between tank and filter. Install a Tee fitting and attach a vacuum gauge. There should be very little vacuum

 

[Tristan1985]

Thanks,
The pressure readings you summarised are correct.
My understanding is that the pressure should be 43 psi at the rail, as that is what the manual sais and what the regulator we purchased is set to.

You have got me thinking but - when the engine is running and I’m getting a reading of 33 psi at the rail and I can squeeze the return line to raise the pressure, that means the regulator is releasing fuel when the pressure is less than 43, yes?

Is that how it should normally function? Or should it only release fuel to the return line when pressure exceeds 43?

Is it possible that air in the system can be altering the functionality of the regulator?

I just can’t imagine that the reg is faulty as we are have replaced it with the correct part and still no change with that pressure.

Noted your tee idea in the fuel line. I think prior to doing this I will install a portable tank and test that myself. I might even connect the tank upstream of the fuel filter and run the return line to seperate tank to isolate the filter from the system.

With the vacuum line that connects to the fuel rail, which I’m yet to locate and haven’t checked it yet, could the vacuum line somehow be causing the issue?

Thankyou Alldodge

 

[alldodge]

Reg should release pressure above 43 (at least no lower then 40)

Is your gauge accurate? Use air pressure with known good gauge and regulate pressure to compare

I don't see it being the filter, you replaced with one FT mentioned and there was no change

With Reg not getting any vacuum then pressure should be at it's highest point. With Vac connected to Reg pressure should be tad lower. The Vac line runs from Reg to under plenum

 

[Tristan1985]

Reg should release pressure above 43 (at least no lower then 40)

Is your gauge accurate? Use air pressure with known good gauge and regulate pressure to compare

I don't see it being the filter, you replaced with one FT mentioned and there was no change

With Reg not getting any vacuum then pressure should be at it's highest point. With Vac connected to Reg pressure should be tad lower. The Vac line runs from Reg to under plenum

The gauge is new. The system was originally tested by mechanic who got same reading as us. When I pressure tested everything upstream of the fuel filter with bike pump the pressure gauge was still connected to the rail and got a similar reading as the bike pump gauge. So I believe it’s good.

So if the gauge is correct and the reg is releasing fuel in the return line I suppose that means it’s a faulty or incorrect reg? Ahhh

I’ll undertake some more testing when I get to boat next.

Thankyou

 

[alldodge]

So if the gauge is correct and the reg is releasing fuel in the return line I suppose that means it’s a faulty or incorrect reg? Ahhh

This is my assumption. Only way I know to verify is by doing a air pressure regulator test I mentioned previous. I know this works because @tpenfield has done it and it works

 

[vroom ZOOM]

The gauge is new. The system was originally tested by mechanic who got same reading as us. When I pressure tested everything upstream of the fuel filter with bike pump the pressure gauge was still connected to the rail and got a similar reading as the bike pump gauge. So I believe it’s good.

So if the gauge is correct and the reg is releasing fuel in the return line I suppose that means it’s a faulty or incorrect reg? Ahhh

I’ll undertake some more testing when I get to boat next.

Thankyou

What is the fuel pressure doing if you turn on the pump? What is the reading? What is the reading as soon as the pump turns off? How fast does the pressure drop to 0 after the pump turns off?

Did you check for WATER in fuel at the rail?

 

[Fun Times]

The pressure is consistently around 33 PSI when running at 1800 rpm. When we turn the key a few times without starting engine the pressure can climb to around 40 but then as soon as the engine runs it drops to 33 and the pressure gauge needle does jump around a bit.

How does the engine/boat perform while on the water? Have you tested the fuel pressure while underway too?

Need to know what happens when you pull off the small rubber vacuum line at the fuel regulator nipple connection point.

It's still possible to have a compromised fuel pulse damper that's up on the fuel rail... Search online fuel pulse damper purpose for additional general guidance.

Plus, this past membership comments with repairing his bouncing fuel pressure always seemed to intrigue me to share with some even if not fully knowing if it was really true or not, something fixed it though.

Re: Fuel pressure bouncing????
The reason the fuel pressure was bouncing was because of two burned up intake valves. The compression was leaking back into the intake and making the fuel pressure go up and down.

Fuel pressure bouncing????

I have a 2002 Mercruiser 6.2mpi. I have been trying to solve some engine problems for some time now in like 2 other dead threads.... I have a fuel pressure gauge hooked up to the fuel rail. I turn the key over to let it prime and the gauge goes up to around 43 psi and holds then drops to...

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