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Posted by hoyboi, Apr 11, 2010
Newbie Forum - Beginner/newbie/general DSM modification questions. First mods, how to run 10's when you haven't run 12's yet, any tech question that doesn't fit in another tech forum.
In my experience, idle surge is most often caused by excess air bypassing the throttle plate. This can be from the BISS, FIAV, throttle body gaskets, throttle shaft o-rings, ISC o-ring, intake manifold gasket, craked vacuum lines, brake booster vacuum line, BOV vacuum line, EGR, etc.
If you can fully close the BISS without affecting the idle surge, you have a leak somewhere else. You should do a boost leak test to see whether or not you have one or more leaks in the air intake path.
www.DSMISC.com thinking that the newer style ISC was compatilble with my '90. It even says so on their website. Wrong! The connectors fit, but that's where the similarities end.
With the new ISC installed, my car would idle surge like crazy. I came to find out that the '90 ISC and newer style ISC's work in opposite directions. When commanded, the new ISC was actually opening when it should have been closing and closing when it should have been opening! You can imagine what this does to the idle!
I replaced the newer style ISC with a '90 ISC and the problem was solved. I'm pretty sure this is only the case with the '90 model ISC, so if you have a different year engine, this probably doesn't apply to you. Good luck.
i dont actually know what year the 6 bolt is from but i am using the iac from the 97 is that a problem
VFAQ Site - Visual Frequently Answered Questions under intake idle surge FAQ scroll to the bottom it tells you how to test your ISC with a voltmeter
It's only a problem if the ISC is from a '90. Does your ISC have a pigtail (wires) with a connector at the end? That would be from a '90
The other years had no pigatail, only the connector moulded into the ISC body. Which do you have?
In my case, disconnecting the idle position switch made the situation better. The engine would rev up high to about 1500 as is did during the idle surge, but it stayed constant and didn't surge up and down. It was still annoying, but definitely more tolerable than the surge.
That sounds like a wiring error. Since the 90 ECUs and 91+ ECUs both work when connected to the same car and since the ISC is a stepper not an electric motor it moves in and out just depending on the order you pulse the coils. If you get the coils wired wrong then the pintle will do nothing or move in the wrong direction.
It's not a wiring error. When I put the '90 ISC back in (using the newly extended wires), it worked perfectly. They both worked perfectly, just in opposite directions. I have verified this with another member on the "Talk" forum who experienced the exact same thing I did- his '90 ISC also moved in the opposite direction of the later version ISC he was trying to install.
I tracked the expected/commanded movement of the ISC using my logger and sure enough, with the new ISC, a zero commanded position was actually fully open (increased idle speed). On the '90 ISC, a zero connanded position was fully closed (decreased idle speed).
The only reason I even changed the '90 out was because I thought it was causing my idle surge when in fact, the surge was being caused by a leak elsewhere.
Not saying I don't make mistakes, but I'm an engineer and have many years experience with wiring and a soldering gun. I checked, double-checked, and tripple-checked my wiring before connecting the new ISC. I've been successfully using the '90 ISC for nearly a year without touching/changing the wires I had to extend in order to allow the connector to reach the new (non '90) ISC. With no vacuum leaks, my idle is rock-solid using my 20-year-old ISC.
Perhaps it's a documentation error but on the 90 ISC the white and black wires are swapped when compared to the 91+ ISC's. So instead of pin 67 from the ECU going to pin 4 on the 90 ISC it goes to pin 6 on the 91+ ISC and ECU pin 68 goes to ISC pin 6 on a 90 but to pin 4 on a 91+ ISC. If it's not a typo in the FSM then it would cause the 91+ ISC when connected to your wiring to have a set of coils swapped.
Putting a 91+ ECU in a 90 car with a 90 ISC doesn't cause the ISC to run in the reverse directions and a 90 ECU in a 91 car with a 91 ISC also doesn't I have to guess it's not a documentation error but the change in the wiring that wasn't accounted for.
It may be an invalid conclusion but I don't have a 90 ISC here to verify it and see if perhaps the pigtail from the ISC doesn't wind up swapping the phasing back.
Good info, Steve. I didn't swap any pins or wires when I changed ISC's. It sure would explain the reversed movement. I simply went by the information from www.DSMISC.com stating that the two ISC's were identical, save for the pigtail of the '90 and the need to extend the '90 wiring harness to reach the new ISC.
Based on what you said, and if the documentation is correct, if I swap pins 67 and 68 I should be able to use the '91+ ISC in my '90? I'll keep this in mind for now. My old '90 ISC is still up and running. I'll keep the newer '91+ ISC as a spare and swap the pins as needed.
To the OP. Since you have a '91 TB and a '97 ISC, I'm assuming you also have a 91+ ECU. If that's the case, you should be good in the ISC department. You should look for leaks elsewhere.
Do you have the coolant lines to the throttle hooked up? If they are and there is no difference cold or hot to the idle the FIAV on that TB may be not functioning.
Demon33 posted in #4 a pretty complete list of things that can leak additional air past the butterfly and cause surging. Some are external leaks and some are internal ones that won't show up doing a standard boost leak test.