MAF meters and open-loop.

[Daniel Houlton]

I have an Isuzu 2.6l engine that has a Hitachi EFI system using 
a hot-wire type of MAF.  I've recently added a turbocharger to it
and I'm noticing something a bit odd when I go to WOT.

I'm using on of those Cyberdyne A/F ration monitors (uses an 
LM3914) with the 10 LEDs.  When driving under boost (about 5 or 6
psi) if I keep the throttle under about 3/4, the meter shows a
slightly rich condition.  But somewhere just after 3/4 throttle,
the meter drops completely off (full lean).  

It seems to act like an on/off switch for the meter.  If I go past
this spot in the throttle, it goes full lean.  I then immediately
back off slightly and it goes back to slightly rich.  It does this
consistently and repeatedly so I don't think it's a quirk in the
meter.  By moving the throttle up and down about 1/4" I can get
the meter to basically flash on and off as it goes from slightly
rich to full lean.

This spot seems to be about where I expect the WOT switch would 
kick in and switch the computer to open-loop mode.  Why would it
go so lean though?  I can understand the computer compensating for
the boost through monitoring the O2 sensor, but why would open-
loop mode screw up?  The MAF meter is still measuring the air 
coming in so it seems the mixture in open-loop should still be 
pretty accurate.

Does an MAF meter not measure accurately in turbocharged 
applications?  The meter is upstream of the turbo BTW.


thanks
--Dan
houlster at bigfoot.com
http://www.inficad.com/~houlster