91'z28 idle
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91'z28 idle
I have a 91 z28 with 5.7 (350) that I am restoring. I have port fuel injection. I cannot fiqure out how to adjust my idle down to the factory setting of 700 rpm. This is my first PFI camaro all the rest were carbed or TBI. I start the car and the rpm goes to 2000-3000, after a few seconds it comes down to about 2000 and staying there. Thanks for any help 
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BCR,
Welcome Aboard!
There really is no procedure for adjusting the idle RPM. The ECM controls the idle RPM based on inputs. About all you can do is to make sure that the inputs are correct and the throttle position is correct so the ECM can do its job.
Another thing to realize is that the tachometer on the dash cannot be trusted for a diagnostic reading. If you are testing and adjusting, you need to have a separate tachometer connected.
That being stated, any air leak will cause a higher RPM. Potential air leaks include vacuum hoses, EGR, the brake booster, gasket leaks, sticking or misaligned plates in the throttle body, PCV system, a throttle stop screw that has been inadvertently adjusted, and probably others I'm overlooking. It is also possible that the TPS sensor is not working correctly, but a '91 should have a fixed TPS, so there is no adjustment, only testing. However, before digging into any of that, there are a few things to check.
Connect your diagnostic tachometer. Start and warm the engine to operating temperature. RPM should start high as soon as the engine fires, then gradually decrease as coolant temperature increases. Once the coolant is at 160° of above, you should be able to start testing. Set the parking brake, block the wheels, and put the transmission in DRIVE. If you don't trust your parking brake, have an assistant hold the service brake while you test in gear. If the RPM is still higher than 700RPM in DRIVE, you need to continue.
Disconnect the throttle cables, to make sure that the throttle, cruise control, and TV cables are not holding the throttle open. Try backing off the throttle stop screw a turn or two. Count the turns so you can return the screw to the proper place if necessary. If the RPM changes with the throttle stop screw, the minimum air position is incorrect and you'll have to reset it. If the RPM does not change significantly with the adjustment of the stop screw, you may have another air leak or a sticking IAC valve. Return the stop screw to the original position and place the transmission in PARK. Shut off the engine and read on.
Throttle Minimum Air Position
Tools needed:
1. Torx driver # T-20
2. Paper Clip
3. Small Punch
4. Tachometer
In order to successfully complete the adjustment, the IAC air passages and pintle need to be clean. The throttle plates and bores need to be clean as well. If this is not the case, you'll need to remove the intake air bellows from TPI engines to gain access to the area to be cleaned. A spray-type carburetor cleaner works well for this. With the engine idling, direct the spray cleaner in to the IAC air passages and around the throttle plates. Shut off the engine and continue cleaning the throttle plates by opening the throttle manually. Once everything is satisfactorily cleaned, replace the intake air bellows. Many times, this alone can solve IAC/idle speed problems.
If this doesn't solve the problem, you may need to remove and clean the IAC stepper motor. If the IAC appears to be clean and functioning properly, continue with the adjustment procedure.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Idle Air Control Cleaning
You can remove the IAC and service it. Remove the electrical connector from the IAC. Unscrew the IAC unit from the throttle body.
You can gently rock the pintle back and forth and allow the spring to extend it until it comes apart in your hands. Clean everything with lint-free cloths and a mild solvent. Harsh solvents can affect the insulation of the stepper motor coils. It's generally the dirt and buildup on this worm shaft that causes sluggish IAC operation.
When the worm gear on the pintle shaft is clean and dry, apply one drop of clean light oil to the shaft and work the pintle back into the rack gears of the motor by the same rocking motion. It takes a while to get the pintle back into the worm gears, but you'll get it. It is important to get the pintle fully retracted into the housing so that the pintle is not forced against the gears when reinstalling the IAC unit in the throttle body.
While the IAC is out, clean the air passages in the throttle body. The orifice in the TB where the IAC resides is the seat that the IAC valve closes against, and it can accumulate a lot of carbon, dirt, and debris. The easy way to do this is with carburetor cleaner and a small stiff brush.
When everything is clean and dry, replace the gasket if it is damaged, apply a little anti-seize to the threads, and torque the IAC to the proper specs. (30 in/lb for 1990-later TPIs). Proceed with checking the TPS and setting minimum air position.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Locate the ALDL connector under your dash panel, in the driver's footwell area. Remove the plastic trim cover (if it is still there).
Cut and form a paper clip into a "U" shape. Insert the clip ends into the ALDL in the 'A' and 'B' sockets.
Turn on the ignition, but don't start the engine. This will force the ECM into its diagnostic mode. Wait 30 seconds to allow the IAC pintle to fully extend. Under the hood, remove the electrical connector from the IAC, then turn off the ignition and remove the paper clip jumper from the ALDL. With the IAC pintle fully extended (closed) all idle air will be controlled by the position of the throttle plates. Some manuals indicate that the EST bypass connector should be disconnected for this procedure, while some make no mention of it. While timing is a factor in idle speed, the EST should only operate as a function of engine RPM, temperature, and detonation sensor inputs. To remove all doubt, disconnect the EST bypass connector if your car is so equipped.
Locate the Torx screw on the left side of the throttle body. It may be equipped with a protective metal cap from the factory. This was intended to discourage adjustment. If the cap is present, use a small punch to knock it out. Once the screw is accessible, start the engine and place the transmission in DRIVE. Adjust the throttle stop to obtain 450-500 RPM with the transmission in "DRIVE" on an automatic transmission car, 500-550 in neutral on a manual transmission car, rotating the Torx screw clockwise to raise speed and counter-clockwise to lower speed. Once the idle RPM is set, place the transmission in PARK and turn off the engine.
Re-connect the electrical connector onto the IAC. Start engine. Idle speed should be governed by the ECM at approximately 650-700 rpm in "DRIVE" (for unmodified cars). Idle speed in NEUTRAL or PARK is less significant, and will be higher.
Throttle Position Sensor (TPS)
Tools needed:
1. Digital Volt-Ohm-Meter (VOM)
2. Breakout jumper wires or probes (make your own)
3. AutoXray, Diacom, or similar scanner will replace the VOM and jumper wires.
Turn on ignition, but don't start the engine.
With a diagnostic scanner: plug in the scanner and read the TPS voltage. It should be below 1.0 VDC on 1990 and later TPIs.
/If you are using a voltmeter instead of a scanner, connect the VOM to the TPS electrical connector terminals ‘A' and ‘B'.
With a breakout jumper: Disconnect the electrical connector from the TPS. Install the breakout in-line, between the TPS and wiring harness connector. Connect the meter probes to terminals 'A' and 'B' on the connector. (‘B' is the positive connection, ‘A' the signal ground, or negative.)
With probes: If you have very slender probes on your VOM, you can back-probe the TPS connector while it is attached to the TPS. If you have made probes of large dressmakers pins or a similar item, you can back-probe the connector as well. Connect the meter probes to terminals 'A' and 'B' on the connector.
Turn on the ignition to read the TPS output voltage at the idle position. The reading should be below 1.0V on a 1990 or later TPI. Using the throttle lever, rotate the throttle to WOT (wide open throttle). The TPS voltage should be over 4.0 volts. Close the throttle again, and then slowly open it to WOT, observing the voltage reading. It should increase progressively and in a linear fashion. If it sticks or jumps or falls off at all while doing this check, the TPS sensor may be failing.
After verifying the desired setting, turn off the ignition switch. Remove all jumpers or the scanner and reconnect the TPS connector as required.
Reinitializing the ECM
If you set a DTC during the procedure, the SES light should be illuminated on the dash. This ECM retains DTC data for the previous 50 engine starts, so the codes will eventually be cleared. If you want more immediate results, after shutting down the engine disconnect the negative battery terminal for five minutes. This will clear the ECM of all diagnostic trouble codes. Clearing the ECM also clears any data learned about your engine, and clears the radio presets. If you have a Delco-Loc or Theft Loc II radio, make sure you follow the procedure to unlock the radio protection before disconnecting the battery. This five minutes is also just about long enough to clean both battery cables. Reconnect the battery. When you first start the engine after clearing the ECM, the engine will operate with base parameters programmed into the ECM PROM. These parameters may not be optimum for your engine, but the ECM will enter a Block Learn Mode soon after the engine is warm and enters Closed Loop Mode. The ECM will write new data tables specific to your engine and will eventually rely on those tables instead of the base tables of the factory program. You can expedite this process by driving the car for 20 minutes under varying conditions to allow the ECM to initialize. Or you can wait and drive the car normally at your convenience. The BLM tables are constantly being updated as sensor input ranges change, but the greatest change will occur within the first twenty minutes of Closed Loop operation.
Once you have ascertained that the throttles are clean, moving freely, and the TPS is working, you should be able to control the idle RPM with the IAC and minimum air position. If the idle is still high, you may have a vacuum leak, failing temperature sensor, or other problem. Chances are that the reset procedure will solve your problem. At worst, you will at least know what is working, and can continue your diagnosis from there.
Welcome Aboard!
There really is no procedure for adjusting the idle RPM. The ECM controls the idle RPM based on inputs. About all you can do is to make sure that the inputs are correct and the throttle position is correct so the ECM can do its job.
Another thing to realize is that the tachometer on the dash cannot be trusted for a diagnostic reading. If you are testing and adjusting, you need to have a separate tachometer connected.
That being stated, any air leak will cause a higher RPM. Potential air leaks include vacuum hoses, EGR, the brake booster, gasket leaks, sticking or misaligned plates in the throttle body, PCV system, a throttle stop screw that has been inadvertently adjusted, and probably others I'm overlooking. It is also possible that the TPS sensor is not working correctly, but a '91 should have a fixed TPS, so there is no adjustment, only testing. However, before digging into any of that, there are a few things to check.
Connect your diagnostic tachometer. Start and warm the engine to operating temperature. RPM should start high as soon as the engine fires, then gradually decrease as coolant temperature increases. Once the coolant is at 160° of above, you should be able to start testing. Set the parking brake, block the wheels, and put the transmission in DRIVE. If you don't trust your parking brake, have an assistant hold the service brake while you test in gear. If the RPM is still higher than 700RPM in DRIVE, you need to continue.
Disconnect the throttle cables, to make sure that the throttle, cruise control, and TV cables are not holding the throttle open. Try backing off the throttle stop screw a turn or two. Count the turns so you can return the screw to the proper place if necessary. If the RPM changes with the throttle stop screw, the minimum air position is incorrect and you'll have to reset it. If the RPM does not change significantly with the adjustment of the stop screw, you may have another air leak or a sticking IAC valve. Return the stop screw to the original position and place the transmission in PARK. Shut off the engine and read on.
Throttle Minimum Air Position
Tools needed:
1. Torx driver # T-20
2. Paper Clip
3. Small Punch
4. Tachometer
In order to successfully complete the adjustment, the IAC air passages and pintle need to be clean. The throttle plates and bores need to be clean as well. If this is not the case, you'll need to remove the intake air bellows from TPI engines to gain access to the area to be cleaned. A spray-type carburetor cleaner works well for this. With the engine idling, direct the spray cleaner in to the IAC air passages and around the throttle plates. Shut off the engine and continue cleaning the throttle plates by opening the throttle manually. Once everything is satisfactorily cleaned, replace the intake air bellows. Many times, this alone can solve IAC/idle speed problems.
If this doesn't solve the problem, you may need to remove and clean the IAC stepper motor. If the IAC appears to be clean and functioning properly, continue with the adjustment procedure.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Idle Air Control Cleaning
You can remove the IAC and service it. Remove the electrical connector from the IAC. Unscrew the IAC unit from the throttle body.
You can gently rock the pintle back and forth and allow the spring to extend it until it comes apart in your hands. Clean everything with lint-free cloths and a mild solvent. Harsh solvents can affect the insulation of the stepper motor coils. It's generally the dirt and buildup on this worm shaft that causes sluggish IAC operation.
When the worm gear on the pintle shaft is clean and dry, apply one drop of clean light oil to the shaft and work the pintle back into the rack gears of the motor by the same rocking motion. It takes a while to get the pintle back into the worm gears, but you'll get it. It is important to get the pintle fully retracted into the housing so that the pintle is not forced against the gears when reinstalling the IAC unit in the throttle body.
While the IAC is out, clean the air passages in the throttle body. The orifice in the TB where the IAC resides is the seat that the IAC valve closes against, and it can accumulate a lot of carbon, dirt, and debris. The easy way to do this is with carburetor cleaner and a small stiff brush.
When everything is clean and dry, replace the gasket if it is damaged, apply a little anti-seize to the threads, and torque the IAC to the proper specs. (30 in/lb for 1990-later TPIs). Proceed with checking the TPS and setting minimum air position.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Locate the ALDL connector under your dash panel, in the driver's footwell area. Remove the plastic trim cover (if it is still there).
Cut and form a paper clip into a "U" shape. Insert the clip ends into the ALDL in the 'A' and 'B' sockets.
Turn on the ignition, but don't start the engine. This will force the ECM into its diagnostic mode. Wait 30 seconds to allow the IAC pintle to fully extend. Under the hood, remove the electrical connector from the IAC, then turn off the ignition and remove the paper clip jumper from the ALDL. With the IAC pintle fully extended (closed) all idle air will be controlled by the position of the throttle plates. Some manuals indicate that the EST bypass connector should be disconnected for this procedure, while some make no mention of it. While timing is a factor in idle speed, the EST should only operate as a function of engine RPM, temperature, and detonation sensor inputs. To remove all doubt, disconnect the EST bypass connector if your car is so equipped.
Locate the Torx screw on the left side of the throttle body. It may be equipped with a protective metal cap from the factory. This was intended to discourage adjustment. If the cap is present, use a small punch to knock it out. Once the screw is accessible, start the engine and place the transmission in DRIVE. Adjust the throttle stop to obtain 450-500 RPM with the transmission in "DRIVE" on an automatic transmission car, 500-550 in neutral on a manual transmission car, rotating the Torx screw clockwise to raise speed and counter-clockwise to lower speed. Once the idle RPM is set, place the transmission in PARK and turn off the engine.
Re-connect the electrical connector onto the IAC. Start engine. Idle speed should be governed by the ECM at approximately 650-700 rpm in "DRIVE" (for unmodified cars). Idle speed in NEUTRAL or PARK is less significant, and will be higher.
Throttle Position Sensor (TPS)
Tools needed:
1. Digital Volt-Ohm-Meter (VOM)
2. Breakout jumper wires or probes (make your own)
3. AutoXray, Diacom, or similar scanner will replace the VOM and jumper wires.
Turn on ignition, but don't start the engine.
With a diagnostic scanner: plug in the scanner and read the TPS voltage. It should be below 1.0 VDC on 1990 and later TPIs.
/If you are using a voltmeter instead of a scanner, connect the VOM to the TPS electrical connector terminals ‘A' and ‘B'.
With a breakout jumper: Disconnect the electrical connector from the TPS. Install the breakout in-line, between the TPS and wiring harness connector. Connect the meter probes to terminals 'A' and 'B' on the connector. (‘B' is the positive connection, ‘A' the signal ground, or negative.)
With probes: If you have very slender probes on your VOM, you can back-probe the TPS connector while it is attached to the TPS. If you have made probes of large dressmakers pins or a similar item, you can back-probe the connector as well. Connect the meter probes to terminals 'A' and 'B' on the connector.
Turn on the ignition to read the TPS output voltage at the idle position. The reading should be below 1.0V on a 1990 or later TPI. Using the throttle lever, rotate the throttle to WOT (wide open throttle). The TPS voltage should be over 4.0 volts. Close the throttle again, and then slowly open it to WOT, observing the voltage reading. It should increase progressively and in a linear fashion. If it sticks or jumps or falls off at all while doing this check, the TPS sensor may be failing.
After verifying the desired setting, turn off the ignition switch. Remove all jumpers or the scanner and reconnect the TPS connector as required.
Reinitializing the ECM
If you set a DTC during the procedure, the SES light should be illuminated on the dash. This ECM retains DTC data for the previous 50 engine starts, so the codes will eventually be cleared. If you want more immediate results, after shutting down the engine disconnect the negative battery terminal for five minutes. This will clear the ECM of all diagnostic trouble codes. Clearing the ECM also clears any data learned about your engine, and clears the radio presets. If you have a Delco-Loc or Theft Loc II radio, make sure you follow the procedure to unlock the radio protection before disconnecting the battery. This five minutes is also just about long enough to clean both battery cables. Reconnect the battery. When you first start the engine after clearing the ECM, the engine will operate with base parameters programmed into the ECM PROM. These parameters may not be optimum for your engine, but the ECM will enter a Block Learn Mode soon after the engine is warm and enters Closed Loop Mode. The ECM will write new data tables specific to your engine and will eventually rely on those tables instead of the base tables of the factory program. You can expedite this process by driving the car for 20 minutes under varying conditions to allow the ECM to initialize. Or you can wait and drive the car normally at your convenience. The BLM tables are constantly being updated as sensor input ranges change, but the greatest change will occur within the first twenty minutes of Closed Loop operation.
Once you have ascertained that the throttles are clean, moving freely, and the TPS is working, you should be able to control the idle RPM with the IAC and minimum air position. If the idle is still high, you may have a vacuum leak, failing temperature sensor, or other problem. Chances are that the reset procedure will solve your problem. At worst, you will at least know what is working, and can continue your diagnosis from there.
Last edited by Vader; Mar 31, 2018 at 08:59 AM. Reason: Updated links
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