I've been confused now, and unfortunately I don't have my factory service manual with me right now to figure it out, but does the IAC have anything to with the plenum on 90-92 TPI's? I've heard that an IAC passage runs through it, and I've heard that one doesn't. Which is correct? I honestly don't remember what much of it looked like in there.

I ask because my IROC (as it sits in storage) has an idle surge issue than came about when I swapped in the L98, but it idled perfectly fine with the LB9. Nothing was changed other than the longblock, aside from me porting the plenum to open up the TB passages and getting rid of the wall right behind them, and an EGR/AIR delete (well, and the T56 swap which has nothing to do with it).

Even with the same LB9 prom and 19lb/hr injectors it had idled fine with, it was surging. I thought maybe that was the issue until I burned an L98 .bin (only changed for the 19's and EGR/AIR delete), and it still didn't go away. I've since tried relentlessly to tune it out to no avail. It does get slightly better when it's real cold out, but is very much still there. Other than that, it runs great, as I have part and full throttle tuned in pretty decently on the $8D (even now with the 42 lb/hr injectors).

So I'm wondering if the ported plenum has anything to do with it, but that doesn't seem to be an issue with other people. Any idea if one of those passages is for the IAC, and I disrupted it too greatly? Any help is appreciated.

 

The Iac passages dont go into the plenum, ( as far as i know/have seen) But the passages from the iac do allow air into the plenum for idle air hope this helps

 

Sounds like a real bad vaccum leak. Since you took the whole plenum off and everything, it's possible you could have ripped a gasket or missed a vaccum line on the back of the plenum.

 

the iac passages only run through the tb they dump out just behind the venturies.

 

I definitely have no vacuum leaks.

So the general concensus is that there is NO WAY that my surging is caused by the porting? I'd like to hear from someone who has ground the wall down as thoroughly as I have and still doesn't have problems.
Even if the IAC doesn't run into it, there may have been enough flow disruption to cause an unsteady air stream, affecting the IAC. It's one theory anyway.

I just don't want to waste money on a plenum if it won't help at all, ya know?

 

I had an 88 IROC, which had a stumble upon returning to idle.
Later I took it in to dealer to diagnose a bad coil wire, which they rebuilt the distributer and all other things to the tune of $$$$ before getting it right, but one of the things bad was the EGR?/IAC motor.
I don't know if this helps you. But seems I read that a bad EGR/IAC solenoid/motor causes idle problems. Mine seemed to almost die when going back to idle after lifting the gas rather quickly during part throttle cruise.
I doubt porting did harm or is the problem. I'd try to see if the IAC is working.
Sorry if I didn't read thread closely enough, e.g. there is zero IAC in the circuit now, but maybe this info helps. Crap, rereading my post, clueless if it was a IAC or EGR solenoid. Well, anyhow hope it gives you some ideas. (was like 10yrs ago so gimme a break please)
brian

 

I removed the EGR wall in my porting project. Had a noticeable decrease in SOTP torque, gained around 1.0mph in the 1/4 traps. NO difference in idle quality. It was a good trade-off and a good mod for what I wanted. Stock '89 L98.

Dave

 

Have you reset your minimum idle per mfg. specs. That can cause your surging. It's possible that with all the mods you have upset the calibration. It's not difficult to check and may cure your problem. See this link: https://www.thirdgen.org/tpimod2

 

You know, for some reason I was thinking that I had reset the IAC, but upon reading that again I don't recall doing it at all this time. That will definitely be the first thing I try when I get back to the car this fall.

I know the TPS values are fine, as verified by my datalogs.

Thanks for the advice and help everyone. I'm sure I'll post more if that doesn't help, but that should be the next thing I try before I can figure out more.