Pulled my 98 a4 ws6 out of storage. It fired right up and idled fine. Drove it around a bit, put some gas in and checked the fluids. Car drove nice.

Got it home and noticed that if in Park or drive the rpm fluctuates up and down about 100-150rpms or so. Checked the major vacuums and they appeared to be snug. I did put on a ported throttle body and did the tb bypass mod this winter.

Any suggestions of where to look? Could it be a symptom of the gas sitting around for the winter? I know these issues are almost always vacuum related I just find it odd that it idled fine for a half hour before I took it for a drive. It also idled fine at the gas station

 

This is very reminiscent of an issue that I dealt with on my '98 as well. Took me a couple years to figure it out, since it was so intermittent at first. I made one or two huge threads about it years ago.

Basically, here was the deal:

- After getting fully warmed up, and driving for a while (especially on the expressway), when coming to a stop the idle would hang up 100-200rpm too high and surge about 50rpm. Eventually, I was able to work out a driving routine that would generally reproduce the issue on demand.

- I discovered that I could instantly clear/fix the issue by shutting the engine off, then turning it right back on (have you tried that yet?) Based on this, I became certain that it was not a hard vacuum leak but rather a sensor or electric motor issue.

- New IAC didn't fix it, a different EGR valve had no effect, and TPS readings seemed fine, though the IAC counts would always be stuck at 0 (indicating too much air to keep idle under control).

- Running out of other options and having already replaced/checked everything else that was likely, I finally replaced the EVAP purge solenoid. This seemed to do the trick, as I was no longer able to reproduce the condition and it's been many years since swapping out that part without any return of the problem. Apparently it was opening further than commanded at idle, thus allowing more air than it should and causing an increase in idle speed that the IAC was not able to compensate for. Turning the engine on and off would reset the solenoid and correct the condition until the next warm-up cycle of the engine.

Having said all that, you did mention that you swapped in a ported throttle body. Based on this, I'd be more inclined to replace the IAC motor first (or did you reuse your original?) since it's a variable at this point.

 

When I did my ls6 intake last summer I replaces the evap purge solenoid because I broke the old one while doing the swap.

I sent my tb in to be ported so I'm assuming all the original parts were reused. I'm in the process of moving across town and the car is at my old place until later tonight.

I read the following on another forum. I did have my battery disconnected all winter.

"Have you done the TPS and IAC reset? When I was cammed sometimes my car wouldn't grab an idle after I disconnected the battery. I would reset the TPS and IAC and the car would idle fine.dum

You unplug both the TPS and IAC. Key the car to the ON positiong. Don't start it. Wait 30 seconds, key off. Plug the sensors back in and start the car.dum

When you're adjusting the TBdumsetdumscrewdumto get your IAC counts into the right range you have to do this each time. Perhaps yours is slightly off and just needs to be relearned."

 

So maybe the set screw position was somehow disturbed when the TB was in for porting?

Since the battery was disconnected, have you tried the standard idle re-learn? For a '98 it's a simple process that calls for allowing the engine to reach operating temp (176°F+), and then idling for 5 minutes in gear, 5 minutes in park, then again with the A/C on for each.

 

Haha that's quite the process. I will give it a shot