Question about TBI IAC on 87' 454

[AusTx68]

I did a complete stock rebuild of my 454, broke in my cam then realized it wouldn't idle. If I gave it a little gas it would start and run fine. I pulled my IAC and noticed the pindle was extended. I pushed it back in and reinstalled it. The motor started fine and idled. Over the next 1 1/2 days the idle got worse and eventually stopped. I pulled the IAC again, pushed in the pindle and it idle. I drove it, parked and it wouldn't idle.

Now here's the confusing part. I decided to unplug the IAC. Started the truck and it fired right up. Took it for a drive and it runs like a champ. Plenty of power, no hesitation (previously hesitated), strong idle but slightly higher. Not annoyingly high. So I'm wondering why it runs so good with the IAC disconnected. Does it serve a purpose at engine speeds above idle? I figured my IAC is out so I ordered a new one. Just wondered how long I can drive it unplugged.

[79k30dually]

The IAC is primarily for idle control only. Above idle the ECM sends the IAC to a programed position. You can use a test light to monitor the position change, DO NOT stab the connector. Use small diameter paper clips to insert into the connector, the four wire are control circuits for 2 motors. I don't have a print in front of me right now, so you'll have to do some guessing. If I remember correctly the wires are light green and dark green, 2 solids and 2 with tracers. Place the testlight between 2 similar colored wires, the light should blink, same with the other 2 wires. The wires are continuously changing polarities. If the light doesn't blink, very fast, there may be a wiring issue or Ecm problem. I installed a 87 454 tbi in my 79 dually, I love the drivability in cold weather. Good luck, Ron

[Hulk Will Smash]

Isn't there a reset procedure when you disconnect the battery?
Posted via Mobile Device

[rfmaster]

1. Install a jumper wire between Pins A & B of ALDL diagnostic connector.
2. Turn on the ignition (but don't start the engine)
3. Wait 30 seconds.
4. Leave ignition on and disconnect the IAC connector (four wire plug).
5. Remove the jumper wire from the ALDL connector.
6. Start the Engine. The engine probably will run rough and may stall. Just restart and try again.
7. Adjust the throttle stop screw to set the idle to 500 RPM +/- 50 RPM. - stock camshaft!
8. Turn the engine off and reconnect the IAC connector.
9. Turn ignition on and adjust TPS to proper settings. (0.60 - 0.65Vdc at idle is typical, but 0.3 to 0.9Vdc will work)
10. Repeat procedure until the 500 RPM +/- 50 RPM (stock cam) in step 7 can be maintained while TPS is at proper idle setting. Performance camshaft may require higher idle RPM!

[AusTx68]

I understand steps 1 & 2 will cause the pindle to extend which it did in my case. It extended until it fully closed which meant no air could enter. So when I got to step 6, it failed to idle. It started but would not idle. This was with a new IAC. I gave up, pushed the pindle back in and disconnected the IAC. It starts, idles and drives. I will try again when my ALDL to USB cable comes in. I will be able to see what's really going on.

[rfmaster]

Originally Posted by AusTx68

I understand steps 1 & 2 will cause the pindle to extend which it did in my case. It extended until it fully closed which meant no air could enter. So when I got to step 6, it failed to idle. It started but would not idle. This was with a new IAC. I gave up, pushed the pindle back in and disconnected the IAC. It starts, idles and drives. I will try again when my ALDL to USB cable comes in. I will be able to see what's really going on.

Correct.
At step 4 IAC is disabled and in fully closed position. There is no air reaching intake manifold plenum through the idle air passage which by-passes throttle plates. However for the engine to idle some air must enter.

Enter minimum idle screw covered by the anti tamper plug!

This screw allows to move throttle blades so that at least some air can enter intake manifold plenum - allowing engine to idle. Since IAC is disabled and in fully closed position the minimum idle speed should be adjusted to maintain engine idle below target idle speed. ECM takes care of this by controlling IAC position so as to maintain target idle RPM. Usually on a stock engine minimum idle speed is around 500 to 550 (with IAC disabled per step 4) and 600 to 625 RPM with IAC reconnected. When you get WinALDL you should see between 20 to 30 IAC counts on a stock engine.

Things get a bit more complicated if you have a performance camshaft. Do you know your bumpstick parameters??? Generally speaking SBC performance camshafts with duration @ 0.050 over 210 deg need higher idle speed. BBC can handle a bit more duration, but not much more before idle quality is compromised. LSA also affects engine idle quality.

In your case - 1) post you cam spec (if known). 2) pop anti temper plug covering TORX adjustment screw and try couple of turns so that the engine would idle 550 to 600 RPM if possible. Then reconnect IAC once you have a steady idle. Idle should increase to about 600 to 625 RPM - that's what stock EPROM is programmed at.

When you turn ignition off ECM moves IAC to a wide open position ~140 counts. You should be able to see this after completing reset - the IAC passage should not be blocked by the head. If this does not happen - either IAC, harness or ECM are defective. If you had SBC to BBC swap please note that there is a small harness difference between the two which will render IAC inoperative (easily fixed).

//RF

[AusTx68]

Hey RF,
Thanks for taking the time to further explain the process. I was able to get the IAC set correctly and it's running right at target idle.

About the TPS, how do you go about measuring the voltage without an ALDL cable? Mine is still in the mail.

[rfmaster]

There are couple of ways of measuring TPS voltage - it is on Pin B (Dark blue wire). The simplest and the most difficult (funny how it is) is to 'back probe' the TPS plug. This involves getting a very small diameter DVM test lead to get around the weather pack seal, without damaging and making contact with plug pin! In the past I used a modified (unfolded) paper clip, but it also required an alligator test lead. After having enough of it I got a set of Pomona Electronics (POM6411) Backprobe Pins and never looked back.

TPS voltage should be around 0.65 Vdc but 0.3 to 0.9 Volts is an acceptable range at idle position.

//RF