2007 Crownline 21SS 5.0 Mercruiser

[Lamb 1]

Started backfiring through the intake and losing power under load. When trying to plane boat it starts to backfire before she is even planed out, around 3000 rpm. Starts and idles fine. I have changed plugs, distributor cap, rotor, fuel filter and fuel pump with no success. Help!!

 

[tpenfield]

:welcome:

Carb or MPI?

got a serial number for the engine?

Did you check the ignition timing?

 

[Lamb 1]

It is a MPI (throttle body). Checking the timing is my next step. Checking for trash in the gas tank, changing the sensor in the distributor, checking the vent for gas tank, changing the map sensor on the throttle body and changing out plug wires. This engine has never been touched before. Gas is clean by dumping out fuel filter and checking. Burned all of gas left over from last year and filled with a new tank with sea foam added. I'm stumped. Serial number is 1A604906.
Thanks

 

[alldodge]

Suggest before you do anything, replace the cap and rotor, they go bad all the time, and can look perfect. If it isn't the cap, keep it in the tool box for when it does happen

Edit: you already said you replaced the cap.

Check fuel pressure

 

[Palone]

MPI is fuel rail direct injection. EFI is a throttle body. Sure sounds like a timing issue to me.

 

[Scott Danforth]

if you have the crab cap (plug towers out the side), replace the cap and rotor. they need replacing every 2 years or so.

 

[Lamb 1]

Thanks for the feedback. As I previously posted I have replaced cap and rotor. I will check timing then replace map sensor. I think it is a fuel issue but I'm just a shade tree mechanic.

 

[alldodge]

The timing of your motor is not checked with a timing light. Bring number 1 piston to TDC then check the position of the rotor to the cap as seen below

Your fuel pressure should be 43 psi at 1800 rpm