[V8] Intermittent High Idle Speed

Carol, Dave, and Zach Weiss cdzweiss at charter.net
Wed Nov 2 21:25:20 EST 2005

For those that love a puzzle--  ;)


When coming to a stop, clutch in, engine speed occasionally catches
somewhere between 2000 and 3200 rpm (usually closer to the low end).  "Idle"
is rock solid at those times, just high.  Revving the engine has no
effect-engine returns to whatever speed it's seemingly trying to idle at (at
the time).  Turning the car off returns the idle to normal.  The engine runs
well other than this-plenty of power, no misses, etc.


I let my mechanic look at it, and he at first thought it was the accelerator
cable.  I didn't see how that would explain the off/on reset, and sure
enough, although there was a definite hitch in the cable, the problem is
still there after replacement.


Now he says it's a combination of engine speed sensor (he got an
intermittent code, although I don't know which one) and a slow ISV.


Seems like speed sensor or slow ISV would cause idle to be erratic.
Sticking ISV or throttle plate should cause code 2231 idle adaptation out of
range.  Could this be the idle switch (but if so, wouldn't it cause a 2121)?
If not, is it likely an ECU problem?


Recently rebuilt top end eliminated excessive oil consumption.  Ned's chip
has been installed for years.  Only other problem has been an occasional
miss on lift throttle that disappeared after a new O2 sensor, long ago.


Dave Weiss

'91 5-speed

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