2000 Mercruiser 350 cutting out under heavy load

[twilson616]

Hello all,
​Got an issue with a 2000 Merc 350 Mag MPI Ski "cutting out" under heavy load and when boat is really weighted down. It's almost like a miss and will drop a 100 rpms or so and then recovers and repeats. My believe now is that it's fuel related, but not positive. Any recommendations on how to test fuel regulator and pump?

Thanks!

 

[alldodge]

Agree, sounds like a fuel supply issue.
What is your motor serial number?

Seeing fuel pressure a\should be at 30 psi

 

[kenny nunez]

In addition to ADs advice on fuel delivery which has to be checked. Also if this is one of the ski boats that can ballast down to make a large “table” and you have a full amount of passengers it may
be possible that the “guardian mode” In the computer is kicking in to protect the engine from being damaged.

 

[twilson616]

Serial number is 0L398364

​​​​​I'm heading to check the fuel pressure at the manifold today. Another question - I see that the fuel pump is only available with the whole gen 2 cool fuel setup as per the service bulletin. Anyone know what changed to prompt this?

 

[alldodge]

Some where corroding in salt water, but you can still get just the pump, just not from Merc.

 

[twilson616]

Tested fuel pressure on the manifold:

With the engine off and key on - 26psi
Engine idling 24psi
Higher rpms drops to 21psi or so.
Remove the vacuum line while idling from the regulator and it goes to 33psI. Does that mean the regulator is doing it's job and the pump is bad?

 

[alldodge]

You should be closer to 30 psi, spec does list 1800 rpm at 30 psi

Take a can of clean gas and a piece of gas hose. Attach the line to the fuel filter inlet and stick the hose in the can and check pressure. If pressure comes up take it for a run that way

Remove the regulator from the cool fuel. Us a air compressor regulator and attach air to the bottom side of the regulator and slowly increase pressure. The reg should open 30 psi or higher

If Reg tested good need to test the pump it would need to be on the bench without the regulator. The pump should put out 50 to 60 psi if blocked off.

 

[twilson616]

Very much appreciate the info!!

 

[twilson616]

Regulator bench tested perfect. Opened right 30psi

​​​​​​Couldn't get pump totally blocked off, but it tested over 100psi? I did have some small leaking in my setup

Any thoughts?

 

[twilson616]

Oh and the pressure didn't change after some idling on a fresh can of fuel. Still 24 or so at idle.

 

[alldodge]

Haven't seen a standard Merc pump get much above 70, so 100 is hard to understand. Hope your gauge is good

I would think its the regulator.

 

[twilson616]

Just tested the fuel pressure gauge with air and it matched with my regulator within a pound or so up to 100psi. Pump has 861156a1 stamped on it so presumably it's the original or oem replacement.

Dumb question.... How do you normally get a sealed connection on the output end of these pumps. Just an appropriate sized hose and clamp?

 

[alldodge]

Have to have the right fittings.

The path for fuel supply is filter, pump, Regulator and fuel rail. Anything over 30 psi goes back to the filter and repeats.

 

[twilson616]

Wanted to post a follow up with the resolution for anyone that searches this and finds it later....


Fuel pump and regulator bench tested perfectly. I replaced them and the corresponding o rings anyway because I already had it apart. Did not fix the issue. Over time I figured out the issue was more about components getting hot and less about being under load. Once the issue started happening - usually after 30-45min - I could reproduce the issue by getting it in the rpm range of 2700-3500 even without ballast full and the boat under no real load.

While taking fuel pressure at the manifold it was constant at around 29psi even with the issue happening.

​​​​​​Running out of options, I went to replace the ignition sensor in the distributor. Found the plastic broke on one side of the pickup. Replaced and haven't had the issue since.

If I could do it again, I would take fuel pressure while it was having the issue first. Also, the abruptness of the miss probably points more toward ignition than fuel in hindsight.

 

[spunkyman]

thx for this ! Gives me some options with my problems...