[Gmecm] Re: Gmecm Digest, Vol 11, Issue 11

[Cowen]

Steve,

I'm certainly no expert, but I'll toss in what came to
mind.  This problem could be from one of two causes:
Load change (and hence RPM drop), or engine movement.

I was wondering if your Blazer had this idle problem
in reverse?
If the problem happens in reverse too, I'd think maybe
the problem was related to the load on the engine
increasing, which affects RPM, so the ECM is trying to
compensate with the IAC.  And it is compensating with
the IAC!  But, the question I wonder is, why doesn't
the ECM know the idle has gone up appropriately?  You
have TBI, so no CPS I think, could the distributor
reference line to the ECM be shorting/seeing RFI from
somewhere/ corrupting the signal to the ECM so it's
not sure what RPM it's really at?   

I don't think MAP would be much affected by the
minimal load change going into gear (MAP value might
rise slightly as the RPMs fell and the vacuum
decreased, but if the ECM was responding to that, it
would respond to the RPM increase too!).  Certainly
the TPS, CTS, and IAS (motor itself) don't care about
load, so I think you can discount them as well.

It's not the IAC itself, because that works fine in
Park or Neutral.  It shouldn't be the PCV, EGR, etc,
because they're the same in drive or neutral, so if
the work in Park, they'll work in Drive.  

If the problem does NOT happen in reverse, it's
probably not associated with load, but maybe with the
movement of the engine as it torques in it's mounts. 
If the problem doesn't show up in reverse, I'd
definitely start looking for which wire is rubbing
through and intermittently grounding/shorting.  How
about the power steering pressure sensor, or an AC
compressor request, or CTS grounding, or that
distributor wiring, or???

Of course, you could just pop it into Neutral every
time you want to slow down, stop, or idle! :)


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