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Discussion Starter · #1 ·
I have an Edelbrock/BBK 58mm TB installed on my 383 and have some ongoing IAC/TPS issues at cold startup that appear to be more mechanical than programming. Specifically the cold idle rpm. LT1 Edit data indicates an actual idle rpm appox 200 below the "desired" idle at that temp.

I have heard that the in the BBK TB -vs- stock, there is a hole in the base of the stock TB that is missing in the BBK; that this hole is interconnected into the IAC passageway in the base of the TB. Also that an 1/8" hole can be drilled after the TB is removed and that this will eliminate the problem after reinstall.

Then I have heard that this is not necessary, that increasing the blade angle a small amount with the idle stop screw to allow more air through the TB at idle will cure the problem.

Although these engines seem to run rich at cold startup, mine seems more so than most.

Any input from people who have experienced and solved the problem would be greatly appreciated.

Discussion Starter · #2 ·

You might take a look at your IAC count. Not sure off the top of my head what normal is, but a quick look in the manual should tell you. If it's off the scale trying to bring the idle up, then you might need to modify the throttle body; otherwise, it might just be a programminmg issue.

If you can't find the correct IAC count in the manual, maybe you can fool with the idle screw and see if the count drops once you have the engine idling where you want it.

Discussion Starter · #3 ·

Had the same issue with the BBK 52mm after head/cam swap. It worked fine on the stock motor. Spoke with Ed Wright about my problem and he said to drill the hole.

If you open the blades, your TPS will be off. It should read .75-.8 vdc when closed and 4.5 vdc with blades fully opened. You can get away with a 1/4 turn or so on the idle screw but more than that will throw off you TPS count.

IAC count depends on where the tune is set for, typically 32. If can be slightly lower.

Remove TB and drill the same size hole as stock, or 1/8". I had to try different sizes working my way up one drill bit size at a time. 15/64" was where I wound up. Don't take big steps, one drill bit size at a time. If it is to big the IAC will read to low and not be able to compisate with the IAC valve. While doing this it would be a good time to clean the IAC valve and passage. Remove the IAC valve to do this.

To get the car to idle right, you must stabalize the IAC and TPS counts to within a range the PCM can deal with. I definately had a "WTF" attitude when told I had to drill a hole in a $325 TB to make it work. It does work. I did drill the hole to big in my search to make it run right. I then had to put in a small bushing to reduce the size back down.

I bought Auto Tap as a scan tool to monitor my progress. You must have a way of reading the IAC and TPS while the engine is running. The car needs to be completely warmed up to get correct readings.

good luck.

Discussion Starter · #4 ·

Sorry, for me 15/64" was to big. that is a size that did work for someone else. I then had to drill 1/4" to put in a 1/4" OD to 3/16" ID bushing. 3/16" was the right size for me.

Discussion Starter · #5 ·
Bill, I know I told you a long time ago, to try adjusting the TB stop screw first, to see if that produced acceptable results, before drilling the hole.

But, since then, I've sort of revised my thinking a bit, in that there is some advantage in directing more of the idle air into the manifold's IAC ports, than the plenum. From talking to a few folks with more radical combos than mine, this seems to be more of an issue with larger cams.

Also FWIW, I've measured a few stock TB holes at about .161-.162" diameter, which is equivalent to a #20 drill bit. Might consider starting with that, and then working your way up.

Discussion Starter · #6 ·
Some guys get a little nervous about drilling the TB.
If you open the throttle blades, you can reset the TPS by slotting it with a rattail file to make it adjustable.
The proper voltage at idle is .67

Discussion Starter · #7 ·
5/25/04 Update:

With Scott Barnes doing all the work and me providing input and positive reinforcement we:

Drilled a hole slightly smaller than the one in the stock TB (a 5/32" hole - .1562 versus stock which is .1610) front-to-back thru the webbing in the BBK today.

Reset the TB blades by backing turning the stop screw all the way in so that there was no contact - then screwing it out to just make contact plus a little less than one full turn open.

Then removed the TPS sensor and elongated the mounting holes in it allowing it to be adjusted.


Set it up with a scan tool: the TPS voltage at .7v closed and 4.7v WOT and the IAS count at 34. Starts and idles perfectly. In gear and out. Idle is a fraction (maybe 50 rpms) high, but will fix that with LT1 Edit. Throttle tip-in feels good and these changes also eliminated that loud screeching, sucking sound from having the IAC at 160 (whew!).

Thanks to all that responded for their help.
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