7.4 mpi motor not getting full fuel pressure, runs out of gas

crustycrab

crustycrab

New Member
Mar 29, 2012
49
Wantagh , NY
Boat Info
2000 Sundancer 270, 9.2beam
Engines
454 Mercruiser w Bravo 3
ive got a 2000 270da with 7.4 mpi , im getting 39lbs of pressure when i turn key to on possition , (needs 43 lbs) as soon as it starts fuel pressure drops to 35 LBS, so far ive changed all high pressure hoses , changed fuel pump and regulator , changed fuel seperator , filters and blew out low pressure fuel lines, tried to run it off a 5 gal tank , all with the same results , any and all ideas are welcome ... anyone??? ty for any and all help , im out of ideas
 
gauge is good , motor idles perfect , and it revs up to 4700rpm no problem , it runs out of fuel.... these motors need to run at 43lbs at all times , and im not getting it , take it out and romp it its fine for about four minutes , slow down it stalls and will not restart , there is no fuel at all , im being told its running the fuel out ... takes a long time to restart ... fuel pump has to repressurize the system, oh and btw i also checked it before the fuel rail and i still get the same 35lbs of fuel pressure , so i dont believe its going to be the injectors bleeding down the pressure
 
Last edited:
I suspect you have a cool fuel system. The fuel pressure regulator (sensor) and basket filter are housed there. Have you looked there for problems?

Can you obtain a code reader and see what it tells you?
 
Quint4 said:
I suspect you have a cool fuel system. The fuel pressure regulator (sensor) and basket filter are housed there. Have you looked there for problems?

Can you obtain a code reader and see what it tells you?
Click to expand...

had the codes read , its not holding any codes at all , i have a cool fuel , but its not a cool fuel 3 system (box style) the regulater was also changed as was the whole cool fuel system .... didnt do anything , same result with new cool fuel pump and regulator and cool thru pipe (copper flow), i have got 35 lbs with original and or the full new system....
 
scoflaw said:
Ethanol damaged rubber lines, tank to pump?
Click to expand...

thats exactly what the mercury pro said to me so we changed all the high flow lines and also took the lines off the fuel tank ran the boat out of a 5 gallon fuel cell to prove it isnt the tank.... im still listening ... thanks for the replies
 
I'm talking about the rubber lines you didn't change, you said you blew them out. The ethanol could have a swelling effect on the interior of those hoses that could restrict the volume of gas passing through it.
 
scoflaw said:
I'm talking about the rubber lines you didn't change, you said you blew them out. The ethanol could have a swelling effect on the interior of those hoses that could restrict the volume of gas passing through it.
Click to expand...

no i didnt change the low pressure lines , and i guess that is possible , ill have to check that out as well i guess
 
so im told that if it was the return lines that were clogged , it would show more fuel pressure not less , so im looking for more answers anyone .... ill try anything , i miss my baby
 
Can't speak for your 7.4 but my old 33 DA had another fuel filter in line after the cool fuel box. Check for that. Also, automotive related, my son replaced the fuel pump on his car a couple of years ago. Still would not run right. Took it to a mechanic and they found that the new fuel pump from whatever store was bad from the get go.

Just a thought,

Gary
 
Those aren't the return lines I'm refering to. I'm talking, FEED. low pressure tank to pump lines
 
Im gunna shoot for a bad or corroded pump relay. If your getting 35 psi and your running your engine dry or fuel im thinking the relay is corroded letting some current through but not enough then it gives out after a couple minutes then the start up has enough current to kick it back on. My reasoning is this. If your getting 35psi at startup your still getting fuel and no matter what rpm you are at you will still be getting fuel just running like crap but still running. If there is no fuel at all after 4 min or so then something is shutting your pump down
 
I'm going to shoot for simple stupid because something similar happened to me with the same boat. Right above the fuel tank there is a shut-off knob. A miniture hose spigot. Mine started to close. Don't know if it was vibration or if I had hit it. I had been running the boat at a good clip not WOT. So I didn't really feel any stumbling. I shut down to wait for a friend so we could meet up. When I went to start the engine no go. Would fire, stumble, stall then wouldn't even fire. I thought I had heat soak issues. eventually I tried that fuel shut off and noticed it was partially closed. Opened it up, boat started and ran better than it had before.

I was lucky that I found this when I did. I don't have a fuel pressure reading though. Never know take a shot.
 
Have you measured the maximum pump pressure and flow rate with it disconnected from the fuel rail? If the pump is weak (I read it was changed out) or the inlet side hoses at fault it should show up with these tests.

It sounds like either a faulty pressure regulator or a stuck injector. How much fuel is flowing out the return lines? If the regulator is good (yeah, swapped, i read) then there should be hardly any if the pressure is low, pointing to a stuck injector. If there is lots flowing then the regulator is faulty. Neither of which explains why you run out of fuel... Unless there is an air lock or air leak on the inlet side of the system.

I am must guessing, put it seems you're ready to reach the wild guess stage.
 
Even with clogged lines you should be able to get your 43#, key on engine off. Pressure will bleed off with a leaky injector, should hold steady.
If your not getting your 43, I'd say bad regulator or bad gauge
Bleed the system down, hook up the gauge, key it up.
Does it pump right up and shut off at 35? Does the pump struggle, and not sound right
I would start there with the pressure first and then look at the volume after
 
Partial blockage of vent system. Does the fuel pick-up in the tank have a "sock"? If so maybe that is a restriction. Trying to think of all avenues
 
thanks to all that replied , as of now i changed all that i had mentioned and it didnt want to stay running , it must have still had some kind of blockage that i never found , but i took it out for a ride , ran it up to 4800 rpm , my max trimmed out , dropped it down to idle , did this a few times and it must have passed whatever was giving it a problem .... i took it out today (saturday) run it for a few hours at all different RPMs .... motor is purring .... thanks all
 
Maybe it was an airlock and eventually it passed. I can't think of anything else that would pass through without other problems emerging. Glad it's running well. Fingers crossed that doesn't happen again.
 
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