Jekyll and Hyde idle behaviour

For the past week I've been having issues with either poor starts and/or stumbling in first gear from a stop, or slow roll (like turning a corner). Its just the oddest thing that there is no consistency to it. Take yesterday for an example- Went to move the 911 for my wife to go to work, and the car started fine, idled for about 20 sec or so, then put it into first and try to move and the car wants to stall and stutter, unless I rev the engine to over 2000 rpm before taking off. But a few minutes later, move the car back and its fine. An hour later go out to go to work- takes 2 or 3 attempts to start, rough idles for about 5 seconds, then fine. Put it into reverse, and drive off normally. Later at lunchtime, starts fine, but sputters and stalls a couple times., then fine the rest of the drive. A little history- after sitting a good many months, I replace my AOS in May. got a P0430 code back then (attributed to the old gas) and when I cleared that code, did note I also had a P0455 code. That code came back once or twice more and I've noted some poor starting for the last few weeks (poor as defined by not starting on the first attempt without touching the gas) and maybe one time it stalled on me when backing up. Before going out of town last Thursday I brought the car into the shop for a brake fluid flush and oil change, I believe they had spotted the evap purge valve had come loose and reconnected it. No more CELs since then. But the car sputtered big time a few blocks from the shop, stop to fill up, sputtered once more, but was fine the rest of the way, and ran well on the 200 mile trip up to Ohio, registering ~25 mpg on the computer estimate. Basically runs fine over 2000 rpm all the time, but you just don't know when Dr Jekyll will show up- can be somewhat embarrassing when its sputtering and lurching - once or twice seemed like even a backfire ( not from the exhuast), but then it would be all nice and Mr Hyde like. One thing I was thinking was maybe the AOS vent tube was leaking (I worried that I might have crack it trying to snake it in or out), but it seems like a vacuum leak should give me a consistent, constant problem. I haven't had the chance to look inside the engine compartment yet, so what do you suggest I look for this weekend?

Sure sounds like a misfire due to a coil pack going bad. I had exactly the same symptoms and the coil pack below the leaking coolant reservoir was the culprit. Changed it without changing the plugs, it solved the issue. Last weekend (1.5 years later) I finally got around to replacing the rest of the coil packs and all the plugs. I haven't posted this yet.....but the cylinder that was misfiring was the one in the upper right of this picture.....you can see the corrosion on the threads, as if the spark was not going to the center, across the gap to the threads, but leaking around the plug and going to the top of the threads (bypasing the gap, hence no spark). Here is that plug compared to a new one:

Here is my misfire thread: https://rennlist.com/forums/996-foru...1-misfire.html

Did you throw a code with that misfire? Edit: Ahh, yes I see in the other thread you did. I my case I don't see any problems with my OBD scanner, albeit using two free phone OBD apps. (which have read many a code in the past)

How many miles on the car and have the plugs or coil packs ever been changed?

I've got 103K miles on it and the plugs and coil packs (I believe all 6) were replaced at 85K.

Quotes source post: 14340152

Any pending error codes? If the poor running only occurs upon take off or when cornering that suggests a fuel supply problem. There can be a fuel leak due to a bad hose (split hose) or loose connection in the fuel tank and when the car is subjected to g-forces this leak becomes worse and fuel supply and pressure are momentarily affected. On a related note be sure the vacuum hoses in the engine compartment are ok. A bad hose that has a crack/split in it that only opens up under some driving conditions can also account for this behavior as the engine can run lean. Another thing that can account for the behavior is a bad coolant temperature sensor or an intake air temperature sensor (this from the MAF). If these don't report the right temperature the engine can be fueled incorrectly. If the engine spits back through the intake that's a sign of a lean mixture. Thus a temperature reading warmer than actual temperature has to be considered. (It can also be a fuel supply/pressure problem -- the above mentioned fuel system leak inside the tank or a bad vacuum hose.) Data logging intake and coolant temperatures, and fuel trims, can be of some value.

"If the poor running only occurs upon take off or when cornering that suggests a fuel supply problem." ^^This is my first thought^^

It does so on take off maybe 20% of the time at most (probably closer to 10% when I think about it) and no problem with true cornering at speed. Only a couple times while taking a corner at low rpm still in first gear. I'll monitor the temps for a bit. Seems to be a little more pronounced during the first few miles of a drive (irregardless of whether the car is cold, or hot) then after its been running > 15mins - I don't think its ever happened at the end of a drive. The shop thought maybe a leak at the left intake manifold, but maybe that is coming from the fact that I had it off to do the AOS- but already 6 weeks ago now. And no pending codes. Of course, I've never had the OBD scanner on when I have one of these events. Guess I should turn it on every start for the next few days.

update- car seems to be running generally okay: Monday, ran fine on my drive to and from work, although did stall one time when pulling out of the gym parking lot before heading home. Wednesday- started fine and okay to work (cooler morning) Drove it quite a bit around town in the warmer afternoon, and maybe had 2 or 3 hiccups out of about 50 stop-and-go.- Thursday/Friday- one day took a couple tries to start, other day fine. One stall, and maybe one time stumped and sputtered out of first gear. (warmer, more humid weather) Bottomline: probably less then 5% of starts,and stop and go's are effected. Doesn't really seem to be serious, just annoying. I did check maybe twice the water temp vs air intake temp readings and they seemed normal. I'm curious whether the outside air temp has an effect, but need more data points.

Only a matter of time before it leaves you stranded somewhere.

What year is your 996?

2004 w/ 103K miles

Just change out the damn fuel filter!

You may want to try a e-gas recalibration (covered many times here) and get the DME scanned by Durametric to check for misfires and fuel trims. Sounds like some kind of vacuum leak, which should show up in the fuel trims.

Quotes source post: 14341809

Air intake and coolant temps seem consistent with the conditions. I do see one oddity in the fuel trims- one of them rarely changes- long term fuel trim bank 1. While the other 3 sensors vary each second +/- around 0%, however this one stays near + 10%, and might change only once, maybe twice during a 10-15 minute trip. (always in the 8-11% range) Bad sensor or just bad communication thru the OBD tool, but the bottom line is the car runs fine 95% of the time even with this non-changing fuel trim number. How should these short and long term fuel trims respond to varying conditions, ie @idle, acceleration, coming off the gas, etc.

Ok, based on what I've read in the last couple days, here is what I think I understand about the fuel trims- correct me if I'm wrong. Short term trim is a snap shot of what the engine is doing at that moment. They respond pretty quickly to throttle changes and generally stays below +/- 5%. Long term fuel trim results from the PCM correcting errors it thinks it sees in the short term trims., ie, if one bank is consistently + 10%, it will adjust the long term trim on that bank up a % or two so that the short term returns closer to zero. LTFT accordingly change much more slowly. So my Bank 1 LTFT not changing much is not irregular, but the +10% (actually up to 14% now) is indeed a problem and indicates the PCM thinks the engine is running too lean. One common cause for a engine running lean could be an air intake leak and bank 1 does indeed correspond to the manifold that was removed to replace the AOS. (Bank 1 is the left, drivers side correct?) Why did the engine take a few weeks to start acting up? (after replacing the AOS, if that is suspect in all of this) My theory (could be wrong) is that initially the Bank 1 LTFT was near zero where it should be, and had taken a couple of weeks of continual correction to get up to 10%. Of course the high LTFT does not explain why the car could run fine some days, and stumble, hiccup and stall on others. Certainly the high LTFT has been about the same day to day, and might not be the cause of the problems, but could be related. I have read that you can reset the fuel trims be either disconnecting the battery, or by clearing CEL codes -which if you have none, you can create one by disconnecting the MAF. So I would be curious if I reset the LTFT, will it run better initially, and will LTFT bank 1 gradually begin to increase every week, along with increasing engine events. If so, I'll probably look into reseating the left intake manifold ( and replacing the AOS corrugated tube that runs through it). Whatcha think?

So are you saying your bank 1 LTFT is ~14% and the other 3 are around 0 +/- a few %? What you said is mostly correct. The STFT maps to RKAT (in Durametric), which is the fuel trim during engine idle. The LTFT maps to FRA which is the fuel trim under load. Since your STFTs are close to 0, it's unlikely you have an intake leak. LTFT of 14% means bank 1 is running lean under load but not when idling (STFT ~0} When under load, there's less vacuum so any intake air leak will not affect the fuel trim as much. l wonder if it's one of the injectors in bank 1 has restricted flow. Have you tried running seafoam/techron in the gas tank?

Yes, the other 3 are pretty much at zero when at idle. (LTFT #2 stays around 1.0%) I've attached a OBD output from yesterday's drive to work- which of course, there were no issues. I guess that is a good point you make that STFT can return to zero, so unlikely a vacuum leak. I guess I could try a bottle of techron. I did note a bit of oil in the plenums and manifold when I change the AOS, so maybe some of that crap got in? Too bad I had just change my oil as some suggest the Techron/Seafoam treatments are best followed by an oil change. Thanks for the input!