509 fuel presure

baja383 · 10-03-2006, 10:17 AM

I need some help here. I have a 509 with the 741 cam EFI motor. It currently set a 48 PSI at idle. It is running really rich and leaves black soot all over the back of the boat. What is the base fuel presure for a 502 at idle.

Thanks Dave

kennyo · 10-03-2006, 10:37 AM

I'm running 32 psi at idle on a 509 with an M3 sc procharger.My regulator ramps it up to 70 psi under boost. I'm not sure what your wot psi should be but 48 is high for cruise and idle.

mjb · 10-03-2006, 10:44 AM

The "base" fuel pressure is really a function of the ECM(and injectors). You did not mention whether yours has been remapped. It certainly should be with that cam change.
The stock fuel pressure depends on which MEFI system you have. I'm pretty sure that the pre '98 systems used 36 psi while the '98 and latter used 43 psi....but again your ECM should be remapped to match your cam and any other engine changes and whoever does your ECM should advise you on what fuel pressure to use.

baja383 · 10-03-2006, 05:21 PM

The ECM was remapped by ASM when the motor was built. I called them and they said to either send back the computer or turn down the presure 3 Lbs. With them agreeing to turn down the fuel presure would it be safe to assume their first recommendation of 48 PSI was to keep it Rich to fend off and lean conditions. Anyone know of Marine ECM tuners here in Nor-cal.

Thanks for you responses
Dave

mjb · 10-04-2006, 09:35 AM

ASM also did my ECM's for my 509's with a similar cam. They also told me to use 48 psi. That pressure seems to work well for my set up. No evidence of black soot on the transom after 2 seasons and it idles well.
There are only a few people in the country that have the ability(software license) to remapp the mercruiser ECM's. The 3 that get mentioned on this board all the time are Jim at ASM, Mark at Precision and Tyler Crockett.