Diagnosing GM efi..

[John Hess]

Yeah!

Fix the vacuum leak and then start working on the rest of it.  Your MAF is
sending false signals if you have a leak bypassing air into the system that
the computer doesn't know about.  I had the same problem when I first
rebuilt my Corvette.  Fix the leak and most of your immediate problems will
go away.

At 01:32 PM 6/17/98 -0400, Christopher Bruno wrote:
>Hey all,
>	Finally got the efi setup going in
>my truck. At first, started up nicely
>and idled nicely and drove OK.  Using the
>scanner, the BLM was up to 158, so I'm
>thinking I have a huge vacuum leak (however
>they are only 18lb injectors on a stock
>350 - fuel pressure bumped up to ~52psi)
>Then it wouldn't start easily (started, 
>reved up and then stalled).  After some 
>long drawn out testing, I concluded the
>IAC controller is OK, I cleaned the valve
>itself and when started up in diag. mode,
>the IAC motor does 'spit out' the actual
>valve (drives it out). ESC is OK and TPS
>seems OK.  So then I unplug the MAF and
>it starts good and idles well (with no MAF)
>but it runs like crap when I put it in
>drive and try to drive it (except under
>WOT).  Now, it still has the PROM for a
>305 but I figured it ought to run better
>than it does with the stock chip and a
>vacuum leak.  It hesitates and coughs and
>spits when there is any load on the motor.
>If in park/neutral it revs up nicely (though
>possibly a little poor on the throttle
>response).  The interesting thing is that
>with the MAF unplugged, the scanner (Diacom)
>indicates that MAF reading is going up (???)
>as the motor is revved.
>
>I have also read that the TPI's actually run
>quite well with the MAF unplugged (TPI engine
>swapping, by JTR).  Now, not only does this
>seem to not really apply to me, but how could
>this even work well? 
>
>Comments?  TIA
>-- 
>Christopher Bruno
>84 S-10 Blazer 4x4 w/350
>    & 85 ECM,TPI for 305
>	Car is currently wearing cone shaped 'bra' and facing
>	corner for running so poorly...
>
>