Idle Air Control Valve (IAC) question - Chevy Impala SS Forum
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post #1 of 17 (permalink) Old 03-30-2017, 05:57 PM Thread Starter
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Idle Air Control Valve (IAC) question

Used low mile 94 LT1 engine just installed will new gaskets,opti, valve springs and 1.6 roller rockers, 2400 stall, cold air intake,
My IAC counts drop to zero after warm up and slight surge at idle. I watch it on my scanner go down quickly after warmed up. I have used a smoke machine and still cannot find a vacuum leak. Plugged off ever line off including brake booster, egr,evap and opti vent. Pulling around 23 vacuum at idle. EGR valve held vacuum fine. Switched complete TB with another one with no known issues.
TBS Sensor was around .57 at idle, I up it to .88 by moving the TBS and it idles fine now but IAC still drops to zero.
I pulled out the IAC turned on the key and watched it move, so i know the wires are working.
I reset the IAC several times per procedure.

Car runs fine, all the BLM look fine and everything else looks good. The IAC is around 40 at 1800 rpm
Fuel pressure is great.
Setting the TB blades at .88 is not the right thing to do, but I guessing I am opening the blades just enough to make it idle with out slight surge.

When the IAC is at zero, I think it is closed all the way.

Any suggestion, I would like to set the TBS back to normal and have the IAC working correctly.
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post #2 of 17 (permalink) Old 03-31-2017, 10:59 AM
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More information is needed.
Quote:
My IAC counts drop to zero after warm up
Do you mean that the PCM has gone into closed loop control?
Are the O2 sensors working at this point?
Quote:
I have used a smoke machine and still cannot find a vacuum leak.
Does this mean you have checked for a intake manifold to block air leak? A throttle body leak? A leak between the TB and the MAF?
Has anyone changed the PCM tune? Have you tried a stock tune?

If you are having a closed loop control problem you could try unpluging the MAF. This would force the PCM to run on the tables. If the engine runs OK this way you might want to check sensors.

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post #3 of 17 (permalink) Old 03-31-2017, 12:13 PM
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I think the throttle body probably has had the adjustment screw turned in just a bit. make sure the butterflys close completely..

94 RMSB 170K Sat 5 years, ran for 1 of those then remained untouched for 4. Sat because it was slapped by a deer and the owners didnt want to fix it for whatever reason.
Mods, all the free good ones.
Future? Probably pull engine, reseal it, and run it a lot more. Looking to get 250k out of it. PRAYING for 300k.
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post #4 of 17 (permalink) Old 03-31-2017, 12:17 PM Thread Starter
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Thanks for reply,
Yep closed loop only, as soon as closed loop happens IAC falls rapidly to zero, new delco O2s working perfect.
Yep smoked the intake and Tb gasket,ect
New exhaust manifold gaskets with new grade 8 bolts.
All unplug the MAF and see what happens.

Just did some more checking this morning,it does the same thing in the Open loop, IAC slow goes down to zero before close loop.
I unplugged MAF and IAC still went to zero.
It really seems like a vacuum leak, im going to smoke it again.

Last edited by KDB50SS; 03-31-2017 at 12:52 PM.
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post #5 of 17 (permalink) Old 03-31-2017, 01:23 PM
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Yeah,but you moved the throttle blades. PCM is likely trying to pull the idle speed down,and can't...

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post #6 of 17 (permalink) Old 03-31-2017, 01:24 PM
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Is your brake booster leaking? Try clamping the line closed.

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94 RMSB 170K Sat 5 years, ran for 1 of those then remained untouched for 4. Sat because it was slapped by a deer and the owners didnt want to fix it for whatever reason.
Mods, all the free good ones.
Future? Probably pull engine, reseal it, and run it a lot more. Looking to get 250k out of it. PRAYING for 300k.
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post #7 of 17 (permalink) Old 03-31-2017, 01:54 PM Thread Starter
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Did the brake booster block off already. Yesterday I blocked off ever port on the intake except fuel pressure regulator and no change IAC dropped quickly to zero after start up and was operating temp. Funny thing is in open loop the IAC does not drop as fast, but soon as close loop happens IAC counts fall fast.
I just put this engine together with all new gaskets, valve springs and 1.6 roller rockers, intake gaskets, ect This thing has good vacuum at idle that is what make no sense.
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post #8 of 17 (permalink) Old 03-31-2017, 03:54 PM
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Quote:
Originally Posted by KDB50SS View Post
Did the brake booster block off already. Yesterday I blocked off ever port on the intake except fuel pressure regulator and no change IAC dropped quickly to zero after start up and was operating temp. Funny thing is in open loop the IAC does not drop as fast, but soon as close loop happens IAC counts fall fast.
I just put this engine together with all new gaskets, valve springs and 1.6 roller rockers, intake gaskets, ect This thing has good vacuum at idle that is what make no sense.
my stock 96 has a vacuum of 18-19 inches . what are you seeing ? my fuel trims are zero % . idle 600 RPM. in gear or neutral....
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post #9 of 17 (permalink) Old 04-01-2017, 06:23 AM
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Hey just some less sophisticated out-o-tha-box offerings, which may be entirely unrelated but still worthwhile checks regarding a new build:
-First off I'm sure you've gotten another set of eyes under the hood to catch anything obvious.
-If using stock ex manifolds then confirmed they don't have the famous hidden crack under the heat shield.
-Weak or wonky O2s? Again, have no idea whether involved but wonder if implicated as cascading factor(s).
-Wonky FPR or pump which manifests itself only at higher revs?
-A smoke machine sounds professional for checking leaks, but never seen one used. Have you ruled out ether or throttle body cleaner for maybe better test?
-A whole 'nother ECU PCM would quickly eliminate that from the mix if you have one or more spare ones.

Sure interested in what it ends up. GL

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post #10 of 17 (permalink) Old 04-01-2017, 12:01 PM
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what year car is the donor 94 motor in?

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