Idle from Hell

[LikeMy6]

ok guys this one is for the site gurus. Did the search for this topic already, and followed what I had found to no avail. Here is the situation, engine runs fine down the road both part throttle and wot. Problem is when I come to a stop it runs like total crap no matter if in park or in gear. Already reset the IAC with the ALDL and reset the ecm by removing the ecmb fuse even set the idle manually with set screw. It fixes the idle and it runs perfect even turning on all accessories, lights, wipers, turning steering wheel side to side etc. Engine still runs smooth as glass. However once I kill the engine walk away for a few minutes then restart it its back to running like crap. Meaning the idle fluctuates up and down drop it in gear and it usually will stall especially once up to 190 degrees. Now get this if I reset the IAC and leave it unplugged the engine runs fine even after I kill it walk away then restart it later still runs fine, drop it in gear doesn't stall still running fine. WTH is going on? Is it ok to just say piss on the IAC and leave it unplugged? Oh and yea its a new IAC valve because the last one did the same thing so I'm out $60 also too. Lets see the expert diagnosis guys!

TIA

 

[Eric Sy GN]

Re: Idle from Hell

You mentioned that it runs good after you reset the ECM. This sets the fuel trims (BLM, INT) back to 128. Maybe something is causing the fuel trims to learn incorrectly, like a bad O2 sensor or something.
Maybe reset it again, drive around a bit, then stop, put it in park, and look at the BLM, INT, and IAC while its idling.
Eric

 

[LikeMy6]

Re: Idle from Hell

It seems now after I installed the new IAC valve even resetting the ecm makes no improvement. Only idles well if I reset the IAC and then unplug it leaving it unplugged. Also noticed that this new IAC valve makes a different noise while resetting a ticking noise where the original IAC valve made more of a steady hum. took a test light to the IAC plug making contact with each of the 4 metal plugs with the ignition on two light up, and two do not. While I am fully aware for the purpose of the IAC is there any downsides to eliminating if I cant figure out what is wrong with it? The O2 sensor is new, also have a wideband O2 installed and wired to the ECM using the Velosyty chip.

TIA

 

[Don W.]

Re: Idle from Hell

What's your IAC reading (number on scan tool) when it's ideling correctly after reset and again when its bad?

 

[ajt86]

Re: Idle from Hell

You cant just eliminate an IAC valve. What are your fuel trims?

 

[GAShan]

Re: Idle from Hell

Ignition module in the distributor.

 

[gstacky]

Re: Idle from Hell

Pull the cap off and look for corrosion in the distributor, a few of us have had idle issues this spring and it's ended up being the distributor every time.

 

[LikeMy6]

Re: Idle from Hell

Don, the IAC # when it was at its best idle was 41. Bad idle was at two extremes 00 and around 90.
The distributor, coil, cap, rotor, ngk plugs, magnecore 10mm wires are all new.
Checked the vacuum lines replaced with new silicon hose. All new intake and TB gaskets.

 

[LikeMy6]

Re: Idle from Hell

Took a few minutes to start it up and write down the #'s for you guys. Also since this is a Velosyty chip it allows the user to manually adjust the fuel trim. On the few times the truck idled well I noticed the AFR was between 11:1-12:1 when it was Ideling badly it pegged the meters at 16:1, and if I revved up the motor it would produce loud afterfire in the exhaust. However not when it was at the lower AFR. So I adjusted the trim on Idle only to the maximum +21%. It seems to made things somewhat better. Here are the #'s before fuel enrichment. Data read from scanmaster. Idle was set at 900-850, but fluctuated between 900-550, and died when put in gear.
-temp 160*
-map 66
-afr 16.0
-int 128
-blm 128
-tps .74
-iac 84

#'s after enrichment. Idle now raised to 950-1000, and remained steady even when put in gear. When in gear IAC #'s rose to around 10.
-temp 175*
-afr 12:1
-map 43
-int 118
-blm 118
-tps .76
-iac 00

 

[Don W.]

Re: Idle from Hell

Looks like you're set to show MAP in KPA. Reset to "hg if you would. While you're in there check the other settinsgs as well. IAC should be around 30 with things working correctly. Eric's chip can be set to emulate the NB o2 by turning on sw 4. Also turning on sw 2 puts the idle in full time open loop. What happens when you do those two things. Together? Individually? 3 turnes off closed loop entirely. What happens there?

Check the dist as mentioned. Swap a module and check the wireing at the dist. Does the tach dance around a lot?

 

[LikeMy6]

Re: Idle from Hell

Looks like a possible success. Went back one more time reset the IAC valve, and cleared the ecm again. Reconnected the IAC plug and started up the engine. Using my left hand to keep the throttle open, I gradually turned the idle set screw back out until it was at .61 on the TPS. Leaving the manual Idle fuel enrichment at +21% the engine finally maintained a smooth idle of 700-800rpm the BLM read 123, the INT read 128, IAC read 37 in park warmed up. The AFR read 14.5:1. Took it for a drive and it didn't stall out, ran great. Returned home shut it down and waited an hour went back and still running great. Seems the Idle was way too lean before, and with too much base throttle opening.

Would like to thank all who offered advice.