First of all, the problem is that the car, '91 T/A 305TPI, auto, is shutting off at idle combined with light braking or full lock turning. Driveways and parking lots seem to be the worst. At higher speeds/rpm, the car has no problems. I started running 89 octane 'bout 3 weeks ago cuz' of $$$ and the problem has worsened, I don't think octane could screw with a car THIS much?! Anyway, besides switching back to supreme unleaded, I've been told that it's most likely the IAC, Idle-air-control(valve?). Anyway, if anyone's familiar with these, please reply. Some other possible solutions are the fuel filter, throttle body scrub, or the ECM itself. Please help, any info. is apprec. Thanks!
Brian
'91 TransAM 305TPI, auto, flow, smooth 'cept @ IDLE!

 

Does the SES light come on, what code?? It could be the MAF.

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Looking for:

87 IROC-Z L98
84 TRANS AM H.O.

 

Double Post!!

[This message has been edited by IROCZTWENTYGR8 (edited October 11, 2000).]

 

DP

[This message has been edited by IROCZTWENTYGR8 (edited October 11, 2000).]

 

DP

[This message has been edited by IROCZTWENTYGR8 (edited October 11, 2000).]

 

PV,

Get back to baseline adjustments to make sure that everything is adjusted correctly before diagnosing the problem. Check base timing, TPS adjustment, and minimum air position. I'm not certain, but your '91 may not have an adjustable TPS, so that may not apply in this case.

Set the base ignition timing with the ESC bypas connector unplugged. Octane swings will affect performance, and it's important to have the correct base timing so the ECM/ESC can manage spark timing correctly.

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 0.54Volts +/- 0.07 VDC.

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 0.54VDC +/- 0.07VDC. The ideal is the center of the range, 0.54VDC for a stock engine. To adjust the output voltage, loosen the two Torx screws holding the TPS to the throttle body, and slightly rotate the TPS up or down, reading the voltage until it comes into specification. Tighten screws. 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 and could be a cause of stumbling and driveability problems.

After achieving the desired setting, turn off the ignition switch. Remove all jumpers or the scanner and reconnect the TPS connector as required.



Throttle Minimum Air Position

Tools needed:
1. Torx driver # T-20
2. Paper Clip
3. Small Punch
4. Tachometer

GENERAL NOTE: The engine should be at normal operating temperature before performing any adjustments. Never rely on the dash mounted instruments for diagnostics and adjustments. The oil pressure and temperature gauges and the voltmeter and tachometer just aren't calibrated accurately enough for diagnosis, but are a relative indication for monitoring the vehicle while driving.

For this adjustment, the transmission will be in DRIVE while you're under the hood. You will need to securely set the parking brake and block the drive wheels. It would also be a good idea to have an assistant hold the service brake while you perform the adjustments.

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 air cleaner from TBI engines or 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. Cleaning the IAC passages on a TPI/MAF engine will set a DTC, but we'll be clearing that later. 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 air bellows on TPI engines. Many times, this alone can solve IAC/idle speed problems.

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.

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", 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 rpm in "DRIVE" (for unmodified cars). Idle speed in NEUTRAL or PARK is less significant, and will be higher.


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 withing the first twenty minutes of Closed Loop operation.

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

 

Yeah, the ses (service engine soon) orange light goes on. I figure it would go on whenever the engine dies on you for whatever reason, so it doesn't do much in narrowing down the causes. Thanks anyway. Hrm... any other possible suspects besides mr. IAC and the MAF? By the way, would the MAF problem be related to the ecm??? Thanks, Brian.

 

Thanks for the info. + pics. Wow~ really helpful. I dun really understand or want to undertake the first part, the TPS-stuff, I'll try the carb-cleaner on the throttle body, several people have told me this could be the problem. As far as adjusting the idle goes, mine was adjusted to 650-675 in drive, but this will prolly freeup a lil bit once the throttle body (if dirty) is cleaned (?). Would replacing the IAC (motor?, valve?) help?

Brian

 

What code is it giving, if u don't know how to check for them, Take This Link: https://www.thirdgen.org/newdesign/tech/index.shtml and go to Miscellaneous Info.

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Looking for:

87 IROC-Z L98
84 TRANS AM H.O.

[This message has been edited by IROCZTWENTYGR8 (edited October 11, 2000).]

 

Look for "throttle body and intake cleaner for fuel injected engines" instead. It should be next to the carb cleaner... look for a yellow can w/red top by Gumout.


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards