?'s on Drill Mod on PP TB
?'s on Drill Mod on PP TB Having idle issues w my 383 build. Thinking I might need this mod. My ?'s are. Do I need to do both mods, drill & isolate the air to IAC or just start w the drilling the hole bigger? Is this a normal fix for people running PP TB's?
TECH Veteran
Joined: Apr 2008
Posts: 4,985
Likes: 112
you need to scan the IAC counts. they most likely are high which is typical with aftermarket TB's and built motors.
The "drill" mod, start with 1/8" hole, re-scan and if still high drill 1/64" larget. re-scan and if still to high drill another 1/64" larger. often hole is 11-13/64". Take BABY steps as each 1/64" makes a big diffrence
you want IAC counts between 30-35 (32)
The "drill" mod, start with 1/8" hole, re-scan and if still high drill 1/64" larget. re-scan and if still to high drill another 1/64" larger. often hole is 11-13/64". Take BABY steps as each 1/64" makes a big diffrence
you want IAC counts between 30-35 (32)
TECH Veteran
Joined: Apr 2008
Posts: 4,985
Likes: 112
Split BLM has more to do with the idle air inlet passage on aftermarket TB vs stock. There are mods to modify that passage.
Car also idles much different cold vs hot. RPMs jump up to 1500 when put in park hot. Will only be at 1000 cold.
New TPS and IAC, professional products 58mm TB. 383 forged stroker by Golen engines, tune by pcm4less.
Running 30 lb injectors w stock fuel regulator, high flow fuel pump. Starting to think I need more fuel pressure.
Trending Topics
TECH Fanatic
Joined: Sep 2010
Posts: 1,037
Likes: 2
New set up. Idle is crap. Set screw on blades is adjusted as far as it can go. TPS surges between .58 to .71, IAC counts are at 80 with car car running and I've done the drill mod only, not the isolation part.
Car also idles much different cold vs hot. RPMs jump up to 1500 when put in park hot. Will only be at 1000 cold.
New TPS and IAC, professional products 58mm TB. 383 forged stroker by Golen engines, tune by pcm4less.
Running 30 lb injectors w stock fuel regulator, high flow fuel pump. Starting to think I need more fuel pressure.
Car also idles much different cold vs hot. RPMs jump up to 1500 when put in park hot. Will only be at 1000 cold.
New TPS and IAC, professional products 58mm TB. 383 forged stroker by Golen engines, tune by pcm4less.
Running 30 lb injectors w stock fuel regulator, high flow fuel pump. Starting to think I need more fuel pressure.
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
Verdad Gallardo
10 Camaros You Should Never Buy
10 LS Engine Myths That Refuse to Die
Verdad Gallardo
TECH Fanatic
Joined: Nov 2008
Posts: 1,776
Likes: 1
From: South Dakota
Staging Lane
Joined: Nov 2012
Posts: 57
Likes: 0
same problem Did you fix your problem? I have the same problem. Help! I went crazy and drilled the F out of it. I think it went over 1/2". It idles now but when cold it's like 450-500rpm, hot is like 1300. Also pcm4less tune.. What did you do to your PP tb?
Staging Lane
Joined: Nov 2012
Posts: 57
Likes: 0
Never did figure it out. Sold the car. Still ran good though, sold it an older lady, she drives the P out of it and loves it. I couldn't deal with it not idling right. I wanted to put a bullet through it numerous times.
That is becasue your IAC valve wasn't doing anything with the hole drilled that large. Correct hole size starts at 11/64", with the passage isolated to feed into the individual runners. If its still not idling right go one bit size larger - but that is it.
After that start looking at TPS voltage vs throttle position, stuck IAC, vacuum leaks, clogged IAC holes in the runners, etc.
Before I did this I read many different opinions of those stating to widen the hole with the more CID. Even with the 396 I decided to stay w/in the stock diameter and the engine idles perfectly. Before drilling, my problem only occurred after resetting the PCM. Car wouldn't idle for **** for a week's worth of start cycles and then it would be fine. Now it idles fine regardless.
Last edited by SS RRR; Nov 6, 2013 at 12:56 PM.
TECH Veteran
Joined: Apr 2008
Posts: 4,985
Likes: 112
shouldn't just blindly drill the hole larger. use a scan tool to read IAC counts first.
typically with larger aftermarket TB's, especially on head/cam motors with higher idle RPM set in PCM, you need to do the "drill" mod or the crack open throttle blade/slot TPS holes to rotate it. I don't subscribe to the later.
before you do anything you need to scan the car to see wtf the IAC counts are. often they are pegged at 160 with the IAC maxed out. start by drilling 1/8" hole then measure IAC again. if it didn't drop to 30-35 9often does not) than drill 1/64" larger and measure again. "Generally" with a 383/head/cam motor you wind up with a hole 11/64" BUT again you need to be scaning to read wtf they are each step. 1/64" makes quite a big diffrence.
In the post noting 1/2" hole was drilled....WAAAY to big
The PCM can learn/adjust to a point but ideally you want the IAC counts to be 30-35 at idle with engine at operating temp
typically with larger aftermarket TB's, especially on head/cam motors with higher idle RPM set in PCM, you need to do the "drill" mod or the crack open throttle blade/slot TPS holes to rotate it. I don't subscribe to the later.
before you do anything you need to scan the car to see wtf the IAC counts are. often they are pegged at 160 with the IAC maxed out. start by drilling 1/8" hole then measure IAC again. if it didn't drop to 30-35 9often does not) than drill 1/64" larger and measure again. "Generally" with a 383/head/cam motor you wind up with a hole 11/64" BUT again you need to be scaning to read wtf they are each step. 1/64" makes quite a big diffrence.
In the post noting 1/2" hole was drilled....WAAAY to big
The PCM can learn/adjust to a point but ideally you want the IAC counts to be 30-35 at idle with engine at operating temp
Staging Lane
Joined: Nov 2012
Posts: 57
Likes: 0
Lol.. You are correct. I did watch the IAC's as I did it, and I went up until they were at 30-35. I think it some other issue going on. Idk. The lady totalled the car.. I think she is keeping it though and getting it fixed.
Anyone have pics of what needs to be done as I also recall reading about inserting a piece of brass tubing in the tb to connect it to the iac hole in the intake? Is that correct to do that as well?
No reason at all for tubing, unless you drill the hole too large and are trying to cut it back down. If so, use a short piece. If you use a piece long enough to reach the hole in the manifold you will block off the IAC from that hole.
Open the throttle blades as far as you can with it still showing zero % throttle angle before doing any drilling. Just having 396" does not mean you have to drill anything. Normally it is only needed with big cams, requiring high idle speeds. Something like a 396" with a cam about the size of the "hot" cam normally can get by without drilling anything. Take a #20 Torqx screw driver to adjust the throttle stop as far as you can, and maintain zero % TPS, and check your warmed up closed loop IAC counts after letting it idle five minutes after start up. If 30 to 35 in P/N with AC off, your good to go.
Open the throttle blades as far as you can with it still showing zero % throttle angle before doing any drilling. Just having 396" does not mean you have to drill anything. Normally it is only needed with big cams, requiring high idle speeds. Something like a 396" with a cam about the size of the "hot" cam normally can get by without drilling anything. Take a #20 Torqx screw driver to adjust the throttle stop as far as you can, and maintain zero % TPS, and check your warmed up closed loop IAC counts after letting it idle five minutes after start up. If 30 to 35 in P/N with AC off, your good to go.