Ok, so here's the deal. It's a new engine and I'm still working out the bugs.. so to speak. I go out today to attempt to set my minimum idle air so I bump the idle up a bit before I start the car. I start the car cold and it starts to surge back and forth between 1k and 1100 rpms... not a slow surge, pretty quick actually... back and forth. So I press the gas a bit and hold the idle at about 1800-1900 rpms and it idles fine... I let off and it starts idle fine.. well, a little rough but didn't surge. As the engine warms up it starts to idle smoothly at about 1200rpms so I'm satisfied. I put my foot on the brake and put the car in reverse. The car immediately drops to 500rpms and idles like **** for just a few seconds and then immediately jumps to 800rpms and idles great... nice and smooth. I shut the car off for a minute and then restarted it.... it began the surge again.. with the car running about 165 degrees so I got confused. I revved the car toa bout 1900rpms again and held it for about 20-25 secnds and then let off and it started idling smootly at 1200rpms again. Very confusing.
Question one.... what could be causing the surge?? IAC maybe?? when I paper clip the ALDL box and turn the key on, it sounds quite sickly. But at the same time I didn't think the IAC really did anything once the engine got to operating temperature which it was the 2nd time I started it.

Question two... what could be causing the sudden drop to 500rpms for a few seconds before it jumps back to 800rpms and idles fine in gear?? IAC again possibly??



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1986 Trans AM
355 TPI
4 Wheel Disc Brakes
9 bolt Borg Warner Rear (2.77's....oh joy) :P
6" rods, approx. 9.7:1 Compression, Stock TPI (soon to be ported), 24#/hr LTI injectors (cleaned and flowmatched by Cruizin Performance), Hedman Shorty Headers, 3" exhaust with 80 series flowmaster, SBC 993 heads completely redone and modified for 1.6 RR, 1.52 RR (for now), Comp Cams XE262-14 TPI Cam, Holley AFPR, TPIS airfoil, MSD wires, MSD 6-AL Ignition, Hypertech 53,000 volt coil.

Track times to come.

 

Ok, do this: when you get the car idleing smooth where you want it, like 800 rpm, unplug the IAC motor while the car is still running. This way the IAC can't move so if your idle still fluctuates you'll know it's due to some other factor.

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91 Z28
Come see The Vicious...

 

If I get it running smooth at 800rpms then when I unplug the IAC it's not going to fluctuate because it's already running smooth so that won't tell me anything by doing it that way.

Tonight I took the car out and down the road for a very short trip. As I come up to a stop light the car downshifts and idles smoothly on it's own...but I went down a back road and stopped the car and shut it off... I then restarted it and put it into gear and it immediately dropped to 500rpms and died. I restarted it and gassed it a little while putting it into gear so it would stay running and then I took off down the road again. When I came up to the next stop sign I let my foot of the gas and stopped and again the car idled smooth and on it's own. Now to me it seems like the only time I have a problem with the rpms dropping so low and the car dying is when I initially drop the car from Park down to drive. I'm beginning to wonder if it really is my lockup causing the problem. Maybe when I drop it from park to drive the lockup is immediately kicking in and causing the drop in the rpms and the computer can't react quick enough to adjust the idle back up before it dies. OR ... maybe it's not the lockup and the IAC is just wore out and can't move fast enough to catch up before it dies.?



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1986 Trans AM
355 TPI
4 Wheel Disc Brakes
9 bolt Borg Warner Rear (2.77's....oh joy) :P
6" rods, approx. 9.7:1 Compression, Stock TPI (soon to be ported), 24#/hr LTI injectors (cleaned and flowmatched by Cruizin Performance), Hedman Shorty Headers, 3" exhaust with 80 series flowmaster, SBC 993 heads completely redone and modified for 1.6 RR, 1.52 RR (for now), Comp Cams XE262-14 TPI Cam, Holley AFPR, TPIS airfoil, MSD wires, MSD 6-AL Ignition, Hypertech 53,000 volt coil.

Track times to come.

 

No, I meant for you to unplug it and drive it around that way. If the IAC is causing your problem this trick will find it. Otherwise you'll know it's not the IAC. You can drive the car without the IAC plugged in, I did it for about a month when my car was having some 'issues'. The main thing you'll notice if you leave the IAC unplugged is that the engine won't rev high when you start it, only a problem if the engine is cold. After initial startup the IAC's job is to compensate for loads placed on the engine at an idle, and to dashpot the throttle so your car doesn't die when it comes to a stop (it sounds like it's doing this part OK). So unplug your IAC to see if your problem is load related (like TC) or just plain stupid IAC related. But you may be right, it does sound like it could be the TC.

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91 Z28
Come see The Vicious...

 

It sounds like your IAC motor is either dying, the IAC passages are clogged, or the IAC motor is not adjusted to operated in the proper range. If you haven't done so recently, I'd pull off the throttle body, remove the IAC motor and the passage underneath the TB, and give everything a good cleaning (including the TB itself). If that doesn't help, I'd try adjusting the IAC before replacing it.

Since you have a built engine and the idle speed you're requesting is probably higher than the stock one, you'll need to follow a different procedure than the regular one in the tech articles. What you'll want to shoot for is your IAC counts to be in the 10-20 range when your engine is warmed up and your tranny is in Drive. This is generally the condition where your IAC motor will be open the smallest amount, so shooting for a low IAC count at this point gives the IAC motor the highest operating range possible.

One of the reasons why your engine is stalling could be that your IAC motor is maxing out trying to "catch" the engine, that is, the IAC count is hitting 160 (IAC fully open). This could be the same problem when your engine is cold and could be causing the surging, but that could also be the result of not enough fuel when in open loop (or not enough timing, or fluctuations in the timing table). I doubt the lock-up is causing the problem since it can't operate when the tranny is in 1st gear, and it would do it all the time (fatal to the tranny) if it was not working.

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Greg Westphal
'87 IROC 355TPI/A4

 

That sounds like the problem. The IAC isn't compensating quick enough, if you will, when the initial load is placed on the engine when I drop it from park to drive. Then it "catches up" while I'm gassing it to keep it running (hence the 800rpm smooth idle after a couple seconds) and that's why it doesn't die when I come up to a stop sign. Judging from the sickly sound of the IAC when the A and B terminals are jumped and the key is on, I would say this is probably my problem. Next problem... money.

 

I had the TB off when I did the engine swap. The TB has been cleaned. There's a slight problem with removing the underneath passage on my throttle body... when I took the TB off, I attempted to remove it to change the gasket and .. well... one of the heads on those little bolts snapped right off. I've had nothing but trouble with torx bolts.. regardless of size.

BTW... how the heck do you "adjust" the IAC itself?

I'm not sure the maxing of the IAC is the problem. I figure if it were maxing then the rpms would never "catch up" ... and they do after a couple seconds and then the car runs fine.

The fuel and/or timing could be a problem though... I don't really know as this is my first new engine and until now I've never really had to mess with anything. Right now my fuel pressure is at 40psi using 24# LT1 injectors with a stock 305 computer chip. My timing is at about 6*. What do you think??



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1986 Trans AM
355 TPI
4 Wheel Disc Brakes
9 bolt Borg Warner Rear (2.77's....oh joy) :P
6" rods, approx. 9.7:1 Compression, Stock TPI (soon to be ported), 24#/hr LTI injectors (cleaned and flowmatched by Cruizin Performance), Hedman Shorty Headers, 3" exhaust with 80 series flowmaster, SBC 993 heads completely redone and modified for 1.6 RR, 1.52 RR (for now), Comp Cams XE262-14 TPI Cam, Holley AFPR, TPIS airfoil, MSD wires, MSD 6-AL Ignition, Hypertech 53,000 volt coil.

Track times to come.

 

Try the idea of unplugging the IAC first, see where that gets you. If it still idles rough, then you have some hard problems to figure out. Now, as for adjusting the throttle stop (idle speed), its not like a carb, you can't adjust it too high, because then the IAC can't do its job efficiently anymore. Think about it, if the throttle blades are letting enough air in to idle on its own, then the computer can not control the idle speed anymore. I had the dealer set mine up, they hook the diag. computer up and see where the IAC values are... then they adjust the throttle stop until the IAC is in the proper parameters.

Next thing you may want to do, is find another throttle body... you want to clean out the passages where the IAC seats, they get really gummed up.

Have you cleaned the IAC valve yet?

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1988 Pontiac Trans Am GTA L98 5.7L, Black on Black with 114,000kms.

Best ET 14.559 @ 95.25MPH
Custom 3" Stainless Exhaust Y-Pipe back (High flow Cat and Flowmaster Mufler)

 

I think you need a scanner.

It would be really useful if you could see those numbers, like IAC position, injector pulses, etc. Are you very far from KC?

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91 Z28
Come see The Vicious...

 

I live about 4 1/2 hours from KC in southwest missouri... right down highway 71 pretty much. Right now I have Craig Moates free scantool software and I'm working on accquiring the parts for the two transistor communications cable. Then I gotta lug this big @ss PC out to my car with an extension cord and then I can at least see what my car is doing while sitting still. Won't be able to do the "moving down the road" scanning until 1. I get a laptop or 2. I get a power inverter so I can throw my PC in the back seat.


------------------
1986 Trans AM
355 TPI
4 Wheel Disc Brakes
9 bolt Borg Warner Rear (2.77's....oh joy) :P
6" rods, approx. 9.7:1 Compression, Stock TPI (soon to be ported), 24#/hr LTI injectors (cleaned and flowmatched by Cruizin Performance), Hedman Shorty Headers, 3" exhaust with 80 series flowmaster, SBC 993 heads completely redone and modified for 1.6 RR, 1.52 RR (for now), Comp Cams XE262-14 TPI Cam, Holley AFPR, TPIS airfoil, MSD wires, MSD 6-AL Ignition, Hypertech 53,000 volt coil.

Track times to come.

[This message has been edited by 86TpiTransAm (edited November 20, 2001).]

 

OK, sounds like that's not the problem then. Probably a sluggish IAC motor or clogged passages. Oh, I meant to say adjusting the minimum idle air screw so that the IAC motor operates in the proper range. You'll want to open the screw up enough so that the IAC counts are in the 10-20 range in Drive with the engine fully warmed up.

I think you need to swap out the 24# injectors for stock (or SVO) 19# injectors. You're running way too rich with those injectors and a stock 305 chip.

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Greg Westphal
'87 IROC 355TPI/A4

 

Oh I know I'm running way too rich with these injectors and it sucks. I can't put the stock 19# injectors back in because they're all shot to hell. I can't afford a "proper" chip right now. ****, I can't afford anything right now and my MPG sucks.

As far as the IAC, I've come to the conclusion that it's dead. See my last post here at this other topic https://www.thirdgen.org/messgboard/...ML/005419.html


------------------
1986 Trans AM
355 TPI
4 Wheel Disc Brakes
9 bolt Borg Warner Rear (2.77's....oh joy) :P
6" rods, approx. 9.7:1 Compression, Stock TPI (soon to be ported), 24#/hr LTI injectors (cleaned and flowmatched by Cruizin Performance), Hedman Shorty Headers, 3" exhaust with 80 series flowmaster, SBC 993 heads completely redone and modified for 1.6 RR, 1.52 RR (for now), Comp Cams XE262-14 TPI Cam, Holley AFPR, TPIS airfoil, MSD wires, MSD 6-AL Ignition, Hypertech 53,000 volt coil.

Track times to come.

[This message has been edited by 86TpiTransAm (edited November 21, 2001).]