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?