Hunting idle and voltmeter - alternator?

I was just out for a run and the car stumbled as I was heading off from a light. I pulled over and the revs hunted up and down .... and the voltmeter followed in unison. I just had the battery (2 years old) on the trickle charger this morning so it was topped up. I revved it a bit and after a minute or so, it ran fine again all the way home. Could this be a symptom of an alternator on the way out?

how long did the battery sit before charging? or was it on a trickle charger for a while?

It sat only for a week and a half. Trickle charged just this morning for a few hours. I tend to do that if I haven't driven it daily.

Was it raining or did you just wash the car? If so, might be coil packs.

Nope - dry garage storage, sunny day. I thought of bad gas or a bad fuel pump but those conditions are more severe. It was the unusual oscillations of the voltmeter that made me suspect the alternator. Drove it again tonight with no issues but I'm planning a long drive this weekend and I'd like to solve it.

Without Codes it will be difficult to diagnose from a keyboard. Vacuum leak ?

When mine did this, it was a faulty voltage regulator.

I think the voltmeter fluctuations are a result of the idle hunting up and down. What comes to mind is a stuck Idle Control Valve. When was the last time you cleaned out the ICV, Throttle Body and MAF? Any codes pop up?

I checked - no codes. As for the other three, I cleaned the MAF last summer - will do it again. Throttle body - same. But the Idle control valve? I'm stumped. There seems to be one on the MK1 .... but not on my MK2. And no "idle control valve" listed on all the major parts places or in the Bentley manual.

I've noticed something similar recently... when the car is sitting and idling, it seems kind of rough and there is some vibration(more than I've had in the past). Was starting to wonder what it might be, but mine isn't nearly as severe as what you had(mine will fluctuate between 600-800 rpm).

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Could be I guess but I would think a vacuum leak would be consistent once there was a break.

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Couple of things... 1) It gets offered up once in a while but it costs nothing and sometimes it helps. Do an E-gas calibration. This should be covered in the owners manual. From memory: With the ignition off and your foot off the gas pedal for the entire procedure turn the ignition to on. Leave on for at least 60 seconds. (I "time" turning on the ignition with the dash clock flipping to the next minute.) Leave on for at least 60 seconds. Turn the ignition off and leave for at least 10 seconds. The next time the engine is started the calibration is complete. 2) A mildly varying idle (with my Boxster) proved to the be the early warning of a bad AOS. Have to mention the other 2 failed AOS's (with over 310K miles the Boxster is on its 3rd replacement AOS) didn't manifest this behavior as they were failing. (They manifested other symptoms. At least initially. In the end they all 3 caused the engine to emit copious amounts of oil smoke to the point the engine was having a tough time idling.) 3) A more drastic varying idle, dropping from around normal (740 RPMs) to around 500 (as per the dash tach) then rising back up again was a symptom of a failing VarioCam solenoid/actuator. While when this was going on there was no CEL and no active error code there was a pending code (P1341 IIRC). Additionally the short term fuel trim of the affected engine cylinder bank (passenger side) was going crazy while the other bank's short term fuel trim was rather stable. (To view these I had an OBD2 code reader/data viewer connected to the car's OBD2 connector.) 4) With the engine idling check the oil filler tube cap to make sure it is tight and leak free. See if you can move the cap about and if the engine reacts. If it does the cap is leaking and should be replaced. (My Boxster is on its 2nd oil filler tube cap. These last around 150K miles is my experience but I'm sure it is not out of the question one could last fewer miles.)

Thanks Macster - I'll do an egas calibration and check the oil cap when I get home. As for AOS, there has been no smoke on start-up or running that I have seen. Will keep the Variocam solenoid in mind as I go through things.

Once you do what others here have suggested: My bet is that your AOS is starting to go bad. I spent months tracking down my hunting idle that showed up randomly. Replaced IACV, cleaned MAF, smoked tested for vacuum leak, replaced coil packs, etc. Not until my clutch went and I had the AOS replaced while the trans was out for the clutch job, did the hunting idle finally stop happening. I also had the RMS, IMSB swapped out but those have nothing to do with idle. As far as your hesitation while accelerating from a stop, my car only did this once. I replaced the coils and that solved it. Hunting idle remained and then as I said when the AOS was replaced the hunting idle stopped once and for all. And by the way, I had none of the symptoms of a failing AOS, (smoke, severe suction at oil cap.)

Damn, if that's it, I know it's a b!tch of a job to replace. I'll do the easy stuff first ...

Let me make one last point that I may have not made clear... I took delivery of my car on Friday Dec. 11th, 2015. I drove it several hundred miles that weekend without doing any maintenance or mods. The car drove just fine. Then on Monday or Tuesday I detailed the car all day. I washed it, clay barred it, then washed it again, then polished it, washed it again. So a lot of washes, and I wasn't aware to not use a lot of water around the rear as to not get excessive water in the engine compartment. So I believe I really soaked the rear end good. I gave it a final wax and was excited to go cruising. I started it up, drove to the exit of my complex, and gunned it. That's when my car bogged down very severely. Lost complete power then it came back, then lost power again. Did this like 4 times and I almost sh.t my pants. I stopped and took deep breaths and wondered what the hell. I then proceeded to drive for some time and all seemed well. But I believe this is when I started to get the hunting idle, after the car bogged down after washing it. My theory is that I flooded the engine bay with lots of water. Coils probably got soaked, water most likely went down the snorkel and into the filter, etc. And by me gunning the throttle and bogging it down like I did, may have taken out the AOS. I've gone over this in my head many times and it is what seems to make the most sense. There are others on here with way more technical knowledge. Perhaps they may chime in and agree or disagree if this is possible. Sounds to me like you had a very similar event...a severe bog down followed by a hunting idle. Of course no water was involved in your case. Good luck.

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My experience after 3 failed AOS's is the smoking and extreme pressure difference at the oil cap are the late stage signs/symptoms of a failing AOS. Failure #1 started with a stumble off idle and a varying idle speed. Nothing severe just the idle was no longer rock steady. I messed around and towards the end of my "investigation" found I couldn't remove the oil cap with the engine idling. A quick phone calll to the dealer SM and the diagnosis was "AOS", followed by "don't drive the car!" I heeded this and called a tow truck. The engine starting smoking after I started the engine to back the car out of the garage and move the car into position. This was made more diffcult by the inches of snow on the ground. Failure #2 started off with a CEL concurrent with a bit of a stumble while the engien was idling at a stop light after having just left the freeway. This at the end of the day after having driven hundreds of freway miles on a road trip. This happened when I stopped for the night in Flagstaff, then the next night in Albuquerque, but I don't recall it happening the 3rd night when I rolled into a hotel parking lot in Sedalia MO. I of course read the codes -- mixture related but believed the problem a failing MAF which is why I pressed on. The next morning though when I started the engine the smoke just billowed out of the exhaust. The car got another flat bed tow truck ride the dealer. The 3rd failure had no early warning symptoms. Everything was just fine until I took off from a stop light and glanced in the rear view miror and spotted what turned out to be oil smoke. I tried to nurse the car back 10 miles to the dealer but after a mile or so had to park the car and call a tow truck. The smoking had returned with a vengence and the engine was running poorly.

Thanks for your responses everyone. Well, I did an e-gas calibration and checked the oil filler cap and then took it for a spin to see if I could replicate the problem and pay more attention the the symptoms, but it ran fine. So this morning we went on a long drive, trying all sorts of driving conditions - city stop and go, highway, country .... and it ran like a top all day. So either it was the e-gas calibration or .... maybe a little water in the gas from my last fill-up - something I've been thinking about. Anyway, if it re-occurs, I'll dig deeper and let you know what I come up with. Someone suggested that the voltmeter goes up and down with the revs but doesn't. That led me to believe the engine was momentarily cutting out and possibly the regulator or alternator were shorting or cutting out .... but water in the gas might create the same situation. I'm thinking about replacing the alternator anyway as preventative medicine - it's never been replaced in 15 years and may be getting cranky.