496 mag strange intermittent problem
 

please help if possible 496 mag intermittent problem, while driving the boat starbd motor starts to run crappy , black smoke, lower rpms than other motor, if i shut off motor then the battery and wait a sec it seems to reset something, then engine runs fine, seems like an overfueling problem, stuck injector? idk one thing i noticed was when this condition is happening when you turn the key on fuel pump should prime for a few seconds then shut off , while in this condition it never shuts off. any help is appreciated.

Fuel pump doesn't shut off. That is different but when the motor is running the fuel pump will not shut off.
Check fuel pressure and need to scan for codes
Also could use your motor serial number?

yes i know it doesnt shut off while running , i shouldve specified that not while engine is running but when i turn on key to prime the system before i crank motor the pump will just keep running, seems like something is just causing fuel to dump in and not combust i will try to scan for codes and i will check fuel pressure asap. serial number M 025579 496 mag. thanks for any help

I'm thinking the injector driver but need the scan to verify

ok i should be getting it scanned tomorrow, i hope, i will let you know asap.

also is the injector driver an internal part of the computer? just curious?

Yes, it's internal to the ECM

so havent had it scanned yet, waiting on adapter for the 496 then ill see, thanks again Bob

so i switched computer from working good motor to running bad motor, did the same problem to the original running bad motor thus i believe ruling out a computer issue. also test pressure at rail turn key on 43 psi, then slowly drops then start motor stays steady at 36ish psi both motors do exactly the same numbers.

As far as fuel pump staying on, i would suspect relay first ? I think.

Just a heads up, it's not recommended to put a good ecu on a bad engine. If something damaged the ecu, you will have 2 bad ecu's than. Always put the bad ecu on the good running engine.

Just my 02

Just a couple of stray thoughts from working on cars years ago:
If the coolant temp sensor comes unplugged (or wire breaks or bad connection, etc) the ECM will interpret that as coolant being at -55 degs F and enriches the mixture significantly.
Bad connections for MAP sensor (or a bad sensor) can cause a similar issue. ECM thinks eng is under a serious load. (A split vac hose for MAP sensor can cause the same issue but wouldn't go away after a power cycle)

Anyway, something to look at while awaiting the code reader results.

thanks for thatinfo i agree, but i think switching them and the problem occurring again ruled out a computer causing it.

so had the motor scanned, no codes at all, also checked fuel pressure 43 at startup, after running stayed at 38ish, both motors exact same readings, still happening

thanks to everyone and their suggestions i might have figured it out , i replaced plugs and wires, plugs looked good but did them any way, i think wires were original , i think at high rpm the plug wires were arcing somewhere and messing with the computer, seems to be fixed but more test runs with no issues and ill be convinced. thank you all again. bob

.060” is too big a gap. Plays havoc with everything.

What gap is best?

Normal plug gaps. .035”-.040”
.060 was derived from gm for emissions in trucks. Too much for engines working under lots of load and higher rpm (boats) ,all the time.

I believe (double check) the factory plug has .709” reach. If that’s the case, as example, the ngk tr-5 and tr6 has .039” gap. It is conventional with vpower electrode. Who cares about platinum or iridium plugs in boats, especially with such high gaps they need changing as often, or more often, than conventional plugs anyway.

These smaller gaps will make your wires and coils last exponentially longer too. :thumbs