my cappucino machine still goes vroom

[Bryan Lally <bryan@mst.lanl.gov>]

The cappucino effect in my cooling system, which you may remember, was
successfully fixed (new pressure cap).  This problem was only a slight
annoyance in the middle of the miss at speed/vroom at idle problem
previously  described, which I have been unable to fix.  I now ask for
advice of the expensive kind ...

Symptoms in short:  at speed, engine occasionally misses, badly, causing a
shudder in the driveline.  At idle, engine occasionally vrooms up about
3-400rpm, immediately returning to normal.  Either event is coupled with
the voltage at the fault code readout point to glitch upward towards 12v,
from the 1.5v that is normal.  The glitch is very short, the response time
of an analog meter limits the meter to showing ~5v.  An oscilloscope shows
the 12v.  12v is what the test points show when the car is off as well.
This glitch _always_ happens in conjunction with the other symptoms.

Car is an '89 80, 2.0l 4 cylinder, no Q, no turbo, Motronic CIS.  73000 miles.

I've been all over the sensors, the harness, pretty much everywhere I can
think of.  Ruled out just about everything.  It looks to me like I've got
an intermittant in the power supply section of the Motronic unit, and when
it burps, the computer resets.  I can't really test this theory without
another computer, which I don't want to buy if it's not really the problem.
 Have I overlooked something?  It will miss with the idle stabilizer
disconnected.  The O2 sensor shows the normal oscillating voltages, a
resistor of the proper value was substituted for the temperature sensor,
etc.  We watched the 12v in to the motronic, and it looked clean.  Any
theories have to explain the voltage glitch.

So, _any_ suggestions?  Can the computers be tested?  Anyone have
experience with computers from salvage yards?  Prices?  Any idea where I
might get one - not too many of these cars out here in New Mexico.  Or, do
I just take it to mr. dealer, and tell him to fix it (ouch - he'll do all
that I've done, except at $67/hour, and the computer is $7-800, the engine
wiring harness is $1000+, and the car is only worth $4-5000).

If you need more details, let me know and we can take it off the list.  Or,
if you've got a spare computer in the garage ...

And thanks, as always.


	- Bryan

'94 BMW K75S		_red_
'94 Honda XR250L	with extra needles and thread
'89 Audi 80		I think I can, I think I can ...