High surging idle, new isc; whats next?

[Nicker]

1991 tsi awd 102k


Okay I’ve located this problem down to two things:

1. Idle Switch: Making contact with TB, when disconnected car will not surge/pulse but the car will idle at 2500RPM. When connected (tried before engine started and while running) car will surge between 1600-1700RPM. I adjusted the BISS (base idle set screw) as low as it goes and it still idles very high.

The computer will "fight" the attempts to adjust the base idle and this can be prevented by grounding a small female electrical connector that is located just to the left of the cruise control module. But im not sure what connector this is, the idle did drop some when adjusted though.

2. ISC (Idle Speed Control) (like all DSM’s I’ve been reading about):
Tested out the ISC and got this back:
1-2: open
2-3: 32ohms
4-5: 40ohms
5-6: 38ohms

Now I know it has to be replaced, since the 1-2 test is reading bad, I’ve also checked for a Boost leak / Vacuum Leaks (one hose was dry rotted open from TB).

So you guys think that if I get a new/good ISC this will fix the problem (if the ISC was just bad wouldn’t it surge at lower rpms?)

 

[steve]

1. Idle Switch: Making contact with TB, when disconnected car will not surge/pulse but the car will idle at 2500RPM.

2. ISC (Idle Speed Control) (like all DSM’s I’ve been reading about):
Tested out the ISC and got this back:
1-2: open

The fact that your car idles at 2500 RPM with the IPS disconnected says you have a large amount of air getting past the throttle plate. The fact that the ISC is bad means that it could be open quite a bit providing that bypass air.

Get the ISC fixed or replace it and see if that doesn't take care of the problem.

Steve