Three months ago I replaced the IAC with aftermarket one in stock. It worked fine for for 2.5 months, after which the truck would idle at 1400 rpm for a minute or so only on cold start up in the morning. after a couple minutes the idlle would come down to normal and be fine for the rest of the day. So I ordered a better Delphi IAC and replaced a week ago. The first morning it hunted a little then idled normally. Each day it got worse in surging back and forth from 900 to 1400 for the first couple minutes on cold start up. I found if I take the negative battery cable off, turn headlights on for 10 seconds reconnect battery, turn key to on, not start, count to 10, then start, it will start and idle normal everytime in the morning on cold start up. Always idles fine after going into closed loop. It's only in open loop that it surges up and down. Any thoughts appreciated, thanks.

 

After doing the PCM reset, how long before it starts acting up again?

 

I use it for work every day. So overnight or after sitting 12-16 hours. It only does it in the morning or first start of the day. I can be at a job for 1-3 hours and it always idles fine on restart. I have had a CEL that will come on for a couple weeks,then it goes out by itself for a 2-3 months. I looked up the code first time it happened, whatever it was it didn't affect driveability or sound concerning. 248,000 miles on her now.

 

That's gonna be a fun one to figger out...... Normally, I would suspect a vacuum leak, but, your symptoms don't really match.....

 

So this morning it started and idled normally at around 800-900 rpm en was normal on cold startup. Go figure. After about 5 minutes of idling it bounced up and down a couple times, then normal rest of the day. Got someone looking at it with scan tool this weekend. Thanks.