It has 208,000 miles. Year 2000. Got the knock sensors replaced and thereafter this started happening. CEL P171 code has been there forever,
I had the knock sensor hack in the vehicle for several years. Then after knock sensor replacement, I tried to revert back to original. On first start, it had a very slow idle and you could tell mixture was very rich because the exhaust had a fuel smell. Then I got the knock sensor code and vehicle went back to limp mode. So I reverted back to the knock sensor hack.
That seemed to correct the problem. No knock sensor code and not in limp mode.
However, a new issue seems to have developed. I had the IACV cleaned out too.
After sitting for a few days, during initial one or two miles, the car seems to have no power and a very pulsating idle. Then baam....it seems to correct itself after runnign for one or two miles and then it runs normal.
I am at a loss. What to check? What could be causing this?

 

After you mess with IACV, the computer needs to re-establish the proper setting. It is normal for it to hunt and then resettle.

Salim

 

I can tell you, from experience, that "cleaning out" the IACV is often just a temporary fix, and can actually clog up again and/or send the unclogged junk back to the Throttle Body, causing problems again. I went through several "cleanings" myself, the first "professional", and the subsequent two more by me. I got tired of this routine and decided to put on a new IAC, along with bench cleaning the Throttle Body, which was full of coked up junk. That fixed it, no more IAC issues. I posted my work on this site if you want to see what I did. Good luck !

 

The last I cleaned the IACV was in 2007 when the vehicle had 112,000 miles. It has worked till now without any issues.

Any opinion on aftermarket IACV? Or is the answer the same - that buy genuine because nothing else works.

If aftermarket is OK, what brand/seller would have a reliable one?

 

I put a Standard Motor Products one on mine. Has worked flawlessly since install. Got it from RockAuto. That was in September of 2015 at 145,000 miles. Now has 169,000 miles.

 

Once the knock sensor was changed, the vahicle was not running right. Very rich - you could smell gas in the exhaust. And the idle was very stumbly.
He then used a very fancy Snap-on diagnostic OBD2 scanner and reset the ECU he said. After that, the vehicle ran OK till the issue re-appeared.
Cold it be that the ECU programming got wiped out completely?
If yes, do I need to take it to a dealer to reprogram it? Biggest problem is the intermittent nature of the issue. And that makes fixing very difficult. I am at a loss where to take the vehicle for fixing it. I know many dealers just throw parts at the problem - hoping it will fix it.

 

I've never heard of the ECU programming getting wiped out. More likely the "reset" cleared the long-term fuel trims, and your P0171 is telling you there is a trim problem.
Then as the mixture gets adjusted you start smelling it.

There are multiple P0171 threads here. I always recommend this video to help understand what it means and how to diagnose it.

You need to get your own OBD2 scanner. There are lots of low cost options that provide live data. You certainly don't need a SnapOn one.