Ok, so the first issue we found was that the intake had a major vacuum leak contributing to the idle problem. We reset the intake today hoping to clear this up once and for all but another odd bit keep coming back. The system keeps (seemingly) destroying IAC's.
The original one died, then I replaced it with the one from the Holley 670cfm TB, then it died, then I went and bought one and it worked for about 3 seconds then went static and died.

When I made the conversion, Troy at Howell had me switch D2 and A11. But when I cross referenced those two from the FSM schematic, to the 7747 pinouts scanned in on diy-efi, I switched them back. Heres what happens (problem occurs with the pins swapped or not):

I turn the car on and it idles for about 25 seconds at 1100rpm. Then it starts surging between 1300rpm and 1900rpm. If I crank up the manual idle screw to about 2000rpm it idles steady, but obviously WAY too high. I replaced the IAC (it died, no movement at all), then I replaced the O2 sensor (minor improvement). So I'm kinda stumped.

I rechecked and the 4 wires from the IAC and they properly go to C3, C4, C5 and C6. So it
isn't miswired. I'm not getting any codes or a check engine light. So I'm not sure now.

Anyone ever heard of an ECM eating IAC's? Could my 7747 have gone bad from Howell instructing me to swap D2 and A11? And would that cause the ECM to wig and turn to the dark side?

Help Me Obi-Wan, Help Me Obi-Wan....

Any thoughts?

Kelly 'GhoSSt' Rosato

 

Sounds like we've been dancing to the same tune. I just finished attacking my idle problem Saturday and it turned out to be a vacuum leak too. "O" ring from one of the old injectors had stuck in the manifold when I was trying different size injectors a few weeks ago. I never noticed that when I took the old injectors out, one of the "O" rings got stuck.

Anyway, I had Diacom running as I was making some final adjustments and noticed that my TPS voltage readings were a little high. So rather than adjusting the voltage the recommended was with my Fluke Digital voltmeter and the engine off, I just adjusted the TPS readings with the engine idling until I saw the number I wanted.

Then when I looked down at IAC counts, I noticed that that had locked; not moving at all.

I shut down the engine and removed the key. Waited 30 seconds and started it up again and the counts resumed as normal.

I went through the same procedure a couple of more times and found that, for some reason, each time I made an adjustment to the TPS voltage with the engine running the IAC counts would freeze.

Today, I'm going to reset the TPS the recommended way and then reset the IAC using the ALDL A/B procedure.

If I remember correctly, IAC counts on a Diacom or ScanTool are only 'commanded' counts. They don't indicate that the IAC has actually responded to the ECM input, only that the ECM has commanded it to move.

Try jumpering the ALDL to fully extend the pintle of the IAC, then unscrew it as see if it has actually moved to the fully extended position.

Once I even did this with the IAC unscrewed so I could actually watch the pintel extend. I did see one post though, where a guy's pintle came completely out doing it this way. Mine didn't though.

Hope this helps.

Jake

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1986 Corvette Coupe, 415 CID, Edelbrock 6073s, ZZ9

 

Interesting. I did much the same this morning. My biggest problem is that I'm flying blind, I have no scanner and no access to on right now. All of my mechanic buddies have moved on to other things. I did what you said and the IAC did extend and you could feel it 'stepping' with the ALDL jumped. So its really not killing them, but the ECM is trying so hard to compensate for the rich tables that its retracting the IAC fully to the point where it locks back.

I've sprayed carb cleaner around the intake looking for leaks and I may still have a minor one, I don't know yet. The intake is drilled for both pre 1986 manifolds and the newer style, so it seems to want to leack around those holes mostly.

The Howell sent me one chip originally (what I'm running now), I told them of the idle problems and they sent me a leaner one (too lean won't run at all) and so he's sending me one in between. I'm going to try to get with Dan over the next week and get this BIN pulled so that we can look at it and play around some more. But I thought I'd give Howell one more shot.

Kelly 'GhoSSt' Rosato

 

you wasted your cash on the o2 sensor , its ignored unil it warms up and the car surpasses the clsd loop coolant temp threshold

 

Hey, I didn't WANT the damn thing. But there are only a few pages in the dictionary between desire and desperation.

Kelly 'GhoSSt' Rosato

[This message has been edited by GhoSSt (edited January 08, 2001).]

 

I've discovered that these TPI setups are very, very sensative to vacuum leaks. It only take a very, very small leak to cause idle problems, especially on cold start.

Before I found the "O" ring in the #1 injector spot, I had to sit and nurse the throttle to get the engine to stay running until it completely warmed up.

Even then, I had to open the TB bores so as to maintain 1000 RPMs in neutral and 800 in gear.

I saw another post recently where a guy had found a vacuum leak at one of his injectors too, where the injectors fits into the manifold. I'm thinking that over time, the "O" rings harden and a vacuum leak develops when they no longer seal well.

I, like you, started throwing parts at the problem hopeing to find the cause of the bad idle. I put in an new IAC and nothing improved.

I then realized that since I wasn't getting any code, there had to be a problem elsewhere; not necessarily caused by a bad part.

So one day I bit the bullet and started the disassembly and low and behold, the "O" ring was found.

Then the light of knowledge came on and I remembered the engine started and idled perfectly BEFORE I did my injector experiment; swapping to 22 lbs injectors in place of my 27s to see if I could lean my BLMs. It was only after I found out that y experiment with the 22 pounders was a failure and reinstalled the 27 pounders that my idle problem began.

So I suggest that if you've made any changes or modifications to the engine, including washing the engine or changing brands of gasoline, go back in your mind to when it ran well and you may find what's causing the trouble. Same principle as with computer problems.

Keep me posted.

Jake

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1986 Corvette Coupe, 415 CID, Edelbrock 6073s, ZZ9

[This message has been edited by JakeJr (edited January 10, 2001).]