Fuel pressure low.

[woodguy81]

Im trying to track down my low fuel pressure issue. It is a 6.2 mpi with both the lift pump and high pressure pump in the system. When I turn the key on and the pumps run I get 43 pounds on the gauge and when they stop it drops to 38-40 pounds and stays there, When it is running im only getting maybe 35 pounds of pressure and the needle is not steady it is rapidly fluctuating around that 35 pound mark. I can feel a pulsation when I am holding the gauge. Things I have tried with no change.

Ran it off a gas can on a short line to rule out tank or line issues
Changed both regulators with oem units.
I currently have the cool fuel unit off the motor to see if anything was possibly restricting that and I couldn't see anything there.
Changed fuel filter with oem filter (the spin on water separating one.)
I am having the same issue on both engines on my boat so Im hoping to solve one and then apply that to the other one. any ideas?

 

[Fun Times]

What year is the engines? Serial number handy too?

Try running the engine with the regulator vacuum line removed to see if the pressure comes up again and see if the vibration settles down.

Other than this, does the engines run normal?

 

[alldodge]

^^Agree serial number

What problems are you seeing?
Might not be a fuel pressure issue, might be your gauge

 

[woodguy81]

seems to be lacking power. I can get another gauge to double check what im seeing as well. serial number of the original engine is m367508. was a 5.0 mpi. Swapped parts to a 6.2 long block and got a new ecm to complete it. Its tough to really know for sure but im looking at the original engine and performance numbers for this boat with 5.7 engines in it and im not getting close with more motor. I honestly have no reference as I got the boat with twin non running 5.0 engines in it. The boat is a 1995 proline 2950. Also not what it came with from the factory. Just got the diacom software and plan to get the data from that in the next few days to see what is actually going on also. The only thing im seing is low fuel pressure so far. Also worth noting that I had all the injectors serviced and most sensors have been replaced along the way. Its been a very long term project that i just got running after 3 years of owning it.

 

[alldodge]

got a new ecm to complete it

ok, are issues are did you get a true 6.2 ecm, note they were 555

Has the distributor been timed correctly

The diacom will help a bunch to see what is going on

 

[woodguy81]

ok, are issues are did you get a true 6.2 ecm, note they were 555

Has the distributor been timed correctly

The diacom will help a bunch to see what is going on

Yes on the ecm. That was ordered new from mercury based on the original serial number but for a 6.2 through John at boats unlimited. The timing was set correctly as far as I can tell.

 

[alldodge]

John knows what he's doing so we should trust it

Looking for diacom
Could check compression, or see if timing is advancing

 

[woodguy81]

Ill get it hooked up and post the numbers from diacom. going to verify with a second gauge as well. The ecm has no effect of fuel pressure though right?

 

[alldodge]

The ecm has no effect of fuel pressure though right?

correct

 

[woodguy81]

Im not sure if this is correct But i read elsewhere that the fuel pressure will be lower running at idle and should increase running under load in the water. Does that sound correct? Im going to get it on the water today and run the software on both engines but if thats true than my issue may have just been the kinked fuel line on the port engine. Also pulling the vacuum line does increase the fuel pressure by about 5 pounds.

 

[alldodge]

Im not sure if this is correct But i read elsewhere that the fuel pressure will be lower running at idle and should increase running under load in the water. Does that sound correct?

Yes, but only maybe around 5 psi as you noticed (other motors can vary)