Ian,

I had a sagging issue once in our first boat. It had a mildly warmed over 454. We replaced a Holley 650 (how the boat was set up when we bought it) with a Holley 750, thinking it would fix the fuel starving issue. Took it out, and, while it ran better, the sagging came on much quicker with the new carb. We left it at the tech overnight. Came in the next morning to find him standing on his head in the storage area under the rear seat. When he came out, he handed me the tank check valve and said, "Found it!" I was completely packed with 20 yrs of crud and fairly well corroded. We dropped the boat back in the water and it ran like a scalded cat for as long as we wanted to keep the fun stick stabbed to the firewall.

You said you've got one tank, but two lines coming from it. It wouldn't be much of a stretch to suggest both check valves went south at the same time. Especially if you got a batch of bad gas. Might be worth looking into.

Thanks. Brad.
Model Machine and Precision LLC
(937)545-8991

Thanks Brad et al!

I think we all agree ita a single common issue affecting both engines. Its so odd that it does this after a few hours out on the water with varied use ranging from some high speed runs to anchoring. Then, she just decides to act up. Both times has been after being anchored off for an hour or less, then she flops.

For the tank. In the back, is two separate fuel lines off of elbow, one to each engine.. then there is a fuel sending unit. Thats all I can see...

No vacuum or or sound that I could hear from the gas cap.

are you saying 2 lines off a single pickup ?

Yes, please Speedies question answer !

I see two separate fuel lines that go to two separate elbows from the tank. Dont know if or where a fuel pick up would be and if so, common to both engines. Hmmm

Just turned keys on, both engines reading 6psi for fuel, when they should be closer to 40psi. No fuel leaks that I can smell or see or feel. First time this happened was before I changed fuel filters, so I can rule out fuel filter change...

Battery voltage above 12?

yes. like 12.8 or so. I think was like 13 when running yesterday...Fresh batteries of 12 mos ago

something that kills both motors at the same time, its either electrical or , out of gas maybe

edit: remove the filter is it full?

agree! Last time, yes both fuel/water separators were full. Didnt pull the new ones off today, but boat ran well for a good while yesterday till the petering out at the end. on trailer, fuel reads 67 gal. In water, more like 55-60 or so. Good amount of fuel in her. Im going to go get about 35 gallons of Ethanol Free fuel, see if that might blend in a make a difference.

Keys on, now getting 38 psi and 10 psi for fuel pressure, engine respective. Electric fuel pump active when key turned, so seems to be ok.

If it happens again pull the filter. I'm thinking its fuel related.
You had 11 psi on both, then a few more key cycles and pressure comes up. Could be the filter was not full, but cycling some more they filled back up. If filter is full then the pump is shutting off

could be sediment in the tank getting sucked up to the pick ups, the engines could starve for fuel even with the filters full if theirs blockage at the pickup tube, where are you reading fuel pressure ?? id put a gauge on the rail to compare #'s if you haven't already

I'm reading pressures via Smartcraft display. Pulled relays, to check for corrosion, was spotless clean. This is a fresh water only stored inside boat...

Both engines just started at ramp, seemed a tad lean, but fuel psi was 39-41 (Smartcraft). Spent 45 min to get 43 gals ethanol free to mix with the 67 gals 87 octane.

Got back to ramp, both engines fuel psi readings arouund 12. Stbd started for a bit, but electric fuel pump stayed on. Port would start.

Gonna change gear oil in mechanical fuel pumps now...Ruling things out..

what does this mean, are you calling fuel pickup tubes relays ?

No. A 4 prong prong plug/relay, checked it, it was fine and clean.

I dont know where a single pick up would be for both engines, given each has its own fuel line going into the tank.

you have 2 pickups one feeding each engine correct ( single tank ) ? where the fuel line connects at the tank the is a pickup that extends into the bottom of the tank which usually has a sock and a check ball in them


https://cimg3.ibsrv.net/gimg/www.off...c7ba0ed619.jpg

Great diagram!!!! Thanks!!

Thoughts that both my mechanical fuel pumps.have gone out? Blown diaphragms????

Do you change the oil in them when you do the impellers?

Hi. No, looked for it, didn't see. Where is it? That is/was a bear of a job!!!!

Yes, and even more often than that. Just did it yesterday to see if would help, didn't effect anything.

Going to pull the sight tube off the mechanical pump today, see if fuel in it. Will have key bumped too with it off, see of fuel spits out that nozzle.. if so,, bad diaphragm. If not,, on to next possibility.

Pulled sight tube from each mechanical pump. Neither dripped any fuel, nor did fuel spit out of nipple when I turned key to prime system. Boat been sitting, electric pumps come on with key. Port only has 6psi, stbd has 30. Both are below the required 40psi.
Guess I'll check the fuel line pick ups in tank next.

This is an absolute puzzle...

Try replacing all rubber lines from fuel tank to pump. Being a 2007 hull manufacturer might not of used good ethanol hose. Inside will get soft and collapse under suction.

Thank you, will be looking at those today. Owners Manual actually speaks to the federally mandated Anti Siphon valve, that the fuel line connects to top of tank. That has a spring and check valve like ball in it, can get worn, corroded, etc and not be able to collapse the spring to get fuel flow.

Anyone know, is there special thread seal or other that needs to be used when replacing? There's some beige putty color thread seal on there now...

I use this on all automotive/marine pipe thread fittings, gasoline/oil/water/coolant
Available at pretty much every auto part store

https://cimg8.ibsrv.net/gimg/www.off...970bf4ad5.jpeg

So, wasn't able to get a psi reading off the rails, maybe my guage i was using wouldn't depress the shradder valve, but I had three experts yesterday all quickly come to the same conclusion that given there is fuel contamination in my mechanical fuel pumps, that those are the problem.

The high pressure pump can't draw deom the tank, so if a bad mechanical fuel pump isn't providing on the suction side, the hi pressure pump can't pull the fuel around and up to injectors.

New pumps have been ordered, along with new fuel line to be run from the tank to each pump. Even the fuel line manufacturer agreed after 14yrs, its time to replace the lines off the fuel tank.

Will post update once new pumps installed.

What do you mean by "fuel contamination?" They all get gas mixed with the oil somehow. Everyone I have talked to says it's normal for the lube to be diluted with gasoline. That's why it's so important to change the oil.

Exactly, mine has always had that contamination issue, hence the frequent gear lube change for that pump. Merc says it shouldn't have any, but we all know they did a poor design.

For the $150 each engine, new mech pump and fuel line to tank going in, with a prayer it solves my issue...

Well, I replaced the mechanical fuel pump in port engine, and still no pressure. I will say its really odd how the arm of the pump sits in the cam chamber of the sea water pump, and the slip ring is lose. When it was all in my hand, I could spin the ring around the cam shaft. I was told thats how it is. And is slightly eccentric so does its job.

Blead the system via shradder valve on rails. Also, fuel water separator has fuel, but I took it off, topped it off, turn key on, pressure builds. Was able to do this several times and got psi up from as low as 3 to 17, just with key on and off.

Boat is near San Antonio, TX. I dont have a qualified mechanic near me, so will have to find someone and make the haul....haven't changed pump yet on starboard engine.

So frustrating....

Ok, I write with wonderful news!!!! Just as I was throwing my hands up, the last thing that I could do is change the fuel line from the tank to the mechanical fuel pump. So, I did, and OMG!!!! Problem solved!!!! The line was restricted from collapsed inner liner, like a blocked artery! Even the manufacture suggested the change when I bought the OEM replacement line (Trident 3/8"). The fuel line was dry when I removed it and I couldn't blow air through. The anti siphon valve working perfectly. So, with new fuel line, new mechanical fuel pump, new filters, the ole 525 should be singing again! After the change, psi reading went from 3psi to 40psi just with key on!

Thank you ALL for all the responses, ideas, information, etc. This forum is an amazing resource for all of us!

Best,
Ian

Yay!!! Congratulations. Now go boating!:thankyouthankyou::evilb::coolcowboy::bunn ydance::drink::riding::p:party-smiley-048::cheer::circle::):boat:

Appears KWright nailed it.......

KWright did nail it on the head!!! Bam!

Yes, had her out yesterday, dont think she has ever sounded so healthy and responsive!!!

Man it felt good to be back on the water!!!!