7747 IAC problems...help!

**halljam70** · 06-25-2015

BLUF: No matter what I change, IAC retracts to wide open when I shift truck from park to drive.

Backstory: 86 Chevy 305, 204/214 @ .050 rv cam, 416 305 heads, 350 injectors. I set max IAC to 0 or 20 on every table I could find, and no matter what, as soon as the truck is put into drive, the IAC retracts to full open even though winaldl or tunerpro R/T show an IAC count of 20. I can live with the truck if I ground the ALDL and unplug the IAC. However, I'm totally lost on why this is happening. I replaced the IAC, coolant sensor is working, not sure what the hell to try next short of pulling this engine out and putting a completely stock 350 TBI motor in its place. Attached file loaded is is currently loaded program. I've checked all over for vacuum leaks and didn't find any. I'll get a data log in the morning. Appreciate any advice to try and help solve this.

**jim_in_dorris** · 06-25-2015

ASDU is a 350 bin, The IAC is probably trying to add enough air to keep that much fuel from killing it at idle. Try a 305 bin and see what happens. Does it run better with IAC unplugged?

**halljam70** · 06-25-2015

Yes, it runs better when I unplug it, at least it doesn't turn 2K when I shift it out of D with the IAC unplugged, or load itself up against the converter. I did modify it to take fuel out of the VE tables (combined ve1 and ve2) and changed the BPW to reflect that it is a 305. Even driving it down the road the IAC will not close. I'll double check it tomorrow to see what the data logs look like to see how rich it is running. I suspected that it was using the IAC to offset too much fuel, however, I've ran it with both the IAC plugged and unplugged and gotten similar BLM readings.

**Six_Shooter** · 06-25-2015

So the ECM is reporting no change (or very little change) when this happens?

How have you verified that the IAC is actually going full open?

**halljam70** · 06-26-2015

By full open I'm saying that it opens to at least 145 count. I verified it with both TunerPro R/T and WinALDL. IAC counts would be between 30 - 50 at idle in park. As soon as I dropped it into drive, the IAC count would start to rise until it topped out at 145. Kicking it out of gear sent the idle up to 1700 to 1950, and the IAC valve would slowly lower in count until the idle was back down to around 800. I've got the fitting to check fuel pressure but neither my autometer gauge or my normal fuel pressure gauge will connect to the fitting. I haven't had time to run down to Home Depot for a brass adapter fitting.

**edfiero1** · 07-27-2015

Did you solve the problem? I am seeing high IAC counts in my car while idling and even higher when driving. Trying to understand when the throttle blades need to be cracked open more with the adjustment screw vs. what is considered OK with a large duration cam.

**Fast355** · 07-27-2015

Years ago I had the same issue with an almost identically setup 305 TBI in my 1983 G20 van. I started from scratch with a different newer .BIN and the problem went away entirely. It would idle about 1,300 rpm with the a/c on in park and race up over 1,600 rpm with the a/c off. The .BIN I had was corrupted in some way and changing the .BIN with an uncorrupted .BIN made the engine easily tuneable. My setup was a .040" over flat top piston 305, ported 601 heads with 1.94/1.60 valves, 204/214 @ .050, 110* LSA cam, 1.6:1 roller rockers, 1.5" primary long tubes and 2.25" dual exhaust with a TBI setup from a 350 1992 G20 van.

**halljam70** · 07-28-2015

Thanks Fast...here's what I think was wrong with mine (and I meant to update this thread, just been buried at work). After replacing the TPS and O2 sensor (both needed it but didn't solve the problem) I got it fixed by setting the blade angle correct in the throttle body. I used several of the techniques I had found on both this site and 3rd gen. However, on one thread one person mentioned how they used a small strip of paper set the throttle blades. I tried it, and despite having it idle around 700 rpms in neutral, the paper would not go past the throttle blades. After a little trial and error I got it to pass the blades with a slight drag on the paper. Plugged the IAC back in, fired it up, and the counts were around 10 in idle after it warmed up. Put it in gear and they stayed stable without the IAC stepping to 145.
Now that this is fixed, I'm about to embark on swapping over to the 7427 computer. This $500 dollar parts hauler gets more attention than my Nova.