98GST unable to catch itself

[sirsol66]

First of all, sorry for the strange title but this is the best way I can put it.

So, some background: 98 gst, 50K original miles, stock turbo, intake/exhaust/dp and some other bolt ons. Stock pump/injectors, no safc or such things.

First, let me say how the problem first started. I have owned this car for several months and initially the problem showed up as a low idle (about 600). I tried adjusting the BISS and this worked...for a few days. By the third day the idle had reset itself back to 600.

A week or so later, after doing some pulls with my friend's evo we were coming to a light (i'm in third gear at about 4K), I put the car into neutral and the rpms drop down to 500 (bottom mark.) The car does NOT die (gauge cluster shuts off, normal lights during startup come on) but just acts like it. After about half a second it comes back on and acts like nothing happened. It did this about 3 more times that night but never again.

Fast forward to last week and again we are sitting at a light with my friend. I had to change lanes so I put the car into gear and accelerate into the other lane (rather quickly.) My car was again at about 3K rpm and when i put it into neutral, this time the car just died. It basically happens that when you put the car into neutral, the rpms drop normally but the car doesnt (or can not) catch itself and the car dies.

The car does not have ANY idle surge to speak of. Ever since the incident I keep the car in gear (I always have anyways but those two times I messed up) when slowing down and put it into neutral at about 700rpm. WHen done that way, the car is fine. Today, however, after I put it into neutral (from about 700rpm), the car seemed to be having a hard time and the idle bounced between 500-700 for a few seconds before settling to 600.

The final piece to this puzzle is that when I unplug the ISC, absolutely NOTHING happens. I have tried unplugging it when the car is cold, warm, half way, any way. NOTHING happens. I am however unsure of the exact operation of the ISC so I do not know if it would have this effect.

My ideas are this: 1) bad ISC that can not catch the car as the idle decreases too suddenly (if this is even a function of the isc) 2) bad alternator (the car's idle drops low also when I turn on a/c or use alot of electrical things at the same time) 3) crankwalk. My car does not show any other symptoms but someone mentioned it so I figure I should put it out there. I dont think it is but i'm a noob so what do I know, right?

Any ideas are greatly apreciated!

I almost forgot to mention some things:

-Did a boost leak test, nothing showed up
-Car has perfect vaccume, no leaks again I could find
-Car is not venting, I'm running a recirculated 1g on a dave brode UICP


EDIT: I have been doing some searches on this site ( I looked through dsmtalk a few months ago and came up with nothing) and it seems that almost everyone says it is the ISC. Can someone please explain to me the function of the isc and how it would cause this problem? I lost my electrical equipment when I moved to my new house so I will test the isc as soon as I can borrow one from a friend.

 

[Mr Peepers]

ISC stands for Idle Speed Control. Does that answer your question?

It is what compensates for extra load such as A/C or electrical loads.

The first thing I'd do is swap in a known working ISC, you even stated that you could unplug it and it wouldn't change a thing. To me, you have found your problem.

 

[sirsol66]

, thanks for the help

I figured that it was the ISC but I'm just trying to figure out that the isc can even have an effect like this. If anyone knows exactly what/how the isc works, please let me know!