2003 mercruiser mpi 5.0 won't run under load

My buddy and I are stumped on this situation. He has a 2003 bayliner ciera sb with a 5.0 merc mpi. Boat will idle for a few minutes then gets harder to crank and eventually all but won't idle. When put under a load it backfires and dies. Seems like its losing fuel to me but I can't find a shrader valve on the fuel rail to test the pressure. Serial # is 0m641478. Any advice would be greatly appreciated. Has a Mallory distributor msd ignition and replaced iac 2 years ago. Runs same with and without the msd.

Re: 2003 mercruiser mpi 5.0 won't run under load

Has a Mallory distributor msd ignition and replaced iac 2 years ago. Runs same with and without the msd.

Click to expand...

You can't just change distributors to some unknown aftermarket distributor on an MPI engine. The distributor has to be able to talk to the ecm and the ecm has to understand what the distributor is saying.

Re: 2003 mercruiser mpi 5.0 won't run under load

Boat ran great all last year with that ignition set up. Sorry I didn't clarify that part. I feel like the fuel pump may be getting weak. It will rev to about 3/4 throttle in neutral then starts wanting to die, but when under load it wont even idle. Anyone know where the schrader valve on this thing is? I don't see one on either fuel rail.

Re: 2003 mercruiser mpi 5.0 won't run under load

Bump! I'd like to take this thing out today for the holiday. Please help

Re: 2003 mercruiser mpi 5.0 won't run under load

Does your distributor and coil look like this.


The schrader valve for checking fuel pump pressure is on the fuel rail (18 19)

Re: 2003 mercruiser mpi 5.0 won't run under load

it looks like that with the flat cap instead of a standard cap like found on a car. It is a mallory brand and has some sort of plug on the back of it but there is nothing plugged into it.

Re: 2003 mercruiser mpi 5.0 won't run under load

That is not a Mallory distributor, it's the GM High Voltage Switch. The plug on the back is the cam position sensor, but is not used by Mercruiser, Volvo does use it however. It has a composite body. The cap is not designed like any other distributor cap.

Have you checked the water separating fuel filter for water, I don't mean just change filters, check the old one for water.

Re: 2003 mercruiser mpi 5.0 won't run under load

Don S said:

That is not a Mallory distributor, it's the GM High Voltage Switch. The plug on the back is the cam position sensor, but is not used by Mercruiser, Volvo does use it however. It has a composite body. The cap is not designed like any other distributor cap.

Have you checked the water separating fuel filter for water, I don't mean just change filters, check the old one for water.

Click to expand...

Has a new water separator installed this year. I have taken the water separator apart and poured it into a bottle and looks like 100% gasoline to me, nothing separated into the bottom. pretty much stumped at this point if its not the fuel pump.. 25 miles to the slip to check it out so im trying to get my plan of attack together at this point.

I feel like its probably the fuel pump getting weak because it will go to about 3500 in neutral and after that it will start to die and you have to work the throttle to keep it running, if you put it in gear it all but dies. I know that when a load is applied the fuel pump has to do more work so I'd say that is probably the culprit. But my confusion is why it starts and idles perfectly on cold start but then once it dies it's hard to even get it to fire.

Maybe the pump is priming enough fuel to get it to idle but cant keep up with the demand at high rpm or under load and dies and needs more priming to get to start again?