Soot from exhaust when revving the 2.0 TDI Audi engine - normal?

**sclamb** · 14-02-2009,11:52 AM

Hi

When I rev the engine while the car is sitting on my drive (by blipping the throttle) I see some black stuff (which I presume is soot) coming out of the exhausts and I wonder if this is normal. If I blip it hard then the amount of black is more than if I blip it more gently - it is like a puff of black smoke. If I press the accelerator gently to build up the revs then there is no black stuff at all.

Last Saturday I drove to Wiltshire and back, so a lot of fast motorway driving of around 200 miles. Then during this last week I have done five days of a mile to the station in the morning and a mile back in the evening. I read that this is bad for the DPF, and so I took the Audi advice and did a 15 minute each way run up the motorway with the gearbox in Sport mode and with the revs at between 2,500 and 3,000 rpm, to regenerate the DPF.

Having got back home I still see some soot when I blip the throttle to rev the engine, the same as described above. Is it normal to see some soot when you do this?

The other thing to mention is that I have a Chipped UK Fast Road remap loaded. If the soot is normal then that is fine. If I shouldn't see anything black when revving the car then I will remove the map and put the original back and speak to Chipped UK.

Thanks for any advice.

Simon

**toff** · 14-02-2009,05:20 PM

Hi,

You should probably await the opinion of someone more clued-up but here's my take:

Even with a DPF, you will always get some black smoke under acceleration, especially heavy acceleration.

Effectively, the soot appears because the engine isn't getting chance to properly burn all the fuel thats being injected into the cylinders.

There is an optimum mixture of air / fuel that needs to be in each cylinder for clean combustion to occur, when you suddenly blip the throttle, it takes the engine a tiny bit of time to adjust that mixture.

To protect the engine (to prevent pinging / knocking) I think the ECU will injects slightly more fuel (than is necessary for perfect combustion) until the engine has reached the speed you are demanding of it. (See the quote from WikiPedia at the bottom of this page).

I have a Motech performance remap and I notice a little more sooty smoke than I used to. I believe this is simply because the ECU is injecting even more fuel than it would at standard, even under light acceleration, hence (as aforementioned) it takes a tiny bit of time for the air / fuel mixture to be corrected and optimum combustion to return!

I hope that gives you some insight and I hope someone else more in the know answers too

Al.

PS: The DPF is a good idea but surely it's counter productive if you have to go on journeys just to clean the DPF! You're using more fuel, producing carbon dioxide and soot particles by making these special journeys!

Of course, I think Audi expect you to do the "special driving" en-route somewhere, not go out for a drive especially to do it. As you say; if you're only doing short journeys all the time, what choice do you have? I'm glad I don't have a DPF. I think the "Add Blue" system is a much better idea.

Originally Posted by WikiPedia

Knocking is unavoidable to a greater or lesser extent in diesel engines, where fuel is injected into highly compressed air towards the end of the compression stroke. There is a short lag between the fuel being injected and combustion starting. By this time there is already a quantity of fuel in the combustion chamber which will ignite first in areas of greater oxygen density prior to the combustion of the complete charge. This sudden increase in pressure and temperature causes the distinctive diesel 'knock' or 'clatter', some of which must be allowed for in the engine design. Careful design of the injector pump, fuel injector, combustion chamber, piston crown and cylinder head can reduce knocking greatly, and modern engines using electronic common rail injection have very low levels of knock. Engines using indirect injection generally have lower levels of knock than direct injection engine, due to the greater dispersal of oxygen in the combustion chamber and lower injection pressures providing a more complete mixing of fuel and air.

Quoted from Wikipedia

**sclamb** · 14-02-2009,05:54 PM

Thanks for the full response, which sounds plausible. I don't see any black smoke during normal driving, even if I boot it in sport mode. Just seems to be if the car is sitting stationary and I blip the throttle, the harder the blip the bigger the initial black cloud. As an example, if I blip to 3,500 rpm quickly and then hold it there I see the black cloud on the initial blip but then it goes and there is nothing more to be seen.

It does sound like what you describe.

**Flakmunky** · 15-02-2009,08:21 AM

I'm assuming you have a B7...

The 140 bhp TDI doesn't have a DPF and will exhibit the production of soot. The 170 bhp TDI does have the DPF and shouldn't produce as much soot. This is the reason why the 140 TDI has the exhaust tail pipes turned down whilst the 170 has straight pipes...

So the question is... Which version do you have?!

**sclamb** · 15-02-2009,11:40 AM

I have the B7 140PS TDI.

I thought this did have the DPF. There was an Audi announcement in 2003 that stated one of the prototypes was the 4 cylinder 2.0 TDI engine (and I don't think there was a 170PS version at that time):

http://www.dieselnet.com/news/2003/09audi.php

Is it definite that no 140PS TDIs have the DPF?

Simon

**gjrwilliams82** · 15-02-2009,03:28 PM

I'm sure there was a thread on this site about a cabriolet owner who was suffering with smoke, or possibly was torn between choosing a TDI or TFSI and the smoke issue arose.

I thought that the upshot of that was that the TDI 140 DID have a DPF and that it needed to be cleared out periodically by a good blast along the motorway.

**toff** · 15-02-2009,05:42 PM

I thought you'd taken it for a drive because the DPF warning light had come on?

(It looks something like this).

Apparently the DPF warning light is quite prolific, so if you haven't seen it since you bought the car, then you more than likely don't have a DPF!

Al.

**sclamb** · 15-02-2009,06:14 PM

No, I took it for a drive because I saw the soot/black cloud when blipping the throttle on the driveway and it was suggested it looked like the DPF was filling up and needed regenerating.

I have never seen the DPF light come on, but then the car is only on 13,000 miles and I have only had it for a month.

Sorry if my original post was misleading.