Mercury 280 bogs down sometimes when trimming, shifter sometimes shuts off the motor

[Dutch]

the reason for the relay is because back when people ran 260's powering the fuel pump from the key harness the draw from the pump exceeded the little wire most people pulled the power from. the 280's have power coming from the ecu so that when you turn the key on it primes the fuel system and the ecu will cut the power to the pump if it senses the 280 isn't running. I have seen 280 fuel pumps run with and without relays, depends on who you talk to and how much you're twisting it up. A fuel pressure gauge wouldn't hurt, and get a ddt if you want to plug into the motor for some data but it won't save it. You mentioned everything is from a 2.4 except the power head. I would then check your fuel pump that you have the newer style, not the one that the barb out of the pump is only 5/16".

[jwestern]

Just discovered the nut on the positive post on the fuel pump is spinning. Thread must be stripped.

[jwestern]

the reason for the relay is because back when people ran 260's powering the fuel pump from the key harness the draw from the pump exceeded the little wire most people pulled the power from. the 280's have power coming from the ecu so that when you turn the key on it primes the fuel system and the ecu will cut the power to the pump if it senses the 280 isn't running. I have seen 280 fuel pumps run with and without relays, depends on who you talk to and how much you're twisting it up. A fuel pressure gauge wouldn't hurt, and get a ddt if you want to plug into the motor for some data but it won't save it. You mentioned everything is from a 2.4 except the power head. I would then check your fuel pump that you have the newer style, not the one that the barb out of the pump is only 5/16".

Are you saying that if the fuel pump primes and then shuts off, that does not necessarily mean the fuel pump is on a relay? I.e. the ECU is shutting it off because the motor isn't running, and it's not because of a relay? If so, then perhaps I do NOT have the fuel pump on a relay.

I checked the fuel pump hoses are all 3/8".

[jwestern]

@Dutch @25two.stroke

I discussed what you said about relays with a friend. Now I understand better what you're talking about. My pump is not on a relay -- the wires go straight from the ECU to the pump.

Today on a perfectly calm lake I was able to notice small intermittent power loss when completely flooring the throttle at high speed. The amplitude of the power loss is much less than before, maybe 10-15%, so it's probably a separate issue from before. Perhaps the lack of relay is the culprit now.

What are the specifications I need to do the relay? I guess I need to know the power output of the ECU, what gauge wires to use... and understand the wiring diagram, and where to mount the relay.

[tlwjkw]

this or similar.. power tha relay with any "purple".. easiest in tha boat...tape up tha pump feed from tha ecu....

Mr. Gasket 40205G Mr. Gasket Fuel Pump Relays | Summit Racing

[25two.stroke]

I don't think your 10-15% power loss is the lack of a relay; but it could be. Always better in my opinion to put it on a relay. Not good for the ECU to have to put out all that power all the time. I've seen a cheap relay kit like that in the link used and it works...I would look for a sealed relay like what merc uses for the power trim. Mercury sells an "accessory power kit" for use on 14 pin harnesses where you wanna wake up the whole dash by the key. This kit has a waterproof relay and the proper colored wires coming out of the holder. If you don't care about the wire colors being correct then cut up a power trim relay harness.

[jwestern]

Thank you both. I ordered that Mr. Gasket kit before reading your post @25two.stroke . Can I seal the relay using silicone?

I poked around the boat and motor today, and uploaded some photos of the wiring in this google drive folder: https://drive.google.com/drive/folde...usp=drive_link

I can identify two relays. What do they power?

There's also this plug with one white and one blue wire coming out, which was unplugged. What does that do?

I also had a closer look at the fuel tank. It's got a plastic stand pipe, and the connection at the top is a sharp 90 degree bend, rather than a more gradual 90 degree turn like Diamond Marine recommends. Perhaps that flow restriction is causing the 10-15% power loss at full throttle? There is not enough clearance for a gentle 90 degree bend due to the master switch. But the opposite side of the tank has a plugged port and lots of room for such a bend. So I think I will switch the standpipe over to the other side and get a gentle bend in there. This will also shorten the hose length from the tank to the pump, since the pump is mounted on that side.

[25two.stroke]

Just mount the relay where it won't ever get wet.
Relays on the motor are the main power relays. When the motor is keyed-up they turn on and supply power to the CDMs, the ECM and that pigtail for the fuel pump. These items all share power off the Red/Blue wire in the harness, which is why I like putting the fuel pump on its own power source in the boat. If your fuel pump were to short or draw too much its taking power straight from the ECM and the CDMs...not good. If its on its own relay there is never enough power draw to take away from the engine management.
2 wire plug is for diagnostic; leave it.

[skialot2]

Use a waterproof marine relay. Thay are cheap.

Amazon.com: True MODS 1 Pack 5-Pin 12V Bosch Style Waterproof Relay Kit [Harness Socket] [12 AWG Hot Wires] [SPDT] [30/40 Amp] 12 Volt Automotive Marine Relays for Boats Auto Fan Cars : Automotive

[jwestern]

I put the fuel pump on a relay. It did not solve the bogging at full throttle.

While doing so I noticed a few problems with the fuel plumbing, in addition to the sharp 90 degree turn at the top of the stand pipe.

1) The fuel line from the fuel/water separator and the pump inlet is 5/16". It looks to be squeezed over the barbs, like it doesn't fit them.

2) The fitting at the fuel pump outlet is a 90 degree tube (not a sharp bend), and quite narrow in diameter. Not sure how crucial the outlet side of the plumbing is, compared to the inlet side which I understand is rather important. The 90 degree tube is presumably because there's no room to run straight out (there's something else in the way, a starter or trim motor or something, I can't identify it).

Is it plausible that this plumbing is causing the bogging at full throttle?

Also, I might be crazy, but there seems to be an imperfect correlation between mechanical motion of the boat and the bogging. In my last test, the bog seemed to oscillate in strength in phase with the boat's porpoising at 75 mph. Not sure what to make of it -- e.g. I don't think the porpoising was crazy enough to slosh the fuel so far forward in the tank that the stand pipe would suck air. The bogging seems to get worse with time; one run with some friends we topped out at 88 mph due to bogging, an hour later on another run we topped out at 77 mph due to bogging.

Next I plan to empty the fuel tank and pull it forward so I can take out and inspect the standpipe, and get rid of the sharp 90 degree elbow by switching the standpipe to the other fuel tank port. I'll also replace that small segment of 5/16" hose with 3/8" hose. But I'm curious to hear all your thoughts.