Idle Adjustment Question?
#1
Drifting
Thread Starter
Idle Adjustment Question?
Can someone tell me if the torx screw on the left (drivers) side of the throttle body on an '86 L98 is used to adjust the idle? It appears to open the butterflies and increase RPM when turned clockwise. I thought that idle was only adjustable via the TPS. Thanks for any help.
#2
Race Director
Member Since: Sep 2000
Location: The Top of Utah
Posts: 17,298
Likes: 0
Received 23 Likes
on
22 Posts
That screw is to set the "minimum idle" speed. Although I have been informed to the contrary, I have trouble understanding how the TPS alters the idle speed, since THAT speed is programmed into the chip and is regulated by the ECM and the IAC.
RACE ON!!!
RACE ON!!!
#3
Le Mans Master
To set the throttle plates,
My 85 shop manual says to jumper the A & B connections on the ALDL, turn the ignition on for 30 seconds then disconnect the IAC, turn ignition off, pull the jumper and start the car, when the ECM goes to closed loop set the idle to 450, turn off car and reconnect
My 85 shop manual says to jumper the A & B connections on the ALDL, turn the ignition on for 30 seconds then disconnect the IAC, turn ignition off, pull the jumper and start the car, when the ECM goes to closed loop set the idle to 450, turn off car and reconnect
#4
Le Mans Master
Originally Posted by CFI-EFI
That screw is to set the "minimum idle" speed. Although I have been informed to the contrary, I have trouble understanding how the TPS alters the idle speed, since THAT speed is programmed into the chip and is regulated by the ECM and the IAC.
RACE ON!!!
RACE ON!!!
Jake
#5
Race Director
Member Since: Sep 2000
Location: The Top of Utah
Posts: 17,298
Likes: 0
Received 23 Likes
on
22 Posts
Indeed. I made the statement that it didn't, and was rather FIRMLY informed that it DID on an L98. Not having an L98, I backed down, but it doesn't make any sense to me. *I* thought that although the hardware was different, these early C4s worked on pretty much the same principles. The only way *I* see it affecting idle speed, is if the tang is misapplied to the throttle arm and cranking the TPS actually moves the arm. Even then, the IAC ought to compensate for all but the most gross movement. Yes?
RACE ON!!!
RACE ON!!!
#6
Tech Contributor
Originally Posted by CFI-EFI
That screw is to set the "minimum idle" speed. Although I have been informed to the contrary, I have trouble understanding how the TPS alters the idle speed, since THAT speed is programmed into the chip and is regulated by the ECM and the IAC.
RACE ON!!!
RACE ON!!!
The idle speed is programmed. While turning the screw in the throttle body will adjust the idle speed initially, the ECM will adjust the IAC to compensate (at least until they manage to get outside the range of the IAC valve, which introduces a set of different driveability problems). Bottom line...To change the idle speed, the ECM needs a new chip ('94-'96 cars have their PCMs reflashed as there is no chip to replace).
#7
Team Owner
#8
Tech Contributor
Originally Posted by aklim
See this to set idle speed
Re-connect the connector onto the IAC. Start engine. Idle speed is now once again governed by the ECM, but your idle should be smooth and steady, approximately 600 rpm in Drive (for unmodified cars).
To adjust the idle speed during normal operation, you *must* program a new chip for cars older than 1994, or relfash the PCM for 1994 and newer cars.
#9
Racer
Member Since: Apr 2002
Location: Tuscaloosa AL
Posts: 353
Likes: 0
Received 0 Likes
on
0 Posts
Is the "minimum idle air set" procedure the same for modified engines with custom chips? I know the chip programming establishes idle speed, I am just wanting to know about minimum idle air set. Also, after you disconnect the IAC, before you start the engine, do you turn the key all the way off or does it matter. Lastly, after you set idle to 450 rpm do you cut off the motor before reconnecting the IAC or does it matter. After IAC is re-connected is it necessary to drive the vehicle or can you just re-start it?
#10
Race Director
Member Since: Sep 2000
Location: The Top of Utah
Posts: 17,298
Likes: 0
Received 23 Likes
on
22 Posts
Originally Posted by CFI-EFI
That screw is to set the "minimum idle" speed. Although I have been informed to the contrary, I have trouble understanding how the TPS alters the idle speed, since THAT speed is programmed into the chip and is regulated by the ECM and the IAC.
RACE ON!!!
RACE ON!!!
Originally Posted by 96GS#007
It's "minimum idle air", not speed. Who ever told you that the idle speed could be adjusted with the screw in the throttle body or via the TPS is flat out wrong.
Originally Posted by Mark L. Warner
Is the "minimum idle air set" procedure the same for modified engines with custom chips? I know the chip programming establishes idle speed, I am just wanting to know about minimum idle air set.
RACE ON!!!
#11
Team Owner
If it is a stock system, with the IAC disconnected, you set the idle speed to whatever the specs say, shut down, reconnect the stuff and set the TPS. After that, clear the codes and restart engine.
#13
Le Mans Master
Originally Posted by CFI-EFI
Indeed. I made the statement that it didn't, and was rather FIRMLY informed that it DID on an L98. Not having an L98, I backed down, but it doesn't make any sense to me. *I* thought that although the hardware was different, these early C4s worked on pretty much the same principles. The only way *I* see it affecting idle speed, is if the tang is misapplied to the throttle arm and cranking the TPS actually moves the arm. Even then, the IAC ought to compensate for all but the most gross movement. Yes?
RACE ON!!!
RACE ON!!!
I have an L98 and have played with TPS voltage settings from as low as .45 to as high as .69 and about all the voltage readings in between. None of them caused a change in the idle speed.
The different TPS voltage settings DID effect injector on time, as reported by Diacom; throttle response was effected, the engine sounded sharper at the lower settings and less so at the higher ones, but idle RPM remained unchanged.
Jake
#14
Race Director
Member Since: Sep 2000
Location: The Top of Utah
Posts: 17,298
Likes: 0
Received 23 Likes
on
22 Posts
Originally Posted by JAKE
I wish I knew who told you that so that we could hash this out once and for all.
Jake
Jake
RACE ON!!!
#15
Race Director
Member Since: Sep 2000
Location: The Top of Utah
Posts: 17,298
Likes: 0
Received 23 Likes
on
22 Posts
Originally Posted by JAKE
I wish I knew who told you that so that we could hash this out once and for all.
Jake
Jake
________________________________________ ___________________
07-19-2004, 09:14 PM #6
scorp508
Elite Member
Member Since: Mar 2000
Raynham
MA
---------------------------------------------------------------------
Quote:
Originally Posted by CFI-EFI
TPS adjustment shouldn't change the idle speed.
________________________________________ _
It certainly does on an L98.
________________________________________ _________________
07-19-2004, 10:09 PM #8
Corvette0096
CF Senior Member
Member Since: Aug 2000
Vancouver,Wa.
---------------------------------------------------------------------
Quote:
Originally Posted by scorp508
It certainly does on an L98.
__________________
If you care to read them in context, here is a LINK to the thread.
RACE ON!!!
Last edited by CFI-EFI; 08-24-2004 at 02:57 PM.
#16
Le Mans Master
Idle is programmed into the ECM and for an auto, actual RPM should be within 50 RPM of the targeted #. TPS has nothing to do with it, but if the voltage is too high, the ECM will think that the car is accelerating and it won't idle properly. If it's too low, it'll probably stall. The throttle plate does move as you adjust the minimum idle screw - it's connected to it, so the TPS setting changes. Before adjusting minimum idle, the TPS should be set to .54 +/- .06. And after you turn the screw, readjust it to the same setting.