Fuel pressure dropping.

[Baka123]

This is the start of season 2 that I have been chasing this issue. The boat will run at low rpm just fine, gets up on plane and eventually falls off like its starved for fuel. I have replaced all fuel lines, the pickup, the tank vent line, the water separator, the fuel filter, the fuel pump, and have rebuilt the carb. (I did the carb myself and am no expert) I was finally convinced I just needed to buy a new carb as everything else was already new. I decided to put a fuel pressure gauge in line and see what the results were prior to buying a carb. Just to confirm what I was thinking. To my surprise the fuel pressure starts off perfect, but then will quickly fall off. What else could be causing this with everything in the system being new? Video of fuel pressure test attached. TIA

 

[turand]

I know your setup probably isn't like this, but this sounds eerily similar when you forget to vent your gas tank, but you mentioned the vent line.

 

[nola mike]

Assuming you've checked the filter before changing it and didn't see anything? Problem was there prior to replacing the pump? Does the problem start right when the pressure drops, or a bit after? Is the pump still running during the episodes (wondering if you're having an electrical issue)?

 

[Baka123]

Assuming you've checked the filter before changing it and didn't see anything? Problem was there prior to replacing the pump? Does the problem start right when the pressure drops, or a bit after? Is the pump still running during the episodes (wondering if you're having an electrical issue)?

 

[Baka123]

Correct all filters were replaced and none of them appeared to need it. Problem started prior to replacing the pump. It starts after the needle sits at 0 for a moment. Will run as normal until then. I guess I should have added this is a Mercruiser 5.7 with a quadrajet carb (the one with divorced/manifold choke). This is a mechanical fuel pump so I'm not sure that electrical would be the issue, but not positive.

 

[alldodge]

Has the antisiphon valve been replaced?
Have a serial number?

 

[Baka123]

Has the antisiphon valve been replaced?
Have a serial number?

Antisiphon valve removed, same issue, replaced it, same issue. Serial OD427176

 

[Scott06]

How long does it take for the fuel pressure to drop to zero? One thing that hasnt been mentioned is the fuel tank vent being plugged, but this would take several minutes to start pulling a vacum on the tank.
you could test this by blocking off the boat tank and hook an outboard tank to the pump, see if it goes away.

if all else fails may want to take a shot by replacing the fuel pump, perhaps the check valves inside are sticking

 

[nola mike]

How long does it take for the fuel pressure to drop to zero? One thing that hasnt been mentioned is the fuel tank vent being plugged, but this would take several minutes to start pulling a vacum on the tank.
you could test this by blocking off the boat tank and hook an outboard tank to the pump, see if it goes away.

if all else fails may want to take a shot by replacing the fuel pump, perhaps the check valves inside are sticking

He said the vent lines and fuel pump have both been replaced. You should try running off a Jerry can, maybe get a cheap electric pump for testing purposes to narrow things down

 

[Bt Doctur]

Run on a remote outboard tank plugged into the fuel sep inlet and retest.
You may have a "sock screen" inside the draw tube from the tank. If the remote tank show no problem then remove the draw tube from the tank and check for a screen. Another way is to try and pass a flex line/string/mono/ etc into the fitting at the tank and see how far it goes in.

 

[alldodge]

With everything replaced including the pickup tube, it has to be the new pump. That is unless there is a fitting which is clogged.

When the pump was replaced, you didn't happen to replaced the pump rod which rides on the cam? Hope not

 

[Bt Doctur]

 

[nola mike]

This is a mechanical fuel pump so I'm not sure that electrical would be the issue, but not positive.

No, wouldn't apply to a mechanical pump, nor with the symptoms starting after the drop in fp.

 

[Rick Stephens]

Just one more thought: I've seen that on a truck where a flaw in a rubber fuel line (between tank and pump) allowed the inner lining to collapse.

 

[Baka123]

Run on a remote outboard tank plugged into the fuel sep inlet and retest.
You may have a "sock screen" inside the draw tube from the tank. If the remote tank show no problem then remove the draw tube from the tank and check for a screen. Another way is to try and pass a flex line/string/mono/ etc into the fitting at the tank and see how far it goes in.
View attachment 337381

I removed this screen on the old pickup with no change. New pickup does not have one.

 

[Baka123]

With everything replaced including the pickup tube, it has to be the new pump. That is unless there is a fitting which is clogged.

When the pump was replaced, you didn't happen to replaced the pump rod which rides on the cam? Hope not

Did NOT replace the push rod. Wondering if maybe the push rod or cam could be causing the issue? Thoughts?

 

[Baka123]

Just one more thought: I've seen that on a truck where a flaw in a rubber fuel line (between tank and pump) allowed the inner lining to collapse.

All fuel lines are brand new. Replaced metal lines with rubber.

 

[Baka123]

I appreciate all the replies and while I have tried running on a aux. tank, with no change. That was prior to a lot of fixes. So I will try this again just to see what happens. What do the symptoms look like when the cam or push rod for the fuel pump wears down?

 

[nola mike]

Did NOT replace the push rod. Wondering if maybe the push rod or cam could be causing the issue? Thoughts?

That's all I can think of. Some applications have shims behind the pump which control the lever travel, not sure in your application. But if that's the case I'm not sure if the pressure would ever rise high enough, or if it would be a strictly volume issue. Swapping an electric in would answer the question of whether it's pump related at all

 

[alldodge]

Did NOT replace the push rod. Wondering if maybe the push rod or cam could be causing the issue? Thoughts?

Then you should be good. If it was replaced I would wonder if the rod was the same length. I think its your new pump