Q5 TDI Rough idle when cold
 

I am performing due diligence before committing to a '14 Q5 TDI purchase and have a question. Apologies if this has been asked and answered before but, a forum search gave no answer. Apologies also, for the long explanation before my simple question.

When idling while cold, my '12 Jetta TDI consistently will have a series of single "misfires". I use the term misfire to describe it because it feels exactly like a SI engine with a single marginally functional spark plug/wire/coil pack. It could also be described as feeling like a SI lean misfire.

It is very intermittent during the warm up period and only occurs during the first start of the day or after sitting all day in sub 40 degree temps. One of my colleagues has a JSW TDI that does the same thing. Short of letting it warm up for a few minutes, the only way to eliminate the problem is to put the trans in drive and either set the parking brake or hold the brake pedal down with my foot. Without fail, everything smooths out and all is right with the world. The car has ~22k miles on it, has done it since new and has never set a CEL.

My assumptions are that the cold engine is so lightly loaded that an occasional incomplete combustion event occurs. Dropping the trans into drive puts just enough load on the engine to increase the cylinder temp to the point where all 4 cylinders fire correctly.

My question to current Q5 TDI owners is this: has anyone noticed this behavior on your vehicles?

Nope, not even close. 7k miles and not a single issue.

Are you located where it gets chilly in the winter. Say into the 20's F/-7C?

Most of us don't get to "enjoy" sub -40* temperatures. :roll eyes: Do you use any brand of anti-gel diesel additive? Ever hear of an engine block heater? I would imagine what you have experienced would be expected with those conditions. People used to take their batteries indoors at night to keep them from freezing in the old days. Not only is your fuel practically frozen, so is your engine oil and transmission fluid. I'm surprised it starts at all!
As for myself, I live in Tulsa, OK and we, thank God, do not have those extreme conditions. I've never had a rough idle....but my TDI is garaged and it never gets below freezing in there.

Why is this relevant to a purchase decision? I do believe in the concept that people look for different things when they buy vehicles, but I don't understand how this characteristic is relevant to a purchase decision unless you believe it affects the reliability or some characteristic that *is* relevant to vehicle ownership. "It misses during cold idle" just seems to be in the "who cares" bin.

Thanks for answering my question about your experience with your car.

I never mentioned sub (-) 40* temps where simply starting a cold soaked anything would be difficult if not impossible. I was asking about temps below (+) 40* which Tulsa does experience on a fairly regular basis.

Yes, I used white bottle PS in the winter, gray bottle PS the rest of the year in my '01 ALH Jetta and am currently experimenting with XPD in the '12 CJAA Jetta.

Yes, I have heard of coolant heaters. Other than 1 hard starting episode with a bad tank of fuel and (-) 25* cold soaking in South Park, CO with the '01, I have not been to justify the use of one.

Unless your car spends the night outside in the conditions I have indicated, you will not know if it exhibits this behavior or not.

Again, thanks for your response.

Perhaps this question is in your "who cares" bin. Since you took the time to respond, it appears to be in your "cares enough to respond" bin.

It is relevant to my purchase decision if it is indicative of normal behavior for VAG CR diesels. If it is indeed normal, then it can ignored for the time I will own the vehicle.

I prefer to enter into new to me vehicle ownership fully aware of all of it's idiosyncrasies. How many people cared only about the look, color, or performance of the Q5 3.2 before purchase? How many of those people got stuck with the hassle and expense of 1or multiple trips to the shop to have the carbon deposits cleaned from their engines? How many of those people would have purchased something else and been satisfied with their purchase had they spent the time to become truly informed consumers?

Perhaps this misfire I have observed is normal behavior for the CJAA. I was interested to know if it occurs in the Q5 TDI under the same conditions. Does it affect reliability? Does it diminish or increase with time/miles? I don't know the answer to those questions. Is it relevant to MY purchase decision and subsequent vehicle ownership? Seems to be since I asked the question.

Gredi, I can report that I've started my TDI outside in -2*F temps (without a block heater) and had no starting or idling issues. Diesel engine technology appears to have come a long way lately. Hope this helps.

Indeed it does, thanks!

Thanks for the reply.