Car intermittently idles rough, but nothing's wrong?

Just curious as I'm not sure I'm experiencing any kind of abnormal issue as it seems like something the ecm does after idling for long periods of time.

I was in the humongous car was line up the other day and while I was idling waiting for my turn the car would idle a little rough for 30 seconds, then return to normal. Happens probably every couple of minutes or so after idling for 20 min. I've noticed this a few times ever since I've owned the car.

I'm assuming its just some kind of mode the ECM goes into to reduce carbon, or clear out the EGR, or who knows what. I've also noticed it spools up the turbo after sitting at idle for around 5 min which makes sense from a keeping the VNT system free and clear standpoint. I would have data logged it but My VCDS was at home at the time.

Couldn't find much after searching, just curious if i'm the only one that's noticed.

I don't think that's normal.

I'll be watching this one as I experienced the same condition with my wife's car when I was stuck in a border crossing recently. Duration of stumble was roughly (no pun intended ) 13 seconds, occurring about every 2 minutes. Never experienced this before. RPMs appeared to hold pretty steady. It almost felt like the flywheel was wanting to come apart: car wasn't in gear and I didn't have the clutch depressed. I first attempted to push down on the accelerator in response to what I thought was an engine on the verge of dying- RPMs went up but the engine still was shaking.

Other than being a little on the slow side to fire off when cold (compared to my car) it runs pretty well, well enough to be clocking just over 49mpg (last tank).

I'd have heard about this from my wife had she encountered it herself, but no mention from her. She doesn't really do much stopping and idling, in which case it appears only related to extensive idling.

Here's the thing, it's not even like it's coughing, spitting or even a lack of power. I can rev it up just fine, and drive it like nothing changed(stops doing it for a while again after you drive it). Only seems prevalent at idle, but only because the engine shakes a little more than usual.

My ALH runs strong, and doesn't show any signs of anything wrong. No codes, all the sensors I've logged are in spec, I've had diesel purge run through it, oven cleaned the turbo after the VNT seized up a couple weeks ago(it did this before that happened though) and other than that this car is bone stock. Mufflerectomy happened 2 days ago.

I dunno, it's just a little odd, but yet doesn't seem out of place. I just want to know what the damn thing is doing when it does it haha.

Darth, might be an IQ adjustment. I've been running short of cash for VCDS for much longer than I was expecting, so adjustments such as this will still have to hold on a bit longer.

Wife's car has had zero issues. I recently swapped in a clean EGR valve (old one was leaking; replacement was from my car and was working just fine). I can't recall whether I had done the swap before or after encountering this rough idle issue.

UhOh said:

Darth, might be an IQ adjustment. I've been running short of cash for VCDS for much longer than I was expecting, so adjustments such as this will still have to hold on a bit longer.

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Yeah, It's pretty pricey, but after a couple limp modes and bothering people for a scan I've found it invaluable. On that note I've checked the IQ settings and all seems normal. The only thing that seemed a little out is #3 injector but by 0.5 which from what I've read isn't abnormal.

Gonna double check pump timing, and see if the balance has changed at all lately.

Update:

I just spent a good 2 hours with My VCDS trying to see what it's doing.

Absolutely everything checks out, pump timing, injector balance, all the sensors, nothing is out of the ordinary

Timing doesn't even change when it shudders, nor does the IQ. Nothing changes before, during, or after the shudder moment. I had it idling forever and checked so much block data through multiple shudders. Nothing happens, but it still does it.

Changing adaptation settings like the idle speed and the Injector quantity changes nothing.

I dunno, on the plus side I know my ALH is running in top shape at least.

So, I've brought my idle up to help keep it a little warmer in the cold weather and brought the IQ adaptation up a bit to 3.4 mg/str and now I'm calling it a day. Any ECM programmers out there that know what my car is doing?

My car had the occasional ratty idle. Sometimes it would throw codes but often not. It used to just throw codes in the winter but eventually it started throwing codes in warm weather. New pump fixed that problem.

roadhard1960 said:

My car had the occasional ratty idle. Sometimes it would throw codes but often not. It used to just throw codes in the winter but eventually it started throwing codes in warm weather. New pump fixed that problem.

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Ugh, don't get me paranoid about my pump now haha.

What codes were you getting?

I haven't had a single code thrown other than a bad MAF and overboost which I fixed, so fingers crossed that I don't have to get into replacing a pump for a while

Can you upload a video?

I might have experienced something similar once with my 99.5.

If you open the hood, does the engine is rocking back and forth when it idles rough?

Darth, can you readily duplicate the symptoms? If so then see if engaging the clutch does anything. If it's the DMF things should quiet down, shaking should lessen.

I don't have ready access to my wife's car right now so I am unable to investigate further. Need to replace a wheel bearing and the engine/transmission mount on my car before I can swap with her.

I've read that one DMF test is to place the trans in fourth and push forward and backward on the car and that if it moves any more than a little bit, as in gear slop being taken up, that it's an indication that it might be a bad DMF.

I had a DMF start to fail at 300,000 miles, but it was a distinct rattle. It did not sound at all like what you two are describing. The rattle got progressively worse, but the engine always sounded and idled normal.

jorioux said:

Can you upload a video?

I might have experienced something similar once with my 99.5.

If you open the hood, does the engine is rocking back and forth when it idles rough?

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Engine does not rock back and forth, it's not that violent of a sputter. It feels more like when you open up the egr a bit too much and it causes it to run rougher than usual, or when a significant alternator or other sort of load is put on the engine and it struggles a bit. It doesn't even affect the RPM's when it happens. I'd take a video but honestly you wouldn't see much haha.

UhOh said:

Darth, can you readily duplicate the symptoms? If so then see if engaging the clutch does anything. If it's the DMF things should quiet down, shaking should lessen.

I don't have ready access to my wife's car right now so I am unable to investigate further. Need to replace a wheel bearing and the engine/transmission mount on my car before I can swap with her.

I've read that one DMF test is to place the trans in fourth and push forward and backward on the car and that if it moves any more than a little bit, as in gear slop being taken up, that it's an indication that it might be a bad DMF.

Click to expand...

Symptoms are the same clutch in or out. Definitely not drivetrain related.

The only time it happens is when it idles for extended periods of time, then maybe once every few minutes it will idle a little rough for 30 seconds and return to normal. If the car begins to move it will not do it again until it idles and is stationary for a while. Otherwise it never does it.

I'm stumped at this point

I'm still not ruling out the DMF, as I've read plenty of accounts of failing DMFs acting this way.

Other things that could contribute:

Bad tensioner/idler or alternator pulley.

EGR (intake clogging)

Injectors.

Just tossing these out there as they are items that others have identified as causing similar problems. Do a search for "alh signs of a failing dmf idle" and you'll start to see the correlation.

Again, try rocking the car as I suggested above. Little time, no cost. I will be doing this on both my cars just to check and will report back when I have done so.

My car was tossing the error mentioned in this thread. http://forums.tdiclub.com/showthread.php?t=203068 I have not gotten around to taking apart my old pump to see if it is something simple like junk around the solenoid.

Got to thinking a bit more... Is it possible that this might be vacuum-related issue affecting the ASV, perhaps causing it to flutter closed? Low RPMs means the vacuum pump isn't working hard, and with some vacuum loads, such as holding down on the brakes (which was my scenario), the ASV drops out.

ASV operates/closes via a vacuum signal at shut down. It is normally open. It takes vacuum to close it. The controlling relay only gets a signal to close it when the engine is turned off and just for a few seconds. I would have to get a multimeter out to check that statement but the valve is normally open and does not get a vacuum supply when the engine is running. http://forums.tdiclub.com/showthread.php?t=128812