Fine tuning idle

[Jameslleary]

Hey guys,
I know all the proceedures for setting minimum idle air, but i have always wondered, how did they do it at the factory?
The manual says to set the throttle blades for minimum sustainable idle, but how do you set TPS voltage with a non adjustable TPS?
I have oblonged the mounting holes on TPS's before, but it would be interesting to know how they do it at GM without modifying the sensor.
Any ideas?

[1998454]

I would think idle would be set by the IAC valve.

[Jameslleary]

Nahhh, they have to set it at the factory before they plug the curb idle screw, at least enough for fuel to get passed the blades, then when everything is set, is when the ecm and iac control idle. just trying to find out if they use the same proceedure as in the manual, or do they tune it per tps voltage?

[1998454]

I guess i misunderstood you. I thought you were trying to set the idle speed which is controlled by the IAC.

[Jameslleary]

I guess i misunderstood you. I thought you were trying to set the idle speed which is controlled by the IAC.

No worries. I have the idle set.
I am justvcuriouse, because when you go through the proceedures, at the end they usually say to be sure the TPS is at .5v.
If the throttle blades are set correctly, but the tps needs adjustment and is not adjustable from the factory, just wondered wht the General does. They all ran pretty good back in the day.
Its like factory exit voodoo.

[907 Chevy]

No worries. I have the idle set.
I am justvcuriouse, because when you go through the proceedures, at the end they usually say to be sure the TPS is at .5v.
If the throttle blades are set correctly, but the tps needs adjustment and is not adjustable from the factory, just wondered wht the General does. They all ran pretty good back in the day.
Its like factory exit voodoo.

That's a good question, although I wonder if the tps voltage even needs to be adjusted after setting minimum air. Wouldn't whatever tps voltage the ecm sees at key on/engine off be taken as closed throttle voltage?

[Eagle-Mark]

On the PCM that come in trucks from this forum the TPS is set to 0% at startup. As long a voltage is below .90 it will be OK, but ,54 is optimum.

If your engine is stock, TBI never rebuilt or wore out it should be fine for ever. If you have a built engine TBI conversion or some need to do it here's a list of how.

1. Timing
Disconnect the bypass wire and set timing to 0° at idle when warm. Procedure- warm up motor then turn off. Unplug bypass. Start motor. Set timing to 0°. Turn off motor. Reconnect bypass. Unplug ECM to clear fault code.

2. IAC reset/min idle speed- This sets the throttle plate on the throttle body to a specific rpm. It should be set to 100-125 RPM below idle commanded in bin file. At ALDL connector you need to put a jumper between pins A and B of the connector. These are pins A9 (what/blk) and A12 (blk/wht) on the ECM. This puts the ECM into ALDL mode. Turn the key on (not start) and the IAC should drive completely closed. You will normally hear a buzzing from the IAC. If you have the TunerPro running up and running on your laptop, or a Scan Tool you should see the IAC went from 145 to 0. Now unplug the IAC connector. Turn the key off. Remove jumper. Start motor. If the idle is too low you may have to keep it running. On the front drivers side of the throttle body is the adjustment screw. It will probably be behind a silver plug. If so just use a drill bit slowly to drill a hole, not to deep as to damage the torx screw behind it, just enough to use a punch to grab it and remove it. Then use a #20 torx bit to adjust the idle speed to at least 100 rpm less then your warm idle speed in bin. If 600 then set to 500 rpm. You are looking for the lowest consistent idle your motor will do. When set, turn off motor and reconnect the IAC. I try to keep the IAC count to at least 10-20 with the motor warm and idling. Higher number are OK but more air pushing gas past throttle blades on TBI engines is better!

3. TPS Throttle Position Sensor. With the idle set, you need to set the TPS idle voltage. This need to be set to about .54V to .60V
The TPS is on the passengers side and will have 2 screws holding it on. The factory TPS will not have much room in the holes for adjustments so I use a drill bit or file to elongate the holes as needed. Or bend tang on throttle shaft. To adjust, just loosen the screws and twist the TPS. You can monitor the voltage on Pin B of the connector with the key on or at pin C13 of the ECM. You can also see it in TunerPro or Scan tool! Anything away from .54V will also show as a percent of throttle on 1227747 ECM like .2 percent or 1.7 percent...depending on how far from .54 you are. But otherECM/PCM set to 0% when key on. I try to keep the idle setting to within 1 percent do to settings in bin file.

This is also a good time to check your TPS voltage. Once it is set, just watch the voltage as you open the throttle. you should see a steady rise in voltage up to about 4.5-4.8V at WOT-wide open throttle. If it jumps up and down or skips some then you will need to replace it.

Clear codes!

4. Fuel pressure The systems calculates the fuel needed based on the initial settings for injector size at a specific pressure. If the pressure changes then the fuel calculations will be off. I recommend checking the feed and return line pressures at the very beginning. This can save a lot of time later. The pressure on TBI should be 13 psi and 43.5 on TPI/MPFI with stock regulaters.
These are feed side pressures and should not change with rpm or load.

The return should be near 0 psi.

These are basics that need to be done before any tuning or data logging can begin.

http://www.gearhead-efi.com/Fuel-Inj...-Fuel-Pressure

[Doober]

That is some awesome info. I may use this to set the idle on the 383