The IAC acts like cruise control!?

[bobc455]

Spent some time with tyrefryer yesterday tuning his car. Great car, BTW! I love it.

Anyhow, we had a couple of weird things happen, including something strange going on with the IAC that had us very confused. While driving around, the IAC motor was jumping all over the place- up over 100 at times (as viewed on the dashboard). This was really pushing the car along, especially when coasting to a stoplight or even under light throttle driving. Eventually at a stoplight the IAC would return back down to 10-15 and the idle speed would settle down to setpoint, but it would take 15 - 30 seconds to get there. We had slowed down the idle P & D reaction to dampen an idle speed oscillation, so the IAC would act kinda slow, but not that slow!

Checked out the Max TPS for idle, that was set to 11, and the TPS rested at 10 at idle. TPS during driving was well over that, like 20-25, so that shouldn't be moving the IAC around.

Next thing I did was drop the throttle follower to zero all the way across, so that wouldn't affect it.

What else is left? Why would the IAC be going sky high and pushing the car like that? The car was well above idle (both RPM and TPS), there was no throttle follower, yet the dashboard was showing the IAC values to be very high (jumping between 60 & 100).

I can't think of any other settings that would make the IAC move.

-Bob Cunningham

p.s. Engine was a 355 smallblock Chevy with 83# injectors (injector opening time set to 1.5 ms on advice from his injector supplier), belt for the supercharger was removed so we were driving around NA.

 

[ZBass28]

Back when my engine was stock (why did I abandon those days??? ) I once had a very similar problem where the IAC would jump up/down and force the idle speed up. Coming to stops would be a nightmare, almost felt like the throttle was sticking. Turned out to be a vacuum leak at the the throttle body that was causing the IAC to freak out. The TB gasket had a small crack in it. The tip off was that I was able to see the strange fuel corrections with my Scanmaster tuner and knew that there had to be some funky air delivery problem. New gasket fixed that up.

 

[tyerfryer]

I changed all the gaskets on the throttle body no change.I don't know what else it could be . Anybody with any ideas?.

P.S. Thanks go out to bobc455 for the time he spent to help me get the car dialed in.

 

[Craig Smith]

Did the IAC value jump around only when the TPS was at or below the idle threshold? I sure hope you say yes, because the only thing that determines the target IAC position above that threshold is the throttle follower.

Did you try increasing the gain on the IAC motor? I'd give that a try. If it causes the idle to become erratic, richen it up or try and add timing trim. I'd be interested to see what happens with this. Consider it an experiment to determine decent P and D gain settings for this car.

 

[bobc455]

Definately not, which is why I am so confused.

Max TPS at idle was 11, and the actual position when this was happening was 20-25 (maintaining about 40 MPH). And, like I said, the throttle follower was zero'd out. And RPMS were around 2000, well above the idle setpoint (about 800 RPM setpoint).

All I can guess is that:
1) There is some other parameter that makes the IAC motor move
2) The IAC was getting some electrical interference from something else, maybe a spark plug wire or something (but I think the IAC is an open-loop control, which means that if this were happening it wouldn't show up on the dashboard)
3) An Alien force beamed a weird program into the FAST just to play with our minds

The idle was very erratic. We had to dampen the P & D settings quite a bit to get rid of oscillation- I think we started with 14&22 and ended up with 5&7. And I forget where the idle timing ended up, but that would cause an oscillation too (even though we had like a 12:1 A/F at idle). It seemed relatively calm in the 40's, but I think our final value was in the mid-teens (16-18). In the 20's and 30's the idle would start to oscillate all over the place.

ZBass, I just saw a new car (300ZX?) with NH plates that said ZBASS the other day and thought of you. Anyhow, I don't think a leaking gasket would affect it this way, thanks for the idea though.

Confused in Boston,

-Bob Cunningham

 

[Craig Smith]

Well, sometimes alien cosmic rays and time-space continuum warps are the only logical explanation for these sorts of things. I am truly at a loss for an explanation here.

 

[ZBass28]

Originally posted by bobc455

ZBass, I just saw a new car (300ZX?) with NH plates that said ZBASS the other day and thought of you. Anyhow, I don't think a leaking gasket would affect it this way, thanks for the idea though.

Really? Damn, I need to collect royalties! And to think "I" was just looking at those new Nissans myself.

I just remembered (to be more specific) it was the IAC motor housing gasket on mine that was bad and needed replacement to solve my surging, erratic IAC, although tyrefryer already changed all the gaskets so that rules that out. Good luck guys.