I posted a few days ago about a hunting idle problem running a AUJP on a manual trans. I changed to AXXD (305 M/T I think) to run my 350 by moving the VE table over and making some adjustments to it to correct for the different M/T timing table.

Anyway, the hunting is gone now, but I still have a rough idle. It idled this way when it was still MAF too. It has a roughness sort of stumble to it that drives my nuts. My buddy's 89 IROC w/all forged 283 I converted to SD idles like a dream even with a larger cam.

Is it possible I have dirty injectors causing it? They're the original 87 GTA parts with about 65K miles on them. I pulled a set of 24lb "shower head" injectors from a 94 LT1 caprice I've been tempted to try, so if anyone suggests it I will.

Here are the details:
- 92 Chevy K1500 Z71 package (long history, R title, previously abandoned, reconstructed onto new frame myself)
- 87 5.7 GTA Tuned Port w/89 serp brackets so truck accessories bolt-on (motor fit like it was made for the truck!)
- Motor is 100% stock cam and L98 heads
- 730 ECM, truck TBI harness reworked by Matt Howell for MAF, I reworked to SD after finding boatloads of 730s and - 749s in the local yard
- Edelbrock TES headers & Y-pipe
- 3" high flow cat
- Flowmaster American Thunder Super 40 series cat-back system w/dual rear exit
- NV4500 Cast iron 5-speed trans
- 3.42 8.5" 10-bolt rear, Yucon gears & Eaton LS carrier

This is what DataMaster says about my idle:

RPM: 650
MAP KPa: 39
LT BLM: 125
Fuel Trim Cell: 4
TPS: 0.57v
IAC POS: 13
Spk Adv: 22.1
CTS: 201 F

The throttle plates were adjusted for 600 RPM warm idle in neutral with IAC fully closed. The throttle body was soaked and cleaned to like-new less then 3K miles ago, and it's always had this idle from Day 1.

I have no vacuum leaks that I know of, and I have my EGR programmed out until I get everything else tuned right.

Any ideas? What else should I be looking at?

 

I would try retarding the timing to 18-20 and maybe close the throttle plates a little. Idle on manual cars is usually higher than 650, like 700-900. Many people try to get the IAC between 40-60 steps at idle.

 

I tune my idle to 0 IAC steps and adjust spark advance for the highest vacuum reading, which is normally the smoothest idle. I have tuned a hotcam and a LPE219, and both were a bit lumpy but dead consistant idles.