I have been trying to tune my 1227747 and keep having trouble because it gets real lean at 60-90 MAP and 2600-2400 RPM. That is, when I open it up part way, not far enough to enter PE mode, it gets lean, backfires, etc. I pulled the air cleaner and noticed that there was a loud whistle (lots of air flowing) at the IAC hole. I plugged it with my finger, raised the mechanical idle to get it to run and removed my finger and noticed that it wasn't flowing as much air. So, I disconnected the cable, put the air cleaner back on and took it for a drive. No backfiring, only slightly lean, and runs much better.

I have noticed at times that when the engine is warm, and sitting at idle, that the IAC is at 145 and doesn't move. I haven't looked at the IAC in winaldl when it goes lean and backfires, but I suspect that it is open. What would cause this? Any ideas? Any pointers to websites that might have clues? I've printed out and read most of the posts I have found, but this one has me stumped.

Thanks for any help.

A sticking IAC, possibly

Oh the infamous IAC. Yes, some folks like to close it and pull the plug. Understandably at that. There is a paper on the gmecm.org website that describes the '747 ECM's IAC logic. Contrary to the papers statement about not finding a closed loop idle there is one.

Other then that the info there will give you a better understanding of the IAC.

RBob.

Yeah, you'll understand it so well that you'll want to just yank it and keep it on your desk to look at every day . I'm a firm believer of not needing it. I did countless 60-0 hard braking to see if I could stall the car out, never happened. Of course I do have a slightly higher stall TC which probably helps, still, it's a royal PITA isn't it. I have since then hooked it back up and adjusted all the thresholds to limit the IAC to only 50 counts. Throttle follower counts are limited to only 20 counts and the lag filter is set to decay a lot faster. This still caused some problems but nothing like the stock iac. Now the car runs great and doesn't require standing on the brakes when the motor is cold. (also changed the coolant vs iac steps table).
Keep in mind this is with the holley 670 TB, not the GM. There are differences in the design, slight enough to make a noticable difference.

Definately not sticking, the computer is definately moving it. I think that maybe the reason it is so high at idle is that I put the ECM into closed loop idle mode and it must be trying to use it to lean out the mixture???

I'll read the doc on the GMECM sight again.

Thanks for the help.

The only time the IAC directly affects the fueling is during opening stepping. There is fuel (async mode) added. Closed loop idle and closed loop fueling are two different things. can have one without the other and vica-versa.

Best bet is to open the butterflys 'til the IAC steps are ~ 10. This is with a warmed up engine during idle. You will need to open the butterflys then start the engine and reduce the butterfly opening. Otherwise the ECM will think that you have the foot on the go pedal and drop out of idle control as you open the butterflys.

RBob.

I'll give that a try again. I've done it before, but then I go and change the BPW or something and it screws things up. I'm still pretty early in the tuning cycle I think -- I've only tried maybe 20 or 30 prom images.

So, why is it opening up during acceleration, but not in PE mode???

I'll read the doc and try to find out.

This is because there is the min TPS position before idle control correct? I haven't had a chance to play with it but my TPS voltage is at .54 volts at idle. Isn't that within range for my car to go into idle control?

The internal ECM TPS% value is self adjusting (within limits). The ECM is checking the lowest seen TPS value since startup and uses that as 0% TPS. It will always go lower but not higher.

And yep, as the butterflys are opened (w/screw) the TPS increases. Once over ~ 2-3% open then the ECM figures you have the foot on the gas and stops trying to control the idle (unless it goes too low).

RBob.

So, it looks like my backfire problem may be caused by the throttle follower routine desribed in Robert's paper. OK, I can reduce the percent, and I can set a maximum. Buy what, exactly, is the logic behind doing a throttle follower? I can see opening it a bit so that the engine doesn't stall when you let off the gas, but why a percentage?

Another related question. The stall saver seems to dump buckets of gasoline down the engine's throat when it activates. Is this adjusted at location 0x293 (mentioned in the above paper - additional fuel added during IAC opening)? Or is there some other location for adjusting stall saver fuel?

Just a thought and may sound stupid but I had a very similar problem with mine in the begining after intake swap the clip had broke off and every time I plugged it back in it was not in all the way and after weeks of tuning and cleaning and what not I realized if I push a lot harder the connector went in another
1/8 and bingo.
Im sure thats not your problem but never know.

I don't think that is it. The pintle definately moves in and out, as i can hear the whistle change when it moves. In fact, when it was pretty wide open, I raised the idle with the idle screw, put my finger over the opening that it sucks air through and it closed up nearly all the way. That's when I removed the cable. It's still a little open, but the car runs much better.

I'm writing a VB program now to help me adjust the BIN file for all the IAC constants. Once I get it done, I will start playing with values and see what happens.

I think I may have found root cause for why my engine often runs lean. One of my injectors is intermittent. So, losing 1/4 of my fuel flow could definately cause a lean condition. I will try to fixt it today and see if things improve as they should. Then I'll tackle the IAC.

Is that injector on the same bank as the 02?
If not thats probably not the entire problem.

Just a thought. I helped a friend convert to TBI a couple months ago. He was using the 454 TBI. We found that the IAC did not work...I happened to have one from an Isuzu Impulse I ofund at the junkyard.

We put that IAC on the 454 TBI and it still didn't work right. It moved and everything, but it didn't help at all.

Turns out that the pintle on the Isuzu IAC was ALOT smaller than the OEM 454 pintle. The IAC wasn't closing off the passage at all. We switched the pintle and it worked fine.

I wish I had taken more notes (yeah I've heard it all) when I tuned my IAC in. Mine works great and has never caused a problem. My tuning method was keep messing with one thing at a time until it worked the way I wanted it. It took me about 3 months to get it perfect. Cold start was a PITA...

If I remember, both sides of the manifold feed both heads or something like that. So, yes, I think that the injector on the right can impact the mixture on the left (where my O2 sensor is).

The IAC is the right one for the throttle body. This is a holley 4 barrel BTI with integrated IAC.

Well, i put in a different injector and the car runs a whole lot better. Now I think I can begin tuning for real No more backfiring, runs strong except that I have it pretty rich at WOT. I can fix that though!

Thanks for all the help.

I think I've got one better. How's about good fuel injectors but a fuel pump that couldn't even wet the return line with the engine OFF!!! Backfire was an understatement. It would just go pop and shut down. What a joke.