Idle Surge And Stalling IAC jumping around.

Dylan4570

Member
Joined
Jun 28, 2021
As the vehicle warms up the idle surge will get worse and worse, eventually the car will stall. I have adjusted the min air screw to range all the way from 15 to 40 and regardless on setting the idle will not stabilize. TPS is 0.42 or 0.44 volts.

IAC will decrease to ~20 and then suddenly surge to ~70-90 and then the process will repeat, eventually worsening until the car stalls. I can see the vacuum drop and AFR lean out from 14 to 17 as it surges. With a 3 inch LT1 MAF and translator what should my MAF reading be at idle? could the MAF be failing? fuel pressure is ~43 line off ~35 line on.
I have tried a new ECM and it had no affect. Also did a IAC reset and that did not seem to affect it. It finally decided to stall while i was driving it.

I could parts cannon an IAC valve at it but am looking for something a little more certain. sometimes the car will throw a code - 41 when the idle gets really low and surges.
 
Your AF is way off. Mine with a 3.5” MAF & translator is 6 at idle I believe.
I also keep my TPS at .42. Change that & the IAC changes so you have to do the dance. Your car may like .38

I keep my IAC below 20 at idle personally. Have you tried resetting your IAC valve & are your translator settings correct?

 
Your AF is way of. Mine with a 3.5” MAF & translator is 6 at idle I believe.

I keep my IAC below 20 at idle personally. Have you tried resetting your IAC valve & are your translator settings correct?

the 20 ish AF is cruising at 40 mph in 3rd gear. The car wont idle when hot. with this exact setup the car ran fine for 600 kilometers aside from minor surging and the IAC count being to high at idle (46+). The reason I attempted to reduce it is because i needed more IAC headroom for the Air Conditioning to function properly as far as I'm aware. as well as the TPS being 0.48 which is above spec.

I have done multiple IAC resets and the car stumbles at first as the IAC opens then runs fine for about 15 seconds and begins surging again.
 
I see. So at idle your AF is 8.

I’d clean the IAC & try an IAC reset & see if that fixes ya up.

As long as your getting 255 at WOT I’d say your MAF is fine.
 
I would physically check the CAM sensor cap wiring and reluctor ring and screw, first see if it's loose. Then an ohms test to A11 ECM wire, car off, both connectors off the C3I module Pin J yellow wire, and the ECM plug unplugged that has A11 in it.
Could be the module going bad too.
I would also clean and re-seat the chip and socket pin connections if you have the know how and tiny pic. and electronics cleaner etc. Free tests until you have to throw some parts at it.
 
02 sensor says 780mv on scan master and 940mv on meter to ground.

I will try replacing the crank sensor and resetting the cam sensor tonight. I also checked all grounds and they are nice and tight.
 
what are the setting on your translator set at?
3 inch lt1
10% lean at WOT
standard idle

Tried 3.5 inch and it idled like crap and was super lean.

AF at idle hovers around 6-8
Wideband AFR is 14-15:1 and sweeps to 17+ when the lean surge occurs.

I'm going to break out my oscilloscope and start looking at cam,crank, and alternator output tonight.
 
BLM is around 122. Below are MAF waveforms. Yellow is the LT1 pre translator and blue is post translator.

Interestingly, when the O2 sensor is disconnected it doesn't throw an O2 code. I tried a new MAF and it did not have an effect.

I am suspicious that something is affecting the O2 reading. The surge is present regardless of whether or not the car thinks it's idling or not. The issue is only present in closed loop.

20220901_193239.jpg
 
I found the problem!

about a month ago I turned the defrost on while driving to work. Whats happened is that the switch that tells the ecm that ac is requested had stuck on(pin B8 light blue). because the AC system isn't charged the low pressure switch was causing the ECM AC status to rapidly switch from Requested to not requested. This causes the ECM to kick open the IAC to increase the idle. However even with the IAC unplugged the surge persists. The reason for this is that the ECM also does either a fuel or timing kick to help bump the idle up as the ac clutch pulls in.

hopefully this will help someone else someday as this is not something that can be diagnosed with only a scanmaster or power logger. The only way i was able to identify the issue was with tunerpro RT.
 
I found the problem!

about a month ago I turned the defrost on while driving to work. Whats happened is that the switch that tells the ecm that ac is requested had stuck on(pin B8 light blue). because the AC system isn't charged the low pressure switch was causing the ECM AC status to rapidly switch from Requested to not requested. This causes the ECM to kick open the IAC to increase the idle. However even with the IAC unplugged the surge persists. The reason for this is that the ECM also does either a fuel or timing kick to help bump the idle up as the ac clutch pulls in.

hopefully this will help someone else someday as this is not something that can be diagnosed with only a scanmaster or power logger. The only way i was able to identify the issue was with tunerpro RT.
Good show ol chap! That’s some Sh@t though.
 
Right under the front of the throttle body.

Once you make any changes it may throw the TPS settings off & you’ll have to do the dance between the two.

0-20 for IAC & .40-.44 for TPS. A Scanmaster will help you out greatly if you don’t have one for the IAC & TPS values. Also useful for other info as well.

 
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