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Rev falloff


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Do you get a SES code 39 for clutch switch position? 

Sounds like it could be a IAC issue... on the 92/93 5 speed cars they use the clutch position switch along with the IAC to help keep the revs from falling flat during shifting.

So check the IAC but also check and make sure the clutch position switch (at the top of the pedal, not the starter safety on the floor) is in adjustment.

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Interesting thought.  No code, but I'll check that switch at my next opportunity.  I did a quick IAC relearn this morning but it didn't help.  My other idea is to disconnect the battery to clear the calibration, then do an immediate IAC relearn and hope they get along better that way.  I've felt some weirdness for a long time, but now it's really getting on my nerves and it seems like the IAC is out of sync.  I was ready to learn how to reprogram the ECM, but I think this is an actual malfunction rather than a tuning issue.

Edited by crazyd
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It very well could be the iac valve is bad. Possibly the TPS after this as it gives data to the ecm. These are both pretty cheap parts to toss on there. I would think the IAC valve would just  up and quit though so no gradual worsening. I’ve replaced both of these many years ago but not for any particular reason.

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I think I've got it.

So I started off checking the upper clutch switch.  This I'm assuming is the same one that disengages the cruise control, the white one that screws in.  If that's the case, yeah my cruise disengage was working fine.  But continuing on, first thing I noticed was a lot of fore/aft play in the clutch pedal.  I disconnected the plug and screwed it in.  Must have turned it at least 10 times to get rid of the play.  It's nice and snug against the pedal bracket now.

Next, moving to the IAC itself.  This is a procedure I modified from the Corvette Forum for W-body.

  • Step 1: disconnect battery for 10 seconds and reconnect, to reset ECM calibration.
  • Step 2: Insert diagnostic tool into OBD1 port to short pins 1&2.
  • Step 3: Turn on car but don't start it, for 30 seconds.
  • Step 4: Disconnect IAC plug, leave diagnostic tool connected.
  • Step 5: Remove throttle cable cover to access idle adjustment screw (10mm & T15)
  • Step 6: Start engine and allow idle to settle.  It may struggle to idle at all at this point.
  • Step 7: Adjust idle to a smooth 950rpm with T10 adjustment screw. Clockwise=higher.
  • Step 8: Shut off engine, reconnect IAC, replace throttle cable cover and disconnect diagnostic tool.
  • Step 9: Start engine and allow idle to settle.


My idle speed adjustment was WAY off.  It was barely idling at about 500rpm.  Now it starts better, idles smoother and does so without needing time to calibrate.

I'll see how it drives over the next few days, but I expect major improvement.



Edited by crazyd
Correct sequence for step 4 and step 8.
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