Motronic 4.1 newbie--still struggling

[Derek Kaznoski]

I checked the engine temp. sensor, its OK (thanks for the suggestion).

I just want to confirm:  the AFM flapper IS supposed to be slightly open at 
idle, right?

Now for some more novice troubleshooting:

I disconnected the idle air bypass hose from the intake ducting and left 
this end open to the air.  I plugged the hole left in the intake ducting.  
As I expected, the car would start and "pseudo-idle" (with flapper reference 
advanced on the AFM potentiometer to increase resistance, as before).  
However, when I blocked the open end of the idle air bypass tubing, the car 
stalled.  Thus the idle speed actuator must be working. (previously, I had 
just clamped the tubing to test this, but I must not have completely blocked 
the flow)

I disassembled all of the upper intake ducting, and found no holes or 
potential leak spots between the AFM and ISA or throttle assembly.  It seems 
to me that anything causing any leak into the plenum chamber would 
effectively decrease the vacuum in this chamber and decrease the amount of 
air flowing through the idle speed actuator (and thus, through the AFM). 
So,I also checked everything that connects to the plenum chamber (oil vapor 
hose, fuel vapor hose and solenoid, fuel pressure regulator tubing) and 
found no leaks.

I can hear a distinct hissing sound coming from around the bottom of the 
plenum chamber where it intersects with the engine or just forward of this 
area.  This hissing intensifies just after the throttle is blipped and 
before the engine responds.  It continues with decreased intensity as the 
rpm's rise.  To me, this sounds like an air leak, but I've never heard one.  
Could this sound be coming from something else, such as circulating coolant?

When this idle problem first started (weeks ago), I checked the rubber 
sleeves and clamps that seal the joints between the plenum outflows and 
intake runners of the cylinders.  I noticed that all of them were slightly 
loose and had worked their way toward the engine side.  I could even see the 
joint past the edge of one of them.  At that time, I loosened all of them, 
moved them back toward the plenum side, and retightened the clamps (really 
tight).  I guess I could've damaged them in the process since they were very 
difficult to get to.  I can't see any obvious damage, but I can't see the 
underside of them.

What is the best way to localize the leak (specifics please!)-- I've read 
about using a propane torch, but I'm a bit leery of this.  Also, what could 
be other potential sources for the leak if there is one?

Thanks again in advance.
-Derek Kaznoski
'90 Alfa Spider


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