Help My Please ! 500 EFI Mercruiser Backfire !
 

After running after 15 min one of my motors is backfiring out the stainless marine risers , I changed the main harness , new coil , new cap and rotor , new spark plug and wires , 42 pounds fuel pressure off the rail at start up , cruise , and wide open ! Even when it's backfiring ! Checked the tank no anti Syfin ball ! Line clear ! Now taking the injectors out to be cleaned !!!!! Even tried disconnecting the tach !!!!! Help me !!!! 305,496 , 9006 text me or email at [email protected]

Check your headers. Could have a leak letting water in to the engine. It will cause a backfire too.

Will agree with above to ^^^^^

Ignition related - pick up coil and or module in the distributor.


will add but your engine reaches WOT ??? If not then - Possible a exhaust valve problem and or valve train problem - valve springs as well. I would pull the valve covers at the very least here to check for a sucked valve and or valve spring(s) Again, I see where you said WOT so you can hit max recommended RPM's or NO -

could be fuel injectors never rule them out.

Testing ideas as well

Compression test and Leak down test ?

fresh gas and gas samples - anything - crud lifting off the gas tank ?

Scan the engine ? Fault codes / sensor parameters ? Timing ?

You are able to hit WOT meaning that your engine acts like ITS NOT down on power ? If so then that would rule out broken valve spring(s) and or sucked valve(s). Just want to be clear on this.

Thank you guys ! Now I will change the pick up , under the rotor , and sent the injectors out , oil looks new and perfect , spark plugs look great . When you say headers do you mean the stainlessarine risers ?

Quick volt drop test , 2 black wires from ecm it uses to ground the 2 banks of injectors. Some good current draw there at higher rpm , volt drop at red wires to injectors under load.god forbid , dont install a no name module or pickup coil in that boat please.

a dirty injector can cause lean backfire.

how many hours on the engine?

I'm guessing broken lifter or valve seat. You're doing a lot of parts changing with little diagnosing. Slow down and find the problem

Still has not answered if he can run WOT at the MAX rpm range, never seen a marine engine that can reach MAX rpms with bad / and or broken internal components Just saying so we keep throwing darts at this one. Bullseye

Wide open throttle no problem ! Only happens after 15 to 20 min running , just had all the injectors done will install tomorrow . If my stainless marine riser is leaking will that cause a back fire ? And if so why doesn't it do it when I start the motor ?