4.3 Mercruiser fuel supply issue

[Jeff J]

Working one that started out as a “won’t start” on the lake but checked good at the dock after it was towed evolve into “won’t start” after fuel filter change. I installed a primer bulb and it started fine after pumping it up. Power starts dropping off after a minute or so and eventually gets to the point it won’t run at idle. Fuel pressure on the rail is about 30 whether running or not. I did not run it across the lake with the gauge installed. Is there something else I should be checking before I dig my way down to the pump?

 

[alldodge]

How about a serial number?

Mentioned about 30 at the rail so it should be MPI but if that's the case it most likely should be 43

 

[Jeff J]

Serial number 2A496785. Yes fuel pressure should be quite a bit higher. Have a Volvo Penta that is hard to start cold because of low fuel pressure (I think).

 

[alldodge]

Thanks the S/N shows Mercs new 4.5
Pressure should be 43.5 within 3 seconds of key turning ON
Should be 37.7 psi at idle

I would install gauge and leave it when running
https://www.crowleymarine.com/d/tec...ower-unit-technical-data-2017-lit-18616-03-35

 

[Jeff J]

I hesitate to leave the hatch open while running wide open across the lake so I can read the gauge. Hatch removal is a PITA in the shop let alone in a slip.

It pretty much has to be losing pressure. The symptoms are the same as a clogged fuel filter. Let it set when it is primed or pump the primer bulb and it fires right up hot or cold. Runs great for a bit then starts losing power. Reduce throttle and it gets better for a bit before power falls off again. The first time it did it to me I changed the filter and it died just before getting back to the dock on the 5 minute test drive.

Pump is conveniently (NOT) located under the engine on the port side so wanting to “know” I am out of options before tearing apart the boat.

 

[Jeff J]

Decided to dig into it. There is supposed to be a fuel pump here somewhere. That may be it behind the motor mount. Can’t see what holds it.

Some days I believe the only good engineer is a dead engineer. There ought to be a law that requires them to service what they design.

 

[alldodge]

The Pic looks like oil lines to a cooler. Have not worked on a 4.5 but the pump is connect directly to the fuel tank line. Think it's on the starboard side

Check the fuel tank vent line next time it dies. Open gas cap and listen for air sucking in.

 

[Jeff J]

It’s a 4.3. coolant lines, oil, lines, fuel tank, fill, and vent hoses all in the way the fuel line from the filter crosses under the motor and comes up behind that motor mount and goes into something covered by a black plastic box.

After seeing that, I decided to load it on a trailer and take it to the shop.

 

[alldodge]

ok, the cover just pops off

The SN came up as invalid so I change the last digit and it came back as 4.5
Sorry couldn't help

 

[Jeff J]

Just letting me know the cover pops off helped. The diagrams in the book aren’t much help. Thanks.

Motor hot now. Will tie what I can out of the way and lay down in there again after it cools some.

 

[alldodge]

With the pressure being low (should be 43 psi) it's either the pump, regulator or a restriction. If it never gets above 30 even at idle with primer bulb, then either pump or regulator but most likely the pump

The pump is a pain to get at as you can see. The regulator is on top the cool fuel module over the pump

If you disconnect the return line coming from the pump back to the fuel filter (arrow pointing in on filter head). Seal off the filter head and seal off the return line. Have gauge on fuel rail and turn key on/off, maybe a couple times to build pressure. It should go above 43 psi but try not to let it get much higher by using the pressure relief on the gauge.

If it gets higher sealed off then it's the regulator, if it doesn't get higher then its the pump
https://www.mercruiserparts.com/bam/subassembly/31507/13808/60

 

[Jeff J]

Perfect timing! I was trying to find a way to test which is bad. I also just noticed the diagram in the manual also shows a filter under the regulator.

It appears the steering hoses and a cooling hose are my biggest obstacles to seeing and removing the cover. The oil hoses pass above the engine mount and I was able to tie the fuel tank filler and vent hoses out of the way.

 

[alldodge]

The cover snaps on in the middle top and bottom
Need to press inward

 

[Jeff J]

Can’t get the cover off. I don’t have the reach/access to the bottom of the cover. Even if I did there is less than 1/2 inch between the boat and the cover. The only thing I can think of is to pull the motor but I am not set up to do that. Is there a work around for the fuel pump?

 

[alldodge]

There are 2 nuts (should be 9/16) that attach the pump to the motor mount. Remove the nuts and bracket and pump are disconnected. Disconnect fuel line and electric and it can be pulled out

 

[Jeff J]

Thanks, I didn’t lay my head on floor to look past the oil lines.

 

[QBhoy]

Hi. Under the air filter and in the middle at the top of the engine, there is a fuel line cross over from each side. On that cross over pipe there is a small fitting with a black plastic threaded cap. You’ll likely know this already if you’ve had a gauge on it by now…Take this off and press your finger into the valve. If there is air in it..you’re narrowed down a few things and have a direction to go in. I know you’ve checked already. But make sure that vent for the tank isn’t blocked. Make sure the filter isn’t drawing air too. The sound of the pump can also tell a story too.

 

[Jeff J]

I installed a primer bulb between the tank and filter for testing at the dock. It allowed me to start and drive the boat to a ramp. Normally I would think it is safe for me to assume any leaks would show when the system is pumped up tight with the bulb. Is that wrong on these?

I have been a mechanic for ~40 years but not on boats or EFI engines.

 

[QBhoy]

I installed a primer bulb between the tank and filter for testing and it allowed me to start and drive the boat to a boat ramp. Is it safe for me to assume any leaks would show when the system is pumped up tight with the bulb?

Likely not. You’re forming a positive pressure from the normally low pressure suction side of things. Likely bypassing any chance of the normal arrangement, having the chance to draw air or be easily restricted perhaps. You may well be right in thinking that sort of thing isn’t to blame../but certainly I’d suggest it couldn’t be absolutely ruled out, if the thinking is that your method does so. Hope that helps. I’d be definitely having a look closer at the fuel delivery system and it’s ability to easily pick up and flow fuel as it should.

 

[Jeff J]

Thanks, I reworded a little in my last post which messed up a quote but I didn’t really change what I was trying to say.

I can check for air in the fuel rail before I drop the pump. The last time the engine was run was to trailer it. I don’t believe I have turned on the key since.