So this is one primary difference between our issues. When my idle hang issue occurs, it doesn't seem to have much (if any) impact on fuel trims (ST or LT) or O2 readings, nor any marked change in the variation between banks. But then, mine only ever hangs by ~100-150rpm, so even if it was an actual vacuum leak (unmetered air) it shouldn't have a huge impact in the first place.

Either way, if you're seeing highly elevated fuel trims during the condition and extreme variations between banks, that's a pretty big indication of unmetered air entering the system somewhere. In my case I don't see those things, which is why I've arrived at the conclusion that mine is a matter of metered air entering that shouldn't be (meaning a fault in the PCM to IAC command signal - hence a high idle with a 0 IAC count and no signs of unmetered air).

 

Some of the guys in the thread have theorized that there may be a crack somewhere that opens up when the car is hot but closes when it is cool. This is partially compatible with the fact that the condition doesn't always occur when it is hot, but it only occurs when it's hot.

So I talked to the guys who did the smoke test, and turns out that they did it when the car was cold. So the next step is for me to get it good and hot, do some WOT runs and hopefully induce the condition, and then do another smoke test. Stay tuned.

 

I’m haveing the same problem lmk if you find anything looking forward to see

 

Since this thread got bumped again, I just wanted to add some potential new data...

...I'm now willing to revisit the possibility that the issue might have been somehow connected to a problem with the battery/battery cables/voltage. I finally replaced the battery (at 10 years old) because it was starting to sound a bit weak on initial cranking. In so doing, I discovered that the battery cables had come pretty loose, likely because the lip on the battery tray was partially broken so the battery was not as secure as it should be. I replaced the whole coolant overflow bottle/battery tray so the battery would stay properly secure, and I reinforced the lip to prevent this from happening again, and installed a new battery.

I didn't realize how weak the old battery had become until I experienced the difference with the new one. I'm now wondering if the reason this issue had disappeared for so long is that I had, in fact, fixed it with that new EVAP solenoid but then a new issue developed due to either the loose battery cables or just the weak, old battery. All of this occurred to me as I was doing the idle relearn procedure for the new battery, and I realized that, during the procedure, it displayed some of the same odd idling traits - but that it all cleared up once the relearn was complete (even before doing a restart). The '98 PCM seems to be really sensitive to idle relearn/idle parameters (as compared to how my later cars all were), so perhaps an old battery and/or loose connection at the battery post was somehow causing havoc/corrupting learned parameters?

I've only driven the car a couple times with the new battery, but so far all is well. I guess we'll just have to see how things go over this summer, but I'm hopeful that it might have been something this simple because the issue was gone for quite a while (after replacing the EVAP purge solenoid) when the previous battery was still new.

 

This whole saga has been so weird. Since my last post in this thread, I took out the PCV thinking it might be clogged with the orangey gunk that was in my throttle body and intake. It was clear, but I sprayed it with carb cleaner and replaced the hose sections (between the catch can and intake nipple) as the they were hard but not cracked.

The car immediately started to run normally, until one day last week when it completely FREAKED OUT per my description in the other thread: I'm cruising at about 40mph @ 1600 or so rpm with my foot off the throttle. As approach a stop, i shift into neutral to be able to slow down, and the rpms spike to about 2600, all with my foot off the throttle. As the car comes to rest, the idle slowly settles down to about 900 (setting is at 650).

Since then, it has run normally - no extreme events or semi-normal or moderate behavior - but I'm waiting, and course hoping, that it will have finally settled back down. We did find a bad coil and burned plug wire, though no misfires on that cylinder were detected. The conclusion that I and the techs I've been working with is that it is not a mechanical or vacuum related problem, but that the PCM is seeing something it doesn't like and responding to it.

My next step if the issue recurs is to have the PCM connections and grounds inspected and tested.

 

Bringing this back up with an update. The problem gradually disappeared over a couple of months. I even took it on a 400 mile round trip last week with no issues. Then the day before yesterday - Boom! It's back.

So this time I not only disconnected the purge line and capped off the intake nipple, I plugged the purge line and disconnected the solenoid. And it's back to running normally. Opening the gas cap doesn't release any fumes. I'm becoming convinced that the vent valve solenoid gets stuck open, and when the purge solenoid kicks on, it creates a vacuum leak as you have described.

Back in 2016 I had a problem for a couple of months with the gas nozzle shutting off when I tried to fill the tank. My theory was that the vent solenoid was stuck closed. Now I think it gets stuck open at times, and this accounts for the intermittent nature of the problem. Make sense?

 

If you had previously disconnected the purge line and capped off the intake nipple (and this didn't solve the problem), I don't really think it would make any difference (in terms of an actual vacuum leak) whether or not the purge line was plugged and/or the solenoid was disconnected from the wiring harness as well. In other words, what you have done currently is effectively the same thing as what you did in the past in terms of eliminating the EVAP system as a possible culprit. Once the intake port is capped, the rest of the EVAP system can't have any impact on what is happening inside the intake/engine. Or at least I *think* you mentioned, maybe in your other thread, having capped off the intake port previously. If I'm remembering incorrectly, then disregard this.

FWIW, I've driven mine a couple more times since replacing the battery and doing the relearn, and so far the idle has been rock-solid stable. Not that I think that your issue is the same, but I just wanted to update this for anyone else searching in the future since the topic came back up. I still won't consider mine "solved" until it's gone a whole year without returning though.

 

My thought was that if the purge system is active and the vent solenoid is open, it might in effect create a vacuum leak . But maybe that's how it's supposed to work. I'm not sure exactly how the system functions, despite watching an excellent YouTube video explaining it. Grasping at straws here ...

 

The purge solenoid is the regulating factor regarding flow to the intake, regardless of the vent valve status. Either way, no part of the EVAP system can impact the engine or A/F ratio once you've disconnected the purge line and capped off the intake port (unless the cap is faulty and that then becomes the source of the intake leak - but then restarting the engine wouldn't correct this.)

 

True that, and more to the point, it started acting up again this evening, just as in the previous experiment. So the point is moot.

 

I wanted to come back to this now that I've had a whole on-season of testing. None of the odd idle issues ever reappeared with the new battery, so perhaps it had to do with low voltage during cranking, or voltage loss during periods between battery charging, that was corrupting stored PCM data and putting the computer into some sort of semi-relearn process for the idle. The car will soon be prepped for winter storage, so that will be the end of data until spring. Anyway, just wanted to provide an update - looks like my current case wasn't something ultra complicated at all...just needed a new battery.

 

At one point in the movie "Christine" the guy thought everything was ok, lol.

 

Haha! Love that movie! To this day I still have proper Buddy Repperton pork chops. Dead serious.

"Some things can't be helped,...some people too."