Why are my radiator fans on after I've shut off my car?

Ever hear your car make a little extra noise at idle? Or have you ever noticed a slight RPM increase and wondered what was happening? Your car is probably doing a Diesel Particulate Filter Regeneration, or DPF regen for short. This is a process that goes on “behind the scenes”, so to speak, with no warning light or indication unless the filter is very clogged, and you might be interested to find out what’s happening.

Diesel Particulate Filters and Clean Air

All 2009+ Audi and VW TDIs are equipped with a DPF. Its primary function is to filter soot particles from the exhaust gas. It aids in reducing emissions, and VW, like all other companies, has been subject to the requirements put forth by the Clean Air Act of 1990.

   Essentially, VW had to lower the emissions outputs for their TDIs in order to accord with the act’s second tier of restrictions that were phased in starting in 2004. Before that, TDIs could still pass emissions tests, though only barely. They were ranked in “Bin 10” for emissions, the worst ranking for a light-duty vehicle. The DPF was part of a new exhaust system designed to lower emissions of soot, nitrous oxide, and carbon monoxide. This exhaust system required ULSD, or Ultra Low Sulfure Diesel, in order to function. The Clean Air Act also mandated that all diesel be replace with ULSD by 2010.

How the DPF Fits in to the Exhaust System

Exhaust gas from the turbocharger follows this pathway:

1)   It enters the Oxidation Catalytic Converter, where carbon monoxide is turned into carbon dioxide and water;

2)    flows through the DPF, which traps all the particulates;

3)    enters the Nitrous Oxide Catalytic Converter, which converts the nitrous oxide into nitrogen and water;

4)    flows through a third and final catalytic converter, the H2S cat, which breaks down hydrogen sulfide;

5)   and flows out into the air through the muffler.

Active vs. Passive Regeneration

There are two different types of regeneration: active and passive. Passive regeneration occurs during driving that produces high exhaust gas temperature, around 350 or 500 degrees Celcius. Exhaust gas will normally get this hot if you’re driving on the highway or accelerating intensely. The gas will be hot enough to burn the particulates in the filter and turn them into ash.

  Active regeneration is more complicated. If your exhaust temperature hasn’t reached the required 350 or 500 degrees, soot particulates start to build up in the filter. When the filter is loaded to around 45%, or about every 500 miles, the ECM will tell the car to begin the active regen process.

The process goes as follows:

1) The car’s computer senses a certain amount of backpressure, from the gas struggling to get through the loaded filter

2) It shuts off Exhaust gas recirculation

3) Crucially, it squirts a little fuel into the cylinder after combustion (during the exhaust stroke)

4) This fuel then ignites inside the Oxidation Catalytic Converter, and raises the exhaust gas temperature way up to around 600 degrees Celcius.

5) The hot exhaust gas runs through the DPF and burns the particulates into ash to clear the filter.

Additionally, your boost pressure will increase to make up for lost power from the lack of EGR, and your RPMs will increase by around 200. This process usually lasts around 10 minutes.

Is it harmful to interrupt the regen?

Not really, but you should eventually make sure it gets done. Drivers who drive many short trips should be aware of this—their DPF’s will become clogged faster (because the exhaust gas never gets hot enough to do a passive regen). Not only that, but they might be interrupting the regen without ever letting it complete, which could eventually cause problems. If the regen is interrupted by the car being shut of, the radiator fans will continue to run. When you start the car again, after the car goes above 38 mph or the exhaust warms sufficiently, the regen will resume.

When your DPF Starts to get Clogged

If for some reason your car hasn’t done a regen, and the DPF is loaded at around 55%, then the car will try to do a 15 minute regen. If THAT is interrupted, a warning light will come on:

DPF Warning Light

At this point, take your car for a highway drive to assist the regen. It’s important at this point to make sure you clean that soot out of your DPF!

Now, if for whatever reason your DPF starts to get extremely full, say around 75% loading, a glow plug light will come on. If that happens, the DPF needs to be either manually activated in VCDS, a diagnostic tool, or it needs to be cleaned by hand or replaced. After this point, your car won’t automatically do any regens, and if you leave the DPF dirty and neglect the problem, your car will start to produce less power and could eventually be in some serious mechanical trouble. Your best bet at that point is to take it to the dealer or a trusted mechanic.