LS IAC will not close all the way

[5.3LS10]

Hey Everyone,

I have a MS3x running a 2003 5.3L LS engine. I have this strange issue where the idle does not drop below 1000rpm. The IAC will go down to 0 steps but it is still not closed all the way. I have confirmed that if I put my finger in the opening of the throttle body and plug the hole where the IAC gets its air from I can get the idle down in the 700-800 rpm range.

I thought maybe it was a faulty or bad IAC valve so I tried a couple others with the same issue.

I then thought maybe it was the throttle body so I swapped it with a couple others that I had, same issue,

I have tried a bunch of different settings but they make no difference.

I used settings that work great on a couple other MS3/LS builds I have done and those all work fine with no problem but this one keeps fighting me.

Any idea what would be causing it not to close fully. I have played with the homing steps, wide open steps, and the homing direction none have helped.

It really seems like a physical issue preventing the IAC from seating fully but I believe I have ruled that out using different combinations of throttle bodies and IAC valves.

Attached is the msq file.

thanks!
Shane

[seijirou]

Does the idle drop if you just unplug the valve?

[billr]

When you tried "homing closed", how many homing steps were used? Use, plenty of homing steps (500 or more), it won't hurt anything to stall the stepper as the pintle stops against the seat.

I presume you have exercised the IAC valve out in the open, using "test mode", and verified the stepper is moving properly and reliably.

[5.3LS10]

When you tried "homing closed", how many homing steps were used? Use, plenty of homing steps (500 or more), it won't hurt anything to stall the stepper as the pintle stops against the seat.

I presume you have exercised the IAC valve out in the open, using "test mode", and verified the stepper is moving properly and reliably.

Thank you for the response. Homing Steps is set to 300. I believe the valves can travel 250 steps so anythign above that should be enough but I will try increasing it.

Yes when I pull the valve out (and put my finger over the end of it so it does fly out across the garage again) go to the Idle Valve test and run it through the in/out test the valve goes in and out. One thing I did not do was measure the distance inside the throttle body that the valve needs to travel to seal against the opening. Maybe I will do that then run the tests to see how close it actually getting.

One thing that I did not understand on the external tests was it seemed like a larger number running steps made the valve extend which normally should increase idle speed which should retract not extend. But when in the car it idles down as the steps decrease so it must be working the correct direction at that point.

We just got it running yesterday and I was working on it for about 9 hours so I need to look at it again today with a clearer head and see what is going on.

thanks for everyone's input so far.

,Shane

[5.3LS10]

Does the idle drop if you just unplug the valve?

I did not try this. Will the valve move when its unplugged?

[jsmcortina]

Use the Idle valve test mode with the engine running.
If the valve isn't fully closing at 0 steps, then it cannot have homed correctly.

James

[seijirou]

Does the idle drop if you just unplug the valve?

I did not try this. Will the valve move when its unplugged?

I don't think it will, sorry. It didn't click with me that you're working with a stepper motor. Nevermind!

[Yves]

On some engines the iac doesn't close completly, as it creates a lot of vacuum on decel, which has some unwanted side effects.

[gjestico]

On the LS cable throttle bodies the IAC chamber and pintle is clearly visible when the TB is removed from the intake. Being able to see the pintle move and seat while running the test mode is very advantageous.

[5.3LS10]

On the LS cable throttle bodies the IAC chamber and pintle is clearly visible when the TB is removed from the intake. Being able to see the pintle move and seat while running the test mode is very advantageous.

Hmm that is a good idea, I did not tihnk about running it through the test mode when it is disconnected. I will try that tomorrow.

[5.3LS10]

I had some time this morning to play with this. I increased the homing steps and I am now having the opposite issue. From 30 to 200 steps it was barely idling at 600rpm without me having to crack the throttle. I suspect the idle valve is not opening at all now or instead of me focusing on the 0-150 step range I am now at the other end of it. Camshaft is a factory LS2 and usually idles around 20-22" of vacuum so I suspect it will not require a lot of valve movement to keep it idling.

Tomorrow I will pull the throttle body and run it through the test so I can watch the valve move when I run the test and make sure it is doing what the software is telling it to do and what I think it should be doing.

Thanks for the input so far.
,Shane

[5.3LS10]

So after some testing back and forth i found that the spread between homing steps and WOT steps was too large. If I increased the spread the issue got worse. As I changed the homing steps closer to 250 the idle is now able to hit 800rpm with 25-30 steps.

Thank you for all the input everyone!

[LSXCutty]

Sounds Like you have an Early PNPLSX Harness from DIY. You need to swap 2 of the stepper wires. Pins 30 &31 on the Grey connector. And the valve will act as it should. They had issue with the early harnesses.

[billr]

You will also need to change from "home open" to "home close" in the TS idle control settings. It's a worthwhile change, as then that hocus-pocus with the open steps and spread between open/closed is avoided. It positively homes to zero by simply setting ample homing steps... done.

[5.3LS10]

You will also need to change from "home open" to "home close" in the TS idle control settings. It's a worthwhile change, as then that hocus-pocus with the open steps and spread between open/closed is avoided. It positively homes to zero by simply setting ample homing steps... done.

I will look into this, oddly enough the next day I went to go and start the car the issue was back...

I just have not had a chance to look back into it until now.