[urq] the infamous 4000 rpm cutout is back

[Louis-Alain_Richard at computerhorizons.com]

I don't know if I should be happy or sad....

After an evening of dismantling and testing, I can say that yes indeed I 
repaired the car.
Good news.

But I don't know what I did to repair it!
Bad news.

In fact, I followed this list many advices and unplug the ECU multi-plug 
and tested ALL the combinations explained in the IST. No problem, all was 
within specs except one resistance: the idle switch resistance when closed 
was 4 ohms (instead of zero) but it was infinite when the throttle was 
open (so this is correct). 

Plus I searched for a bad connector (for the air temp sensor) and I didn't 
found any (is there really a connector by the right side shock tower? Not 
on my car). 
So I unplugged and checked all the other connectors (speed and reference 
sensors, O2 sensor, coolant sensor, etc...) under the hood and none was 
showing corrosion or broken pins.
The only "suspect" thing I found was this: the air bypass valve (for AC 
fast idle, bolted on the right side shock tower) connector was lying there 
unplugged. Since I don't have AC, I can't say if it was lying there from a 
long period or I kicked it accidentally during my checks. Anyway, this 
should't have any effect on the 4000 rpm cutout, right ?

So after reinstalling everything, I did a road test: Yeehaaaa! as Dennis 
said last week.
I know turbos love cool weather, but Mr. Butt-dyno was showing a nice 
clean powerband, all the way to the redline. I guess new Bosch Silver 
sparkplugs and an open muffler are the way to go for rev-happy engine?

Regarding the plugs, I have a question: this is the first set of plugs 
that are not light-brown in color when replaced. This set was gray-ish, 
and even if all 5 looked the same, I am wondering what caused this non 
regular color? The only difference between this set and all the other 
Bosch Silver sets I replaced is the heat range: the gray-ish plugs were 
WR5DS and all the other sets were WR6DS. Could it be only that?

Anyway, car is well, problem is gone but the cause is unknown!

Louis-Alain
1983 Quattro