IAC & Open/Closed Loop
Thread Starter
TECH Fanatic
Joined: Nov 2007
Posts: 1,745
Likes: 18
From: Lansing, MI via Bowling Green, KY, Dalton GA, Nashville, TN & Atlanta, GA
IAC & Open/Closed Loop Does anyone know the role that the Idle Air Control valve plays, or what happens with the IAC, when switching from open to closed loop?
I've been trying to resolve a relatively new high idle, 1400 or so RPM +, and recently did all of the following on the same day.
1. Replaced the IAC valve. My car has essentially been undriven for a very long time and the IAC was one component that had never been replaced. This did not seem to make much difference in the idle.
2. Replaced the throttle position sensor. Seemed to make no difference.
3 Adjusted the pintle on the new IAC valve per the Factory Service Manual, the steps of which are documented here, 1994 LT1 IAC Pintle Adjustment. I believe we also tried adjusting the throttle bottle blades via the set screw to see if they would close a bit. I'm not sure adjusting the set screw made any difference to the blade position. If it did, it wasn't visually noticeable.
After completing Step 3, which seemed to help but didn't completely address the situation, the car will very briefly act like it's choking out/dying after idling for about 30 seconds. Every time it does this, it throws a code, which hasn't been identified due to not having scanner capabilities, and then resumes running "normally". I'm assuming the timing may coincide with the car transitioning from open to closed loop. I'm wondering what role the IAC plays, if any, in that transition.
I've been trying to resolve a relatively new high idle, 1400 or so RPM +, and recently did all of the following on the same day.
1. Replaced the IAC valve. My car has essentially been undriven for a very long time and the IAC was one component that had never been replaced. This did not seem to make much difference in the idle.
2. Replaced the throttle position sensor. Seemed to make no difference.
3 Adjusted the pintle on the new IAC valve per the Factory Service Manual, the steps of which are documented here, 1994 LT1 IAC Pintle Adjustment. I believe we also tried adjusting the throttle bottle blades via the set screw to see if they would close a bit. I'm not sure adjusting the set screw made any difference to the blade position. If it did, it wasn't visually noticeable.
After completing Step 3, which seemed to help but didn't completely address the situation, the car will very briefly act like it's choking out/dying after idling for about 30 seconds. Every time it does this, it throws a code, which hasn't been identified due to not having scanner capabilities, and then resumes running "normally". I'm assuming the timing may coincide with the car transitioning from open to closed loop. I'm wondering what role the IAC plays, if any, in that transition.
TECH Veteran
Joined: Apr 2008
Posts: 4,985
Likes: 112
a common cause of high idle is vacuum leak. look closely at vac lines
if you messes with the TPS by adjusting the stop screw on TB...don't so you need to restore it where it was. You need to read VDC from TPS and have it around .67vdc closed and 4.5vdc fully open
the IAC adjust idle when load is on engine from AC & power steering, etc to keep idle rpm consistent. It wants to read between 30-35 at idle
you should have also cleaned the IAC orifice when you removed it with TB cleaner
if you messes with the TPS by adjusting the stop screw on TB...don't so you need to restore it where it was. You need to read VDC from TPS and have it around .67vdc closed and 4.5vdc fully open
the IAC adjust idle when load is on engine from AC & power steering, etc to keep idle rpm consistent. It wants to read between 30-35 at idle
you should have also cleaned the IAC orifice when you removed it with TB cleaner
Thread Starter
TECH Fanatic
Joined: Nov 2007
Posts: 1,745
Likes: 18
From: Lansing, MI via Bowling Green, KY, Dalton GA, Nashville, TN & Atlanta, GA
I will have to reset the set screw.
I also need to get a laptop and some scanning software.
I have been under the impression that all air entering the engine during idle was controlled by the IAC, but based upon some additional reading, that may not be the case. I now suspect that whatever code is being thrown may have more to do with messing with the set screw than adjusting the IAC pintle, but that's just a guess.
Any suggestions on places to check for vacuum leaks?
This car has not been driven since 2000. I start it occasionally just to let it run. One day earlier this year, it started idling faster than normal and has pretty much done it ever since.
The exhaust was replaced from the manifolds back several years ago. For a number of reasons, I've never tightened the clamps on the y-pipe and catback, but I'm assuming that wouldn't have anything to do with idle rpms, or could it?
I also need to get a laptop and some scanning software.
I have been under the impression that all air entering the engine during idle was controlled by the IAC, but based upon some additional reading, that may not be the case. I now suspect that whatever code is being thrown may have more to do with messing with the set screw than adjusting the IAC pintle, but that's just a guess.
Any suggestions on places to check for vacuum leaks?
This car has not been driven since 2000. I start it occasionally just to let it run. One day earlier this year, it started idling faster than normal and has pretty much done it ever since.
The exhaust was replaced from the manifolds back several years ago. For a number of reasons, I've never tightened the clamps on the y-pipe and catback, but I'm assuming that wouldn't have anything to do with idle rpms, or could it?
defiantly a vacuum leak, what id recommend is pull one hose at a time off the intake and put your finger over the hole and see if the idle comes down, then you can isolate it.......places to look include AIR solenoid on side of intake, PCV hose on front under TB.....brake booster hose and the booster itself I cant remember the rest but there are more on the passengers side all related to emissions stuff, I haven't run emissions on an LT1 in 15 years lol
Thread Starter
TECH Fanatic
Joined: Nov 2007
Posts: 1,745
Likes: 18
From: Lansing, MI via Bowling Green, KY, Dalton GA, Nashville, TN & Atlanta, GA
I'll check for vacuum leaks. If there are other places to look in addition to those already mentioned, please feel free to add.
I believe I have removed most, if not all, of the emissions stuff on my car.
Thanks for the info.
I believe I have removed most, if not all, of the emissions stuff on my car.
Thanks for the info.
Thread Starter
TECH Fanatic
Joined: Nov 2007
Posts: 1,745
Likes: 18
From: Lansing, MI via Bowling Green, KY, Dalton GA, Nashville, TN & Atlanta, GA
Would vacuum leaks manifest themselves on a scan? I broke down and bought a laptop and an ALDL cable. I'm hoping to scan via the free Scan9495 software sometime over the holidays.
TECH Veteran
Joined: Apr 2008
Posts: 4,985
Likes: 112
scan software will just show codes and engine data (rpm, fuel trims, 02's, etc, etc). It does not identify vacuums leaks
if you have a high idle, the more common reason is a vacuum leak although it is not the only cause of high idle
vac leaks can be in many places. even under dash where vacuum operated HVAC stuff is.
You do want to restore your TB blades back to where they were stock. With that said this is something you won't be able to do with a vacuum leak going on at the same time
If you are reading TPS volts with scan software, you can close blades until the reading is around 4.67 VDC with blades closed. Then measure your IAC counts. They should be 30-35 with motor at operating temp...but again if you have a vac leak your idle will remain high
if you have a high idle, the more common reason is a vacuum leak although it is not the only cause of high idle
vac leaks can be in many places. even under dash where vacuum operated HVAC stuff is.
You do want to restore your TB blades back to where they were stock. With that said this is something you won't be able to do with a vacuum leak going on at the same time
If you are reading TPS volts with scan software, you can close blades until the reading is around 4.67 VDC with blades closed. Then measure your IAC counts. They should be 30-35 with motor at operating temp...but again if you have a vac leak your idle will remain high
Trending Topics
Thread Starter
TECH Fanatic
Joined: Nov 2007
Posts: 1,745
Likes: 18
From: Lansing, MI via Bowling Green, KY, Dalton GA, Nashville, TN & Atlanta, GA
I'm planning to reset the TB blades and I'm hoping it won't be a huge PITA. Visually, I couldn't tell that backing off the set screw did anything at all. I'm guessing it was holding them just a wee bit open. It will be interesting to see what code it's throwing during what I'm assuming is the transition from open to closed loop. I've never used any scanning software. Here's to a user friendly experience. This software seems to have pretty good supporting documentation.
If I do have vacuum leaks, I wouldn't even know what to check. Would it only be things feeding into the intake?
If I do have vacuum leaks, I wouldn't even know what to check. Would it only be things feeding into the intake?
Last edited by BTC; Dec 24, 2015 at 09:29 AM.
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
Launching!
Joined: Aug 2013
Posts: 205
Likes: 6
I'm planning to reset the TB blades and I'm hoping it won't be a huge PITA.
take the intake elbow off so you can visually see the throttle blades.
turn the screw until they're closed, then give the screw just the slightest turn back out.
you don't want them 100% closed or they might bind up, you're looking more for 99.5% closed. you definitely want the screw to be the stop, not the throttle body bores.
but the less air through the plates, the better. a fixed amount of idle air comes through a hole at the bottom of the throttle body, and a variable amount of air through the IAC.
after that reset the ecm's memory by pulling the PCM BAT fuse for a minute
Any suggestions on places to check for vacuum leaks?
- a torn brake booster diaphram
- a torn EGR diaphram
- a stuck open PCV valve
- a stuck open CCP valve
a hand operated vacuum pump like a mityvac will track down any vacuum leak that isn't a damaged gasket.. you just pump it up to produce vacuum and watch the gauge to make sure it holds.
there are some 'by design' vacuum leaks on some engines too, for example the 95+ LT1's vented opti is technically a vacuum leak (just happens to be a very small one which is trimmed out..)
Thread Starter
TECH Fanatic
Joined: Nov 2007
Posts: 1,745
Likes: 18
From: Lansing, MI via Bowling Green, KY, Dalton GA, Nashville, TN & Atlanta, GA
couldn't be easier
take the intake elbow off so you can visually see the throttle blades.
turn the screw until they're closed, then give the screw just the slightest turn back out.
you don't want them 100% closed or they might bind up, you're looking more for 99.5% closed. you definitely want the screw to be the stop, not the throttle body bores.
but the less air through the plates, the better. a fixed amount of idle air comes through a hole at the bottom of the throttle body, and a variable amount of air through the IAC.
after that reset the ecm's memory by pulling the PCM BAT fuse for a minute
every vacuum line and device attached to it is suspect. besides cracks in hoses and fittings, the things i see often missed:
- a torn brake booster diaphram
- a torn EGR diaphram
- a stuck open PCV valve
- a stuck open CCP valve
a hand operated vacuum pump like a mityvac will track down any vacuum leak that isn't a damaged gasket.. you just pump it up to produce vacuum and watch the gauge to make sure it holds.
there are some 'by design' vacuum leaks on some engines too, for example the 95+ LT1's vented opti is technically a vacuum leak (just happens to be a very small one which is trimmed out..)
take the intake elbow off so you can visually see the throttle blades.
turn the screw until they're closed, then give the screw just the slightest turn back out.
you don't want them 100% closed or they might bind up, you're looking more for 99.5% closed. you definitely want the screw to be the stop, not the throttle body bores.
but the less air through the plates, the better. a fixed amount of idle air comes through a hole at the bottom of the throttle body, and a variable amount of air through the IAC.
after that reset the ecm's memory by pulling the PCM BAT fuse for a minute
every vacuum line and device attached to it is suspect. besides cracks in hoses and fittings, the things i see often missed:
- a torn brake booster diaphram
- a torn EGR diaphram
- a stuck open PCV valve
- a stuck open CCP valve
a hand operated vacuum pump like a mityvac will track down any vacuum leak that isn't a damaged gasket.. you just pump it up to produce vacuum and watch the gauge to make sure it holds.
there are some 'by design' vacuum leaks on some engines too, for example the 95+ LT1's vented opti is technically a vacuum leak (just happens to be a very small one which is trimmed out..)
Why is the opti vent considered a leak? Isn't it metered air & a closed/sealed system? Perhaps the opti doesn't seal perfectly.
Launching!
Joined: Aug 2013
Posts: 205
Likes: 6
the air is 'kinda metered' because it's taken after the maf, but that doesn't make it not a vacuum leak.
It's definitely 100% closed right now.
The gasket covers the set screw, so I'll be removing the tb for each adjustment.
TECH Veteran
Joined: Apr 2008
Posts: 4,985
Likes: 112
Just cut off the tab that blocks access to the stop screw. yeah that screw should never really be messed with
replacement gaskets are just a few $ at most auto part stores
Thread Starter
TECH Fanatic
Joined: Nov 2007
Posts: 1,745
Likes: 18
From: Lansing, MI via Bowling Green, KY, Dalton GA, Nashville, TN & Atlanta, GA
So I finally got around to hooking up a scanner yesterday. This may have nothing to do with the idling issues, but the codes it's throwing are 44 & 64, lean condition on both O2 sensors. Would not having the y-pipe clamped to the collectors potentially contribute to this? I'm also wondering whether the ceramic coating on the headers may have fouled the sensors. I believe I have some new sensors I could throw in.
When I first started it yesterday, it idled very high and I'm pretty sure that at least the driver's side header was glowing. Doesn't that happen when the fuel mixture is lean? If the sensors are fouled up, could that screw up some tables used to calculate the fuel mixture in open loop, or are those values set and never updated?
My car has been on jack stands for years. All we do right now is start it periodically to let it run. I do hope to get to the point where I can drive it before I turn 60, which gives me a bit over 10 years. The car is in Ky and I live in Michigan, so I don't get to mess with it very often.
When I first started it yesterday, it idled very high and I'm pretty sure that at least the driver's side header was glowing. Doesn't that happen when the fuel mixture is lean? If the sensors are fouled up, could that screw up some tables used to calculate the fuel mixture in open loop, or are those values set and never updated?
My car has been on jack stands for years. All we do right now is start it periodically to let it run. I do hope to get to the point where I can drive it before I turn 60, which gives me a bit over 10 years. The car is in Ky and I live in Michigan, so I don't get to mess with it very often.
Thread Starter
TECH Fanatic
Joined: Nov 2007
Posts: 1,745
Likes: 18
From: Lansing, MI via Bowling Green, KY, Dalton GA, Nashville, TN & Atlanta, GA
For anyone interested, I believe I finally resolved this cluster.
My initial high idle appears to have been an issue with the IAC valve, which I replaced. The reason that the new IAC did not seem to resolve the issue is because I failed to reconnect the Evap Canister Purge Solenoid to the throttle body. So, I had both mechanical failure and user error/stupidity.
Assuming the issue does not return, I'm actually glad that's all it turned out to be. I'm surprised I failed to recognize the problem subsequent to the IAC replacement. I had the throttle body off 3-4 times and never noticed the Purge Solenoid wasn't connected. After identifying the problem and messing around with it, I'm surprised I didn't both see it and hear it. You can definitely hear it sucking during a normal idle. Maybe it was idling so high it was simply drowning out the sucking/hissing sound.
I have to say that I felt like an idiot when I identified the problem. I guess the upside is that I was prompted to get an ALDL cable and some scanning software, which is pretty cool.
I'm assuming this is also the reason it was throwing the Lean O2 reading codes.
When the car is in Open Loop, and it's running in Fuel Enrichment mode, or whatever it may be called, are the values used to calculate the air fuel ratio fixed, or can they change?
My initial high idle appears to have been an issue with the IAC valve, which I replaced. The reason that the new IAC did not seem to resolve the issue is because I failed to reconnect the Evap Canister Purge Solenoid to the throttle body. So, I had both mechanical failure and user error/stupidity.
Assuming the issue does not return, I'm actually glad that's all it turned out to be. I'm surprised I failed to recognize the problem subsequent to the IAC replacement. I had the throttle body off 3-4 times and never noticed the Purge Solenoid wasn't connected. After identifying the problem and messing around with it, I'm surprised I didn't both see it and hear it. You can definitely hear it sucking during a normal idle. Maybe it was idling so high it was simply drowning out the sucking/hissing sound.
I have to say that I felt like an idiot when I identified the problem. I guess the upside is that I was prompted to get an ALDL cable and some scanning software, which is pretty cool.
I'm assuming this is also the reason it was throwing the Lean O2 reading codes.
When the car is in Open Loop, and it's running in Fuel Enrichment mode, or whatever it may be called, are the values used to calculate the air fuel ratio fixed, or can they change?