I swapped in a 454 ss tbi (2") with either 80 or 90 pph injectors for my stock tbi. Car wants to idle at about 1750 rpm in park, 1100 rpm in drive when I first start it up. After it warms up a little bit the park idle starts surging between 1250 and 1500 rpm fairly quickly, but drive stays solid at 1100 rpm. I checked for vacuum leaks, didnt see anything, so I think it's an IAC problem. Took it out and started the car, pindle moves in and out freely on its own.

Is there a table I should be looking at or a constant that will help get the idle down? Ive been reducing the numbers in the "IAC steps vs. coolant temp" table but it hasnt affected the idle.

I guess even a better question would be exactly what the numbers mean? There's a constant - IAC park position - and it's set at 160, can someone please explain what that means? I did a search and readup on what the iac does in general, that's straight forward, but what the numbers in the tables mean are tripping me up.

Thanks for any help, as always

 

OK

http://www.diy-efi.org/gmecm/papers/747iac.txt

That seems to answer a lot of my questions.

So say my park position is 160. When my coolant temp is 177* F my IAC step value is 51. Is that 160 - 51 = 109 steps from 0 (fully closed), or 51 steps away from fully closed?

 

No prom settings need to be touched to get it to idle down once the engine has past the warm up cycle.

The IAC isn't closing or there is a leak somewhere.

Thats not saying IAC operation can't be tweaked to get driveability to your liking.

 

Have you adjusted the throttle blade opening for the desired idle @ 0 steps? I'm not familiar with the specifics of the 8746. But, the procedure is usually similar across ECMs. I believe there's a procedure on this site under the technical info section...? It may be that your throttle blades are open too far for the IAC to compensate. Do you see values at, or very near, 0 steps at any of these operating points?

The number of steps represents the steps in the "open" direction. 0 = closed, 255 = open as far as possible.

It wouldn't surprise me if you have to modify some of the closed loop idle calibrations to get rid of your surging problem. If the 454 TBI IAC opening or pintle are shaped differently (i.e. bigger orifice or larger/smaller change in airflow for a step change), you'll ultimately have to calibrate the ECM to comprehend this.

 

Chuck!

Did you solve this problem?
I did the same swap and have the same situation. I have no vacuum leaks. If I adjust the throttle blades with the IAC extended the idle still jumps up after the IAC is reconnected. I can back the throttle stop screw off until the blades are closed all the way and the IAC will open far enough to keep the idle high - in the 1100-1200 range. The IAC is clean, I've even tried a different IAC but made no difference.

I started a search and this post came up as most recent. I was suprised at the similarity in the situations.

 

Do you mean adjusting the throttle minimum air position? There's a novel idea. Don't overlook the basics. With a larger bore TB like that, a little opening provides a lot of air.

 

Well, OK, I'll bite.

No, what I'm trying to say is that no matter where the throtle blades are adjusted after the IAC is reconnected, the idle goes back up. With the IAC disconnected (and in the extended position) I can set the throttle blades to either a good idle speed or even close them so far the engine will die. In either case, after the IAC is reconnected the idle goes back up to 1100-1300 rpm.
Suggestions?

 

Some codes require you to drive the car for a VDD reset to accure.
No reset, no workie, rightie (Doc's terminology).

 

I don't have a 454 TBI to measure. But, does anyone know if the IAC opening and pintle have the same dimentions as the smaller bore varieties? It might be that the computer is commanding a number of steps that give a certain airflow that's acceptable in the smaller bore TBI. But, when you go to the same number with the larger TBI, you get more airflow and a higher RPM??

 

Same size.

 

More trivia:

There are small taper, single large, and dual taper Pintles.

From what I've seen all the dual's measure out the same. But, obviously, even between Doc and I there's no guarantee we've seen anywhere near all of them.

 

Are the engines in question up to proper operating temperature? Say 200 f or more? If not then then the idle won't come down, may even be at a cold high idle. Also be sure that the battery voltage is correct. Low BatV and the idle gets kicked up too.

RBob.

 

Thanks guys.
No codes have been thrown.
I have a 180 degree thermostat. With the SB TBI, I had 195. I haven't checked voltage into the ECM. It is, however, a new battery.

 

Good possibility that 180 thermostat is the problem. Try the 195 and sse if the problem goes away. Or, burn a new PROM with the IAC counts lowered in the IAC position vs Coolant table.

RBob.

 

my 7.4L GM TBI is going on soon so this is of interest to me. seems high idle is a common complaint i see out there. CHUCK. have you considered zeroing out the iac vs coolant table just to see what effect is? might confirm vac leak. i in effect have one IAC at zero since stuck closed and am working off of only one functional IAC(crossfire) and see a cold engine startup that is compromised(minimally high idle) but still does not die. in CL i am at zero steps IAC(aldl) and believe the butterflies are bottomed on the throttle stop. and butterflies set at max closed position with screw. no further adjustment. still warm CL idle is a little high without AC on. that tells me some leakage around 'flies or the bore vs 'flies diameter a bit off and some air infiltration.