Fuel pressure drops to zero on anything past idle...help!

[RockandRoll24]

Thanks for the help so far! Got a new issue...

I have a gen 6 502 with the VST fuel system. I have been chasing a low fuel pressure issue and have since replaced raw water sea pump, mechanical pump, VST pump, and fuel pressure regulator. I've taken care of most issues that were dogging me earlier, however I now have a new one. Whenever I increase throttle past idle, the fuel pressure plummets to zero and the engine dies. Sometimes, if I throttle up during the pressure loss, the pressure will rebound and the engine recovers from the stall. It's like the pump catches up for a split second and recovers, then plummets again. On key ignition and pump prime, I have a solid 36lbs that holds. When the engine bumps over, it will idle all day with about 32 pounds. As soon as I increase throttle, pressure drops within about 2 seconds. I measured voltage at the VST pump and it holds solid at 12+ throughout the pressure drop. I'm at a loss at where to check or go next. I've checked and retightened all the fittings I took apart to change pumps.

I bled out all of the air that I saw through the fuel pump gauge relief button until I saw solid clear fuel coming through. Still have this issue...

Any thoughts??

 

[cecho]

Have you checked your water separator and/or fuel filter?

 

[RockandRoll24]

I replaced it last fall (probably has 5 hours on it so far this year). It was full when I took it off to check...

 

[Scott Danforth]

What was it full of? Did you dump the contents into a clear container and inspect.

Have you checked your anti siphon valve?

Your fuel pressure is too low even at idle. You have a fuel system restriction

 

[tpenfield]

The model year and serial number of the engine will help. I assume since it is a Gen 6 w/ VST that it is in the mid-late 1990's range :noidea:

Some of the fuel rail systems are spec'ed to rung in the 32-36 psi range, while others are spec'ed to run in the 43 psi range.

Any history to this engine that would be significant ? (long lay-up, new-to-you, sudden issue, gradual, etc ?)

 

[cecho]

I had a boat several years ago that ran fine until one day it just started clogging fuel filters within as little as 10 minutes of run time. The engine ran great and then it would completely stall and only run at idle. The causes ended up being crap in the gas tank. I remove the fuel tank and completely clean it out. It was a 100 gallon aluminum tank and there were bits of rust (from the fuel level sender) and varnish covering the inside of the tank. I don't know why it suddenly became a problem when it had been running like a champ previously.

 

[RockandRoll24]

The model year and serial number of the engine will help. I assume since it is a Gen 6 w/ VST that it is in the mid-late 1990's range :noidea:

Some of the fuel rail systems are spec'ed to rung in the 32-36 psi range, while others are spec'ed to run in the 43 psi range.

Any history to this engine that would be significant ? (long lay-up, new-to-you, sudden issue, gradual, etc ?)

I rebuilt the motor down to crank bearings last spring. I don't have the serial on me at the moment. It's the K series with MEFI-1. Ran well all last summer. It's always had some trouble with vapor locking, but normally I can stay ahead of it by doing all the recommended shenanigans. This year however, I developed a surge at idle with the occasional stall. No codes aside from a coolant sensor failure (don't think that's relevant). Put a gauge on the fuel rail and found about 30 psi at idle. So...I checked the raw water pump - full of gas and metal sludge. Replaced raw water pump, mechanical pump, VST pump and screens, and fuel pressure regulator. Also replaced the IAC. It idles perfectly now. But this new stall off idle has me stumped. When advancing the throttle, the engine is following the pressure (obviously). What I'm trying to say, is that the pressure drops to zero and then the stall follows, not the other way around.

 

[tpenfield]

I rebuilt the motor down to crank bearings last spring. I don't have the serial on me at the moment. It's the K series with MEFI-1. Ran well all last summer. It's always had some trouble with vapor locking, but normally I can stay ahead of it by doing all the recommended shenanigans. This year however, I developed a surge at idle with the occasional stall. No codes aside from a coolant sensor failure (don't think that's relevant). Put a gauge on the fuel rail and found about 30 psi at idle. So...I checked the raw water pump - full of gas and metal sludge. Replaced raw water pump, mechanical pump, VST pump and screens, and fuel pressure regulator. Also replaced the IAC. It idles perfectly now. But this new stall off idle has me stumped. When advancing the throttle, the engine is following the pressure (obviously). What I'm trying to say, is that the pressure drops to zero and then the stall follows, not the other way around.

30 psi is a bit low, but most likely not the source of the issue. The spec on the 805227A1 regulator is 32-36 psi. So, when you turn on the ignition, but not start the engine, you should see the fuel rail pressure go to about 40 psi for a second and then drop down to the spec range of 32-36 psi and hold it.

To me it sounds like a fuel delivery restriction to the fuel rail. There are the output and return lines from the VST pump to the fuel rail (pic below) and the fuel rail itself. (upper arrow in the pic). Is there a certain RPM coming up off of idle that the pressure starts to drop?

There is also the vacuum line from the pressure regulator (lower arrow). If you clamp that off, just wondering if it makes a difference in the symptoms. If not, I'd check/replace the fuel pressure lines. You might even need to remove the fuel rail and check for clogs/debris. I would also re-check the screens.

did you get the Mercruiser/Quicksilver fuel regulator or an aftermarket brand. I had a bad experience buying an aftermarket regulator . . . had to get the OEM regulator to get the pressure within spec.