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.

 

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!!!

 

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

 

Someone told you the adjusting the TPS alters the idle speed????

Jake

 

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!!!

 

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.

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).

 

See this to set idle speed

 

As pointed out in the article, you're setting minimum idle air. This is not the rpm the engine will idle at once everything is reassembled and in working order. From the article....

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.

 

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?

 

I mis-spoke when I added the word "speed" instead of using the word, "air". I DO realize that it is minimum AIR that is being set. Probably because it is set at a specified rpm, with a tach, that I incorrectly referred to it as speed. Also, I know that not only does the engine never idle at this speed, that it doesn't affect the speed that it DOES idle at. Nobody ever tried to tell me that the "idle screw" as it would appear to be, adjusted the idle speed, but much to my disbelief, I WAS told that the IAC adjustment changed the idle speed on the L98. I didn't believe it then and if I had any doubts, they have been erased, here. I bold-ed the portion of my quote that you cited to illustrate our agreement. A reprogrammed chip, alone doesn't necessarily change the desired minimum air setting. Only if the idle speed has been raised. If the idle speed has been reprogrammed to a higher rpm because of a cam, etc. the minimum air may also have to be raised. Otherwise if, all or most, of the idle air comes from the IAC, there won't be sufficient "headroom" in the IAC travel to affect a cold idle or to boost the idle with the A/C on. [QUOTE=Mark L. Warner] 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? No matter to either turning off of the ignition question. When the engine is restarted, it may not idle well. It should even out with continued idling. On my '84, the Helms says to warm up the engine and drive it at a speed of 35-40 mph to reset the IACs. I don't believe that is necessary on the L98s. Of course, it could do no harm.

RACE ON!!!

 

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.

 

Learning...seems I'm not the only one who has these issues.

 

I wish I knew who told you that so that we could hash this out once and for all.

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

 

I'll look for it. It sure doesn't change the idle speed on a Crossfire, and as I said, I couldn't imagine it doing so on an L98, either. I have played with mine, because all the local experts, got a, "deer in the headlights" look when I asked the affects of altering the TPS setting. At .700 the throttle became VERY sensitive. Small changes in gas pedal position made large changes in speed. Throttle response? Drag strip times were unaffected, and I found maintaining a steady cruise speed on the highway, difficult. Too sensitive. To me, if it didn't help the 1/4, it was useless.

RACE ON!!!

 

I found it. Here are a couple of posts from some of our resident genius's. Both rather prolific posters.

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07-19-2004, 09:14 PM #6
scorp508
Elite Member


Member Since: Mar 2000
Raynham
MA

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Quote:
Originally Posted by CFI-EFI
TPS adjustment shouldn't change the idle speed.
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It certainly does on an L98.


________________________________________ _________________
07-19-2004, 10:09 PM #8
Corvette0096
CF Senior Member


Member Since: Aug 2000
Vancouver,Wa.


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Quote:
Originally Posted by scorp508
It certainly does on an L98.

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If you care to read them in context, here is a LINK to the thread.

RACE ON!!!

 

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.