2000 Camaro with heads, cam, 92mm intake/TB tuned etc. I bought the car used with the mods already on it. It runs great, pulls hard, but it has issues idling on first start up, cold or hot. At first start up, I have to peddle it a bit before it idles on its own. I spoke with the tuner and his response was “that’s the best possible start with that cam. Customer wanted the biggest cam he could get and that’s what he got. If you want a better startup, have to go with a smaller cam.”

I adjusted the TB blade to the point where it now starts on its own with no help from me. It still idles low for about 10 seconds, then catches and idles like normal. My problem is, I read IAC counts should be between 30-60 once at operating temp, but mine sits at 0 according to the scanner due to the RPM’s being higher. I can back the TB screw almost all the way out to the point where it won’t start without throttle, but all the readings stay the same once it idles.

I took a multimeter and tested the TPS plug (disconnected). Blue white has 0.009 and gray/black has 5.00. I saw later I should have checked it with it plugged into the TPS which I can do later today. Does this sound like a bad TPS sensor? Am I on the right track?

RPM at temp is 800-850
Desired RPM - 775
IAC - 0

KOEO
TPS (V) - 0.00
TPS % - 0

KOEO/WOT
TPS (V) - 0.00
TPS % - 99

KOER
TPS (V) - 0.00
TPS % - 0

KOER/WOT
TPS (V) - 0.00
TPS % - 99

 

So the throttle body is not stock, correct? The factory TB blade should be all but closed when the throttle is at rest, open just enough to prevent catching in the bore. But the factory blade has a small hole drilled in the middle to allow a bit of air to pass, not sure if any aftermarket ones have this provision.

It makes sense that IAC counts are at 0 if you currently have the blade open far enough for idle speed to rise above the commanded set point. I can't imagine why "desired RPM" would only be 775rpm in the first place. For reference, the stock idle speed for an M6 LS1 F-body was 800rpm - that's for a stock engine with a stock cam (auto trans was 650 in P/N, 550 in gear). If the cam is THAT huge, I can't imagine why the tuner wouldn't have bumped idle speed higher to eliminate some of the issues.

Sounds like the car runs pretty well after startup with your new idle speed of 800-850. So I would reset the TB blade to nearly closed, have the tune updated to command an idle speed of 850rpm, then check IAC counts (and, if needed, you can further adjust the TB blade until IAC counts fall in the 30-60 range once fully warm).

But there is no point in fighting the commanded/desired idle speed by way of the TB blade (which is what you seem to be doing currently). Use the PCM as intended, meaning tune/command a higher idle speed, and then let the IAC do its job.

I just realized that I missed this sentence the first time around. If the IAC is not responding to elimination of airflow (meaning the closing off of the TB blade) then something is amiss besides just the low-set idle speed. Was TPS voltage showing 0.00 with it plugged in or unplugged? I'm guessing the former, as your data shows that TPS percentage values were still being displayed.

But I wonder how it's able to idle at all with the TB fully closed (unless it has a bypass hole drilled in the blade like the stock TB). If there is an air source anywhere that's allowing idle speed to rise above the commanded set point, then you should certainly see a "0" for IAC count (meaning it's fully closed, trying to regain control of idle speed).

 

I’ll get the specs on the cam once I get home.

TPS voltage was plugged in on all tests, and all the readings are from a Snap-On Solus scanner. The idle part has me baffled. I accidentally started and moved the car with the TPS unplugged and it started and idled the same. I have TB screw set so just one light tap of the throttle and it idles out fine. I’m picking up an ACDelco TPS just to try it and see what it does. I’ll also remove the intake tube and see if there is a hole drilled in it.

It also has a slight stumble from 0-WOT till about 1100 rpm and then it wraps up fine. I don’t really notice any driveability issues either. I was only starting to mess with the throttlebody screw to fix the idle issue at start up and that’s when I noticed the TPS sensor was at 0v. It does smell really rich, but it doesn't have cats.

 

Cam is 240/246 @.50lift
Texas Speed PRC 243 heads

 

So there's different ways around the idle issues you are experiencing, I've gone through several "setups" with my car ie cam only LS1, heads and cam LS1, now 383 with a fast 102...ALL of which have given me some form of idle/surging/stalling problems until I took it to the last tuner. Instead of allowing the IAC to control the idle speed he did the opposite and opened up the throttle blade and pulled timing to bring the idle speed down..it basically does not utilize the IAC at all. The IAC motor is too slow to correct for changes in airflow which is what causes the idle speed to dip and sometimes stall or cause surging. My throttle body is a nick williams 102 with no hole unlike the stock TB. So far i have not experienced one single issue with stalling or surging or cold start issues.

 

It’s a FAST 92 TB and 92 intake. I got it apart, and there is roughly a 3/8 hole drilled into the plate. It looks like there was a second one, but has been siliconed.

I did the the original TPS sensor while on the car, and it all the numbers functioned correctly. Voltage was correct and moving the TB by hand, it all read correctly. Here is what I found:



This makes a lot sense, and I appreciate you taking the time to explain it. I couldn’t figure out why I couldn’t get the IAC to work. As mentioned in the post above, it does have a hole drilled into the blade. I set it back to where it was originally, started it, and it was the same. Had to peddle it a few times for awhile till it finally idled on its on. I then cranked the screw 2%, relearn on IAC/TPS, and now it starts much better. It idled at 500+\- for a few seconds then up to 850 and settles. It does have a bit of the cruise control effect while I’m coasting, but that’s not a huge deal since it’s an M6. It drops back to idle as I come to stop 15mph or below. This is after the car has been sitting for 5-6 hours.


https://youtube.com/shorts/YQAnKivV0ZY?feature=share

 

Find another throttle body for starters.

 

So I have heard. It was on the car when I got it, but what do you recommend? Will I need to get it retuned?

 

I would recommend a Nick Williams and if you have access to HPtuners you would just need to get the idle air counts into the proper range at operating temperature, it needs to be between 30 and 60 counts in order to maintain idle.

 

The only issue with doing this is that you'll have the same sort of minor idle inconsistencies as a performance/aftermarket carb'ed setup, meaning idle speed will be slightly higher or lower as intake air (and ambient temps in the engine bay) cause the throttle body to swing from cool to hot or vice-versa. Excessive load changes may also lead to the same. The Holley on my '71 idles about 50-75rpm higher on a cool evening vs. a hot day, at least until it sits long enough at idle for the carb to heat soak (which can take a bit of time with outside air induction and a phenolic spacer). If you drive the car in a wide range of weather extremes (such as a year-round daily driver), this inconsistency will be more pronounced. Many factory carbs had various provisions to stabilize operation for ambient temp swings or load changes (such as Hot Idle Compensator, A/C solenoid, thermostatic pre-heater hose [though this was not the main purpose for this component], etc.), but you won't find these in use with most aftermarket/performance carb applications due to complexity and/or power inhibiting effects. IAC is used in an SEFI setup to maintain consistent idle speed during load and temp changes that will otherwise have to be averaged when the IAC is rendered inoperative. Probably not a big deal for a weekend toy, but it will likely have to be set to an idle that's a bit too high (during cold weather) in order to maintain reasonable idle speed under heavy load and/or hot conditions.

On a side note, looks like JB Weld was used to close up the other TB blade hole in the picture above. The existing hole looks larger than the one found in a stock TB blade (I think those are 5/32" from the factory, or the closest metric equivalent).

 

Larry from ASSC is who has tuned mine the last few times, with the old LS1 and heads and cam combo and stock LS6 intake/throttle body he used this same basic method, using the IAC as little as possible, had that setup for about a year before I had to pull it, but I never had any idle inconsistencies, now being it was a stock TB it didn't give him as much of a problem to get it to idle nice, but with the larger intake and 102 throttle body they can be a bitch to tune..a lot of guys I talk to with a 102 TB end up just living with surging and stalling which I HATE lol...the car is 99% street driven, for a track only car I guess its not as big of a deal. He messed with the car for a week on and off trying different methods but he said with any LS car with a fast 102 he usually ends up opening the throttle blade instead of using the IAC. As far as idle speed goes its a bit higher than what some people desire..mine is set at 975 rpm..personally I like a faster idle anyway. But I haven't noticed any changes in how it idles, and I've gone anywhere from 90 degrees down to 50 if I have it out on a cool night and it runs the same.

 

Is the IAC still operative at all? Even if just a little, this might explain how it maintains a good average even at the "extremes". But that's probably why the higher idle speed is necessary (to maximize overall stability). If the IAC has been completely deleted then I'm surprised you don't see any change at all in idle speed with A/C on vs. off (unless maybe there is a timing table to adjust for this?...I don't remember), or major swings in TB surface & air intake temp (unless it's pretty much always heat soaked in this application).

I'm with you on the idle surging and stalling though...that **** was annoying as hell in the early '00s when we first started camming these engines, at this point it's just nuts to have to live with that for a street car.

 

My IAC will kick in when I crank the AC.

 

I couldn't say what the IAC counts are in the tune but I know he said he tried introducing the IAC and everytime he did it would want to dip the idle to the point of stalling when coming off the throttle quickly while lightly cruising say through a parking lot or something..so he ended up basically bringing the IAC counts down to nothing once the car is at operating temp and just adjusted the throttle blade open more..during cold start the IAC is still utilized though.