I'm running the Holley 58mm TB with stock IAC housing and valve. I've been chasing a bad stumble on acceleration for a while. Doesn't matter if I'm at idle or if I'm cruising at 3000 rpms, anytime I stab the pedal it stumbles then runs like a raped ape. WOT is great but the stumble is keeping me from opening up the bottle. So, in my troubleshooting of the stumble I came across a low TPS problem and the IAC counts being slammed at 160 all the time. I just fixed the TPS by adjusting it to be within spec and that code has cleared and there are no other codes.

My tune is mail order and the car does run rich. In my many searches I came across a post somewhere that said a rich condition can cause high IAC counts because it's trying to feed more air to compensate for all the extra fuel. It sounds possible to me but I wasn't sure if there might be something else to check.

oh and I just put new O2 sensors in today also since one was not reading so good but that did not cure the hesitation or affect the IAC counts.

thanks

Have you reset the IAC using ShoeBox's tech site? If not, I would do this first. How far do you have the TB blades open with the stop screw? I would try to close them as far as you can...

Closing the throttle stop will cause the PCM to try and increase the IAC counts. You would open the blades up further with the stop screw, to try and pull the counts down.

But before you adjust the throttle stop, which may be little more than a band-aid on problem, check the IAC motor. If the motor is not working, and the PCM feels the engine needs more idle air, the PCM will keep increasing the counts until they reach maximum (160). Shoebox has a check procedure:

http://shbox.com/1/iac2.jpg

You should also check to make sure the IAC passage in the throttle body is not plugged up, and that the pintle on the IAC motor is not crudded up.

The O2 sensors will not affect the IAC counts.

I tried the IAC reset procedure and I should have mentioned that I also replaced the IAC valve yesterday. The old one was very crudded up and the pintle was not moving. The new one is moving but IAC counts are still at 160. The throttle stop is completely backed out and is not holding the blades open at all. I did crack the blades open for a while to see if the counts would begin to drop but they did not.

It idles just fine but is very rich due to the tune. I'm thinking it's trying to add more air to compensate for the rich condition. Does this sound feasible?

I will pull the TB off tonight too and see if the passages are clear to rule that out.

Why is your tune so rich? That's generally only a problem with a huge/high overlap cam, and there are ways around that. It should idle at 14.7:1. I'm not aware of tuners changing that.

The PCM can not correct a rich A/F ratio by adding more air via the IAC valve. It only uses the IAC valve to correct the idle RPM when the RPM departs from the programmed target value.

I'll have to pull up the tune and see what the last mail order tuner did. It's possible the injector constant could be off. My cam is small by comparison..see below

Duration:
Intake: 227
Exhaust: 227

Lift:
Intake: 556
Exhaust: 556

Lobe Center: 112

Centerline: 108

I appreciate the responses

sig updated for all mods

If the injector constant is "off" the PCM should be adjusting the long term fuel trims (BLM's) to correct the A/F ratio..... unless the constant is set more than 15% above actual.

Pulled the throttle body off tonight and I think I can see where the problem might be. I have the plate kit from Nitro Dave's and the passage through the plate looks much smaller than what is on the throttle body and intake. I'll get some pics tomorrow and I'll run it without the plate to see if the counts drop.

Diameter of passage in the Holley throttle body. (0.493in)


Diameter of passage in the nitrous plate. (0.332in)


Comparison...(yep I'll be cleaning that TB out)


I'm going to put the TB back on without the plate and see if the counts drop.

well, there is definitely improvement with the plate removed. I put the TB back on without the plate and started it. I let it idle until the pcm went to closed loop. In open loop the IAC counts stayed at 160 but as soon as it switched over to closed loop they started to drop. I let it idle for a few minutes and get up to full temp and the counts made it down to 78 at it's lowest point. There is still a great deal of improvement to go but I had never seen it go below 157 prior to removing the plate. I really don't want to drill out the plate and there's not much room to do that anyway. I'm thinking I can try to adjust the throttle stop screw out a bit then adjust the TPS back to where the pcm thinks they're fully closed. Hopefully that will bring them down some more.

Still taking suggestions though.....

DM log from tonight's idle session is here ---> http://mafia-motorsports.com/no_plate.uni

Well, even with no responses I'll keep this going so that someone else might find it helpful one day. I know I'm happy to find info when I need it on here.



Worked on it more tonight. First I fixed the injector constant in the tune. I have 42# SVOs which should be set at 44.35 but mine was at 38.02. That explains why it was so rich. It was set for SVO 36#.

I also turned in the throttle stop screw a bit then adjusted the TPS back down. Did this a few times and was able to get the IAC counts down to 56. I'm on the right track but I have a feeling when I put the plate back on I'm going to have to go through this series of adjustments again due to the size of the passage through the plate.

It was a little late for me to do much more than blip the throttle but the hesitation might be gone...or at least diminished greatly. Hopefully get off work early tomorrow and go for a drive.

Watch how far you open the TB blades, you really don't want to get your TPS=0% voltage above 0.75 volts, spec is somewhere between .55 and .65 . If it gets to high it will mess up the TPS=0% status to the PCM that effects a few of the timing and fueling tables. Putting more air through the TB and bypassing the IAC could also cause uneven BLM's at low RPM as the TB air is not distributed through the intake evenly as the IAC air is.

Why don't you want do drill out the IAC hole in the plate? It won't hurt anything. It will simply eliminate a restriction. I'd say make the plate hole larger than the TB hole. That way, its still the TB hole that is limiting flow.

Everytime I turn the throttle stop screw out a little more, I physically adjust the TPS back to .4mV so as far as the computer is concerned, the throttle is closed.

I may end up drilling out the plate but even drilled out I would need to make these adjustments. I'll get the counts down to an acceptable level then put the plate back on and see if it drives them up higher again. If it does, I either have to let more air in another way by opening the blades further, drilling holes in the butterflys, or drilling out the plate. Since this plate is made with internal passages for the fuel and nitrous I'm a little hesitant to go poking holes in it. I will definitely be calling Nitro Dave's for confirmation that I won't damage the plate by opening up that hole.