Rough idle and black smoke on cold start up??

[audi_a4b7]

Hey everyone I'm new to the forum and I have searched the web and forums at length to help diagnose this issue. A bit of background my car is a 2006 Audi A4 2.0 TDI and just clicked over 100,000kms. The car has been regularly serviced and maintained. It has also been at the shop for almost 2weeks [Audi specialist] however they are not sure what the issue is.

The issue I'm experiencing is on cold start up the car idle hunts and puffs a cloud of black smoke. It adjusts itself after about 30seconds or so but is horrible upon start up. The shop has scanned for codes/errors but nothing, they have also tried a compression test which came back fine, checked my injectors - one was swapped and re-installed, cleaned my Exhaust Gas Recycling and the air cleaner changed. The car was also serviced and oil filter replaced.

They seem to think it could be the Dual Mass Flywheel, however the problem is only on start up and I am not getting any usual Dual Mass Flywheel failure noise. From reading up, I also believe it could be the fuel pump - low fuel pressure. Both myself and the shop are baffled by the problem, any help would be appreciated. Thanks in advance!

[shahk62]

When you say hunts -does it fluctuate up and down a few hundred rpm ?


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[audi_a4b7]

Thanks for the reply. The rpm needle does fluctuate but extremely minimal. It's more like the engine wants to choke almost like its about to stall. Only does it on cold start up. Any ideas what it could be?

[shahk62]

Thanks for the reply. The rpm needle does fluctuate but extremely minimal. It's more like the engine wants to choke almost like its about to stall. Only does it on cold start up. Any ideas what it could be?

I've had something similar which involved the intake runner flaps not functioning correctly.
Potentially an intake runner flap issue or a carbon buildup issue. Try using some intake decarb or throttle body cleaner in your intake to see if that will help - but only do it on a warm engine.

[audi_a4b7]

Thanks did you end up having to replace the intake runner flaps or did the clean up sort the issue?

[audi_a4b7]

Seems like a common issue with the V157. Worth a check whether it is sticking and if it has blown a fuse. I'm getting desperate so any help is appreciated. What else could it be?

[mtroxel]

When you say black smoke, I think it's too rich at start up. And since it settles down after 30 seconds, I wonder what you're fuel trims are at start up. Does your shop have VCDS? If they do, then can go in and watch what your fuel trims are at start up.

[audi_a4b7]

They have scannd for codes and faults but nothing. Not sure if they have a VCDS but i hope so as they are an Audi specialist. I'll def ask the question next time i'm in. Any ideas about what could be causing it to run rich at start up?

[mtroxel]

I don't know that motor well, or diesels in general. But if he just has an OBD reader and is looking for codes, I don't think he's your guy. You need more data than that.

[audi_a4b7]

This is my first Audi and Diesel. Unfortunately this hasn't been a pleasant experience for me. I'm sure different people will have had different experiences compare to my personal one. It seems that Audi had a lot of expensive common faults that they recalled. I have put about 4k in the car in the last month and it's ridiculous plus sending me broke. The guys at the shop have been good but they seem to think it is the DMF which i do not agree with because DMF failure or symptons are well documented and i am not experiencing any of them. Doesn't black smoke on start up mean fuel to air ratio is wrong/combustion issue? Anyway the quote for DMF repair is $1700 but the shop is not sure thats the issue however they did say they would keep looking if this doesn't fix the problem and only charge me for the actual fix and labour. I probably should get a VCDS scan and get a second opinion. Sorry about the rant and thanks for the help!!

[mtroxel]

DMF, dual mass flywheel? God I don't know how that makes black smoke out the exhaust pipe. These guys are just spit balling. I don't know how you can be a "Audi specialist" if you don't have VCDS.

[audi_a4b7]

Exactly my thought as well regarding the black smoke. I would be extremely surprised if they didn't have a VCDS and claiming to be an Audi Specialist but it could explain the guessing. I'm going to double check with them.

[audi_a4b7]

That probably confirms that I need a second opinion. I want an accurate diagnosis and if no codes come up start eliminating the small stuff first instead of common know problems for that model and year like Dual Mass Flywheel even though i have no symptons of it. Can someone please confirm my assumption that the black smoke has to be fuel or air related on start up?

[mtroxel]

That's my thought. Black smoke is always a rich mixture. Choke your lawn mower too much you get black smoke. Same thing.

[Okedokey]

Oxygen heater going? they tend to die slowly. If it is not heating up 'fast enough' it may cause fuel air mix issues I assume?