Intermittent "surging" issue

[PunchyTurtle]

This is a 2005 50HP Mercury EFI. It's been having weird fuel delivery issues that have been intermittent and displaying a wide variety of symptoms, making it really confusing and hard to diag.

It started with low idle while warming up, to excess fuel spilling out of the VST at idle (twice), and loss of fuel pressure after shutoff causing hard starting (continuing issue), and now this surging situation has started. It almost seems like a cold cylinder.

It will persist as long as I've let it. But, when I open up the throttle, it will stutter and be doing the same repetitive surging loss of power thing for maybe 1.5 seconds, after which it will seemingly work itself out and run smoothly and entirely normally. It then runs perfectly normally until I shut it off and allow it maybe 10 seconds before starting back up again. It will have lost most of the fuel pressure at that point, and the surging thing does not happen every time.

I attached a video of the surging thing it's doing.

 

[DeepCMark58A]

Fuel quality? Do you have a water separator/filter? How old is the gas?

 

[Jeff J]

Not holding pressure in the fuel rail after shutdown kinda indicates the high pressure pump to me. It should pump right back up when the key is turned on so it really shouldn’t affect starting unless the pump isn’t performing. What pressure are you seeing or how are you checking pressure?

 

[PunchyTurtle]

Fuel quality? Do you have a water separator/filter? How old is the gas?

I have a separator, and the gas is very new. Maybe a month old.

 

[PunchyTurtle]

Not holding pressure in the fuel rail after shutdown kinda indicates the high pressure pump to me. It should pump right back up when the key is turned on so it really shouldn’t affect starting unless the pump isn’t performing. What pressure are you seeing or how are you checking pressure?

I haven't checked it yet, but I will definitely do that.

I just put a fuel pressure meter right onto the shraeder valve on the VST, right?

Also, this motor doesn't have a fuel rail per se. It's just high pressure lines after the VST that go into a distributor sort of thing.

 

[Jeff J]

I just take some fuel hose and tee into the output side of the low pressure pump. I think most fuel injection systems have a port to connect a gauge to somewhere on the high pressure side. It reminds me of a valve stem on a tire. Schrader valve is a better term. Fuel can squirt out of it when the core is pushed. I usually tee in downstream of the VST if there isn’t a port to attach to or none of my adapters will fit.

Pressure readings should be steady around 18 psi for the low pressure side feeding the VST and 40-50 psi for the high pressure side but those numbers should be confirmed. I may be confusing it with a different manufacturer.