FYI, the MEFI3 does have O2 sensor feedback if the option is enabled. GM apparently went cheap on the original RamJets and didnt provide the feedback.....I think that they didnt provide incoming air temp sensor either. I am running a MEFI3 with feedback enabled and it works great......IF you keept the water temp at the right level. For us old guys, it goes against our background to put a hotter thermostat in! GM had problems with idle stabillity on the 3.....caused by their El Cheapo no-sensor approach.

You can get a scan tool at Rinda http://216.230.203.183/marine/mtspec.htm
for this controller.

Arizona Speed and Marine sells an upgrade to enable O2 on a MEFI3. They live at http://www.azspeed-marine.com/

Also, Scott Urshel is knowlegeable. He is also in Phoenix at
http://umiracing.com/

MEFI4 is a later version. It has some dual engine balance reporting that a 3 didint have.

MEFIs were originally marine computers so they dont have all the interlock junk that a normal ECU has. Regardless of altitude, temperature, or gas, mine runs great!!! I'll never run a carb again.

mike in tucson