The car: 87 Vette, high flow heads, edeljunk TPI, cam, ported exhaust manifolds, stock converter.

The problem: If I set the idle to ~850 warm and in park, the IAC is at 0. I've changed the idle speed in the eprom to 800. When I shift into gear, idle drops to ~600 and the IAC goes wide open. Now, if I turn on the lights, A/C, radio, etc, the idle speeds all drop of course - and the in gear idle speed keeps bumping agianst the stall saver but stays alive ~500 rpm. I adjusted the idle to 600 with everything on and it idled fine, but now the park idle is 1000. I've only changed the commanded idle table, the AFR vs. temp (to get open loop fuel close), and cool spark (idles at 34 degrees when cold to stay alive). I've also changed inj constant, tc lockup, and cooling fans, but they shouldn't affect idle. I'm using Winbin and corvette ARAP.

So, with the cam and heads, can I reach a happy medium with the idle - where the IAC is never fully open or closed so it can respond to changes like turning the A/C on? Does the ECM try for the commanded idle whether in gear or not - Should I make the commanded idle match the in gear idle? Should I mess with warm idle timing? Are there other tables I should be looking at?

Thanks for the continuing help and patience.

 

I had a similar problem with an '87 Vette. Are you by chance running a high flow fuel pump? Regardless, check the fuel pump voltage at idle (using your diagnostic scantool). If it's less than 12-13volts, that's most likely your problem.

What happens is that the ECM monitors the fuel pump voltage. It widens the injector pulsewidth to compensate for lower voltage. Reason being the injectors use the same voltage source, so applying a lower voltage to the injectors makes them open slower. Hence, the pulse width is increased to compensate. This false/unnecessary increase in pulsewidth at idle manifests itself as an overrich idle and wacky idle speeds.

Also, the ECM will shoot for a 50rpm higher target idle when the tranny is in Park or Neutral. When in gear, the commanded idle speed is that programmed in the idle rpm vs. coolant table.

Nick

 

Stock pump - for now. I have to rebuild my savings . Fuel pump volts are good according to the scanner.

 

Burned a new one with 650 commanded idle. It idles just under that in gear with the IAC wide open. In park, it idles at 900 with the IAC closed. Should I make the commanded idle lower than what the throttle opening alone will support in gear - so the IAC can then take care of the A/C coming on? Do I need more idle timing. In gear at 600, it idles well, but I want the IAC to control the idle when accesories draw on the engine.

 

Anybody home?

What doe sthe IAC do to the commanded idle? It seems to kick it up.

 

I typically use 0 as a hot in gear IAC count. I also, set the IAC max posistions to their min., so that it doesn't try to ovecomp for things.

Also, use the smallest Alt pulley I can find, the voltage correction is nothing to ignore. A good alter., battery and wirng are a must.

Cammed, MAF, and just a 231 I have a dead smooth idle, and no IAC caused problems, with full lock steering or in / out of gear corrections.

Remember to do what the engine wants, not what you think it needs. Again 0 IAC is fine

 

I've been shooting for 0 IAC, but when I kick the A/C on, IAC goes wide open. Otherwise, it opens a little bit on a loaded in gear idle.

I was incomplete in my last post - Why does the A/C cause the commanded idle to jump?

Also, how can I keep the in gear and park idles nearer in rpm? Timing?

 

[QUOTE]Originally posted by Aaron's 87:
[B]I've been shooting for 0 IAC, but when I kick the A/C on, IAC goes wide open.

Wide open is like a 100+ IAC count. If it's doing that your definetly hitting the stall saver.

Otherwise, it opens a little bit on a loaded in gear idle.
I was incomplete in my last post - Why does the A/C cause the commanded idle to jump?

The A/C draw several HP at idle, and percentage wise for load that is alot, so you have to add some air, to keep from stalling the motor. Might hunt around your hac ans see if there is a A/C IAC adder number.

Also, how can I keep the in gear and park idles nearer in rpm? Timing?

There are commanded idle speeds for both. Just need to edit things.

 

She hits 160 when loaded with A/C - also A/C bings the commanded idle way up.

I don't have a hac for 6E. The IAC goes wide open with A/C, the commanded idle goes up, but the actual idle just drops a little. With the TB set open enough, she'll idle OK with the IAC wide open, but I'd rather have it control the idle than the TB set screw.

The commanded in gear idle is set by the idle vs. temp table, the commanded park idle is 50 rpm higher. But, my idle drops 300 rpm from park to gear. Probably a function of the cam and stock converter. I'm thinking the only way I an bring the park idle down from 900 is to take out some timing.

 

TunerCat has a constant for idle RPM bump with A/C on. I have mine set at 150. It works -- sorta. When the compressor kicks in, the engine bogs a little, then jumps way up (over corrects a little), then settles to the new (higher) desired RPM. Sitting at idle, this is somewhat erratic behavior if the compressor is cycling.

To the ECM, "A/C on" is when the compressor is on, not when the selector has requested A/C. I want the idle bump to occur when the selector says A/C, not just when the compressor is on.

So, I'm considering a wiring modification so that the ECM gets its "A/C on" signal from the selector, not the compressor. I expect much smoother operation that way. Can anyone offer any reason why this won't work? Is the A/C signal to the ECM used for anything else?

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'87 IROC 5.7l TPI - original owner!

 

I sprayed carb cleaner in the IAC passage while it was open and the motor was rich - it bogged the motor. To me, this means the IAC is actually open and it's passages are clear. Shouldn't a wide open IAC give you about 200 rpm as long as you're not really lean?