I'll post this in the Nitrous and blown as well... perhaps someone can give me some assistance...

Since I added the intercooler to my car, I have been getting MIL lights coming on DTC 506 Idle system low IAC responding... DTC 507 Idle Hight IAC responding...

I'm hoping someone who is superchargered with LT1 edit can share if they have had the same experience... I'm due for emission here soon and it's going to be a pain let alone the risk of the light popping on...

The idle seems stable... the other issue I have is that the IAC is going to 0 at idle where most things I have read say that it should be at 25... if I adjust up the idle (throttle body) the idle just increases and I don't see a difference in the IAC. I have also cleaned and replaced the IAC... I'm running a BBK 52 mmm throttle body, and the best I can tell I have no vaccuum leaks...

Appriciate any programming suggestions i.e. the idle and iac settings in the ecm..

Thanks!

mo

 

FWIW, IAC = 0 generally means you need to close the throttle plates, to push air through the IAC @ idle. Did you look at this site?

http://members.***.net/chipsbyal/idle/

It speaks to idle air on an LT1 w/ aftermarket TB. Don't know anything about the mod myself, but Al has a pretty good reputation as a tuner...

HTH

 

Doctor J,

Thank you! That is some great information... I thought I would need to close the throttle plates too, however I can totally back the screw where the blades are completely closed and still get a zero...

I even replaced the gasket between the throttle body and the plenum thinking that I was leaking somewhere...

As I remember (without pulling the TB off) the orifices are the same as stock, obviously the variariable here is the supercharger.

I'll look further into the information you pointed me too... in you "humble opinion" which has been very valuable. Do you think upping to a 58 mm would be advantagious ? Right now the max I have seen in CFMs is only 722 as the block is stock. I'm thinking would not be a benefit, but curious to what you think...

thanks again

mo

 

Mo:

You're welcome! (You should really send the thanks to Al, he did all the work...)

Something doesn't sound right - as you close off the butterfly the air has to go somewhere to get into the manifold...

Three possibilities come to mind:
1. Throttle cable is binding or too short, so plates can't go 100% closed - did you check visually, or if you close the throttle plate all the way does it stall out?
2. IAC unit is defective - the 'steps' reported in the ECM are just feedback from the stepper-motor in the IAC. If the motor or connections are weak(?) the ECM can't register the pintle movement (and so reports '0' even when it has actulally moved off 0).
3. IAC pintle needs to be reset physically to its 'park' position. There is a procedure in the GM manual for 'idle air reset'. I can never remember if it's supposed to park fully open or fully closed....

If that doesn't work, I'll try to think of something else.


Re the larger TB - I dunno, I'd have to dig around and remember how to calculate a Reynolds number for that airflow. Last time I looked at it, I seem to recall that the deltaP induced by the TB was pretty small. If you go up to the larger size it would only gain you a few inches of water column - which I think would be small in comparison to the pressure drop of the I/C, valves, heads, and whatnot.

It's a good question though. I'm booked up this weekend, but if I get a chance next week I'll see if I can do the calc and come up with something more exact.

(BTW, I'm jealous of your set-up.. wish I had one like it to play with.)

DrJ

 

Dr J,

Thanks for the compliment... I'm lucky I guess... I started this project about a year ago thanksgiving...

I replaced the IAC... pricey lil guy... replaced the gasket between the throttle body and plenum. The throttle cable is loose and as plenty of slack. I did have a slight issue with this throttle body, where it seemed to bind and stick at the closed position. I realigned the throttle plates and got the binding to stop. The plates visually are closed to a point the set screw isn't touching the throttle assy.

I might add that also with the stock throttle body I had the same issue with the IAC...

When I scan the system with autotap, the IAC is parking at 160 and after the car reaches about 150 degrees it starts spooling closed to zero... I mean this thing is so close its not funny... I drive 9mph the iac reads 31 I stop it goes to zero. If I start the engine and go through the process per the manual... (I have that too...) it seems to go through the process however it settles back to zero....

Here's another though... this started when I added the intercooler to the system... at the same time I had to replace the bypass valve for a smaller one 1 inch vice 1 3/4 inch... I' wondering if the diameter of the bypass is not dropping the inlet air level enough... sheesh so many variables here... I guess a quick check would be open the output of the bypass valve the engine will go pig rich but it should show if the IAC is making a compensation...

I'm probably wandering here.. I certainly appreciate you point the information my way.. that may end up being what I need to resort too if I cant resolve this...

Please let me know if you have any additional thoughts... I'll keep you posted as well ... thanks again..

Mo

 

Mo-

OK, that makes it a little more clear. Here's today's hypothesis:

It sounds like (setting process engineer hat firmly on head) the IAC & butterfly are working correctly...but a little too much air is getting through the TB at idle, thus the IAC PID loop is out of its control range (by a couple of cfm) & can't go any lower. To put it in perspective, at idle the total air flow is ~30 scfm or thereabouts. The extra air you are getting is 'probably' due to a little extra positive pressure from the blower/spillback valve arrangement stacking up air in front of the TB.

If my hypothesis is right, it's a matter of trying to cut the idle flow through the IAC by a few cfm - i.e. introduce a little more deltaP at the IAC flow orfice. Test that by maybe putting a piece of tape over the IAC inlet (on the front of the TB somewhere) & poking a hole in the tape a little smaller than the air passage. See if that brings the IAC steps up at idle.

Should that not work, send me an e-mail off line - I'll ponder it over another beer (or maybe less beer). HTH

DrJ

 

Dr J,

I think you nailed the same hypothisis as I had... I'll try that tomorrow, another thing i'll try is while i have the hoses off the TB i'll unplug the MAF to keep it from choking and verify that the IAC is reading something above 0.

I'll plug the little hole between the throttle plates and reconnect to see it it helps... The more I meditate in the info you first provided me the more that makes sense to me.

As I may have earlier stated once I'm rolling the IAC is tracking it's just at idle... so the engine is trying to find the right idle point and once its at zero it kicks the code out because it can't go any lower...

Again I'll try your suggestion tomorrow and keep you posted...

Thanks for taking the time to work though this with me...

Best regards,

Mo