I just had a 1995 502mpi, according to ser #, rebuilt, bought it from a guy in pieces. Biggest cam and still be safe w/ cmi headers. I am not sure if the accessories are orig to the block because it has cool fuel and single belt. Eng sounds smooth and runs well but I am running the exact same rpm and speed as my old tired, stock 454 it orig came with. 57 mph 4700 rpm 23 " mirage on a 27' powerquest, sea level,gps. I think am running in safe mode, after 3500 rpm the rest of the throttle is uneventful. Here is I believe the issue- I am getting 2 codes 21 and 42. I have replaced the ign module at the dist. and checked the 4 wires going to the ecm for open and short to grd. Checked tps for smooth resist chg through out rotation and 5 v from ecm. cleared codes, started eng, both come back. ecm part #16234539. Bought used ecm, 16189069 no codes. Now after run still code 21 tps but 42 is gone now getting code 33 map sense high volt. Checked 5.04 v from ecu. Do not have scan tool just meter and test light. Reinstalled orig ecm, back to codes 21 and 42. Any accurate help is greatly appreciated. One last question, are these codes keeping the eng in safe mode, if I can get only code12 flashing will the eng wake up. Base timing set to 8 then paper clip removed from data connector.

Anybody have any suggestions? I am going out w/ a mobile mech today. Hopefully a scan tool will offer some new insight.

throttle position sensor and and a est(electronic spark control),,time to start looking at the harness and if thats not it,,ecm time.

Went out today w/ a mech and scan tool. He never saw code 21 or 42 but saw a code for o2 rich, I did not think my ecm was looking for o2, completely confused. I did learn today , from merc cust serv that the # on the back of the ecm is only the part # for the box and not the software inside. I am seriously thinking of going w/ a carburater. My biggest problem is that the block is a gen 5 1995 but someone put newer parts on it- cool fuel, single belt, roller cam, I don't know what wire harness is on it or if I am running the right ecm.

If you go carb you should at the very least weld a bung in to the exhaust and run a wideband. Since you have the bung EFI may still be an option.

This system has great reviews but is largely untested in the marine environment though.

http://fitechefi.com/30002.html

Thanks, I'll keep that option open but I just purchased a used harness and ecm from a good running eng.