5.0 TBI low fuel pressure after running for 10 to 15 minutes

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Looking for suggestions: 1999 Mercruiser 5.0 with TBI EFI
Boat was running great until very recently when after 10 or so minutes of running the low fuel pressure beeper will alarm and then engine runs rough and/or dies.
Since this problem started I have:
replaced the fuel filter/separator. Problem persists after taking to the lake and running.
replaced the fuel pump in the cool-fuel assembly (not super easy to do). Same problem after testing at lake
I've verified the issue is indeed low fuel pressure by installing a squeeze bulb between the fuel tank and fuel filter and when the fuel alarm starts I squeeze the bulb to build up the fuel pressure and the alarm goes away and engine runs fine, until after several minutes it comes back. I can also visually see the injectors at the top of the throttle body go from a very poor weak spray (runs rough) to a good spray (runs good) after squeezing the bulb and building fuel pressure.
I most recently suspected the fuel pickup in the tank was restricted and would get worse after running for a while and sucking more "junk" in the screen. Since it was difficult to get the fuel pickup out of the tank I tested this theory by using a 6 gallon portable tank to connect to the fuel inlet line instead of the boat's main tank and unfortunately the same problem still persists using a completely different tank and fuel pickup.

Any ideas? I don't believe it is a vapor lock issue, it's not hard to re-start, it just truly loses fuel pressure for some reason? Something is causing it to lose fuel pressure while running. It's happened at both speed and just idling. It's not the fuel pump or the fuel pickup as I've replaced those. It doesn't sound like a pressure regulator issue to me, but maybe?

I've searched around the internet but couldn't find anyone else describing having these same symptoms.
 
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alldodge said:
Your hand pump might be masking the real issue. It might be a collapsed fuel line, but then again it may be the regulator (item 15). Need to check fuel pressure 30 psi. Could be its low to begin with and just gets to low to run later

https://www.mercruiserparts.com/bam/subassembly/31846/4856/90
Click to expand...

Thanks for the thoughts. The hand pump is just something I added after the problem started happening, it helped to verify fuel pressure was actually low and it also helps me run the engine to get the boat back to the dock. Collapsed line is interesting. The only line it could be is the one line from the filter to the pump and I had that line out when I replaced the pump and didn't notice anything, but it may not be visually obvious. All other lines (on the suction side) were different lines when I used the portable tank and the problem persisted.

The more I think about it the more I'm convinced it's somehow heat related. When the engine is cold it runs great initially 100% of the time. It isn't until the boat runs for a while that this problem pops up.

Perhaps the fuel pressure is on the low side initially (I'll check the actual pressure when I can) but whatever the pressure is initially it's enough to run the boat just fine, it's very consistent that after running for 10-15ish minutes or so the pressure drops to a level that's a problem. If this were truly an intermittent problem I would expect it to happen occasionally when everything is cold, but that never happens.

The fuel pump is new and acting exactly the same as the old pump, so I know it's not the pump.

I'm wondering if insufficient water is flowing through the cooler (item #8 in the diagram above) this fuel pressure problem could happen? I wouldn't think so but I don't know that much about these fuel "coolers" and what exactly happens with insufficient cooling. The impeller/water pump is new and the engine isn't overheating but maybe somehow the fuel cooler itself is clogged or water isn't flowing properly in this fuel cooling circuit?
 
alldodge

alldodge

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The coolers are to help prevent vapor lock. That's not your issue and the motor would overheat if there wasn't enough flow. Only way I know to measure pressure on TBI is with the Merc tee fitting
 
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