[V8] Idle Problem Returns to The Twilight Zone

[Carol, Dave, and Zach Weiss]

I owe you folks (especially Steve B and Alex at IA) the bizarre conclusion
to the idle/ECU problem.

 

A couple months ago I posted about an intermittent high idle, where the idle
would hang at some random value between ~2k and >3k after driving a while.
It would manifest itself when coming up to a stop sign (a thoroughbred
demanding its head, perhaps?).  Turning the car off and back on always
brought the idle back to where it should be.  Codes were idle stabilizer
short and engine speed sensor.  Shop replaced both and cleared codes, but
the speed sensor code kept coming back, and the intermittent high idle was
now occurring after only a few minutes driving.  They also rung out all the
wires from the ECU to the sensors, and cleaned connectors in the engine
compartment.  They did check the throttle for sticking, including the cruise
control bushing, but cycling the power contraindicated a mechanical problem,
anyway.  I have the IA chip, but that does not seem to be relevant.  The car
does also have intermittent instrument cluster issues (water temp and fuel
gauges don't read) but they didn't correlate with the idle.

 

The shop swapped in a computer from an automatic they had for sale and the
car ran ok, although idle was consistently around 1100-1200.  The speed
sensor code can also indicate a problem with the Motronic, so I got one from
an automatic from Bruce at Audi^H^H^H^HGermanCarConnection.  Steve and Alex
offered to help with swapping the chip.  However, the "new" automatic
computer did the same as that from the five-speed, but it would reset itself
without having the car turned off-probably a firmware difference.

 

My wrench had mentioned that he could never get his expensive diagnostic
computer to talk to my ECU, but he had read and cleared the codes with a
"cheaper" one.  Unbeknownst to me he now decided that with another ECU in
hand the 5-speed box was not such a precious commodity, and he did get the
fancy diagnostic computer to communicate by giving the original ECU a couple
of love taps with the heel of his hand!  He cleared the codes and the car
has now run well for several weeks.  It has idled once around 1200, but this
might be the air conditioning?

 

Now all that's lacking is eerie music and a Rod Serling voiceover.

 

I'm hanging onto the other ECU just in case.  There's a chance that both
computers have the same problem, but that seems unlikely.  Because some of
the signals to the Motronic come from the IC, I wonder if there's something
going on there.  Any differences between the 5-speed and automatic clusters,
apart from the speedo constant?  I'd like to have spare parts or at least a
chance of getting them before attempting the resolder fix.

 

Dave

91 5-speed