Oops, OK, I have both eyes open now and sorry for misreading your post! I mistook sub forty degrees for minus 40* below zero. My apologies! Unimaginable cold! Yikes!

 

in my case it was not a Q5, but a 2013 Q7 TDI dealer loaner. I perceived some idle vibration, more so when cold. Another order of magnitude different than my W12, meaningfully more than my 4.2 V8 gas, and more than a gas V6 Audi I owned in the past. More than the gas portion of my current four cylinder 2.0T Q5 when running on the gas side. On the other hand, I've been in a Cayenne TDI several times in recent days (as a passenger), and in that application and seating position it seems pretty darn smooth at idle; not meaningfully different than other V6 gas experience in similar conditions.

That Q7 TDI had 2500 miles on it, almost new looking inside and out, and no obvious issues. Q7's are now way long in the design tooth, and maybe the motor mounts or something were another step behind generation wise.It is also at a marginally lower tune than the Q5 TDI, and the Q7 is a real porker weight wise. Still my question there was about the NVH (and possible boost lag), not the heft or acceleration, which was decent. Some incremental idle vibration doesn't surprise me in general given both the higher compression and the self ignition. Much less than older diesels. But it was perceptible in both the steering wheel and even in the pedal some if I was thinking about it. Not like a rude heavy duty/construction type vehicle by any stretch, but also not transparent.

I was interested in it for a possible alternative to a 3.0T, but unfortunately ran into some pretty over the top zealot stuff on the other board where I had the vehicle interest and couldn't get a direct answer to what I had run into w/ the Q7. It was kind of a turn off.

 

Jetta TDI nor Q5 TDI does this.

 

I have a 2003 Jetta TDI that was parked outside in Reno for 2 years... never had an issue starting. You do need to let the glow plug light go out before starting, takes about 2 seconds in real cold temps (0 to 20 F).
I have a Q5 TDI and an A6 TDI, and they both idle smoothly in cold temps (20 to 40 F).

 

2800 miles not a single issue.

 

No worries. Up until after midnight celebrating the new year would make one of my eyes stay shut early in the morning as well!

 

Diesel engines by design will be a little rougher at idle than a gas engine, because of the way the combustion happens.. The moment of compression is much more violent than in a gas engine. Frankly, I'd be a little bummed if I bought a diesel, and it didn't' sound like a diesel! =)

I've spent lots of time in a variety of Q7 TDIs here, and other diesel models in Europe, and there's no question, the Q7 just isn't as well insulated or something, because the A6, A7 and A8 TDIs are much quieter, much less "dieselish".

I have yet to sample the US spec A6 a7 or a8 TDI though, so I'll hold off final judgement till I do.

Regarding the boost lag comment, I think that comes down to turbo design and size. I've never really thought the Q7 TDI was overly laggy, but all diesels will feel a little "slow revving" compared to a gas engine.

Compared to the Cummins engine in our old Dodge truck, the Q7 is absolutely silky smooth. =)

 

By "when cold" do you mean the engine, or the air temps? Does the TDI engine in your Jetta record misfires? I wonder if, since this seems common between your car and others, it's just "one of those things" that they all do? It could be that one cylinder just doesn't warm up as fast as the others? I know my diesel tractor will run really roughly for the first few moments when it's cold, even if I glow it well.. I can tell it's running fine though, no black smoke. With the modern engines, that's a little tougher, since there's so much keeping the smoke from coming out. =)

 

Interesting since our 2 climates are similar as far as cool weather goes.

Thanks for the reply.

 

I have always let the glow plugs cycle on and off even when doing a hot restart. Got in the habit a long time ago and have made no effort to re-train myself.

Thanks for the reply.