Ecotune Decat Downpipe - Exhaust Fumes in Cabin

Have you started noticing exhaust fumes in the cabin and engine bay of your 3 series E46 diesel?

It's likely you have the same common fault I had with my E46 330d, a split flexi-pipe section on your E46's primary catalyst / down pipe. There are a couple of other possible faults that could cause these symptoms though.

Keep reading to find discover all the likely causes of exhaust fumes in the cabin, my thoughts on the Ecotune decat down pipe and whether my car passed it's MOT with one fitted.


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Symptoms - Exhaust Fumes In The Cabin And Engine Bay

Since I bought the car, I had noticed a faint smell of exhaust fumes when the heater blowers were on and the car was stationary, waiting at the lights etc. It gradually got worse until a couple of weeks ago I decided it had to be put right.

After going under the bonnet to replace a missing fixing I noticed instantly from start up that fumes appeared in the engine bay, originating from under the air filter intake ducting / near the front right suspension top mount.

If you squint and don't blink you can see the plumes of smoke in my video below. Please excuse the video quality...


If you missed it in the video, I've highlighted exactly where the fumes were coming from below.


When the heater fans were on in the cabin, diesel could be smelt almost instantly. However, when driving along no fumes could be smelt even with the fans on, or so little fumes were getting in that it couldn't be distinguished. When sat at traffic lights or a junction, fumes once again started entering the cabin. Neither distance travelled nor engine temperature seemed to make any difference to the problem.

As the car is my daily commuter, I needed to find some kind of temporary fix whilst I researched a more permanent one.

Temporary Fix

I found an extremely crude fix, which saves the unpleasant smell and headaches. Press the recirculate button until the green light is on to the top right of the button, as shown below.


It's not an ideal scenario as you have to remember to switch just before you get to the lights or junction to avoid circulating the diesel fumes. Also, you can't just leave it on this setting either, as the windows start to steam up, or at least they do in my car.

Potential Causes of Exhaust Fumes In The Cabin And Engine Bay

I did some searching around on the net and built up the following list as a conclusion.

• Most likely - Flexi-pipe on the exhaust primary catalyst has split - lower section of part 1 on this exhaust system diagram
• Split or cracked exhaust manifold - part 1 on this exhaust system diagram (bad news, expensive)
• Badly sealing exhaust primary cat
• EGR pipe split - part 1 on this diagram
• Inlet manifold split (some doubt whether this would cause these symptoms, also expensive)

Armed with the possible faults I started inspecting my car. After getting underneath and removing the under tray it was fairly obvious that my flexi-pipe section on the primary cat downpipe was indeed blowing, causing a considerable exhaust gas leak. The vast amounts of soot made it quite evident, as you can see.

Permanent Fix

I looked at a few replacement primary catalyst options for my 330d;

• £126.30 - Pattern copy from onlineautomotive.co.uk
• £157.49 - Ecotune stainless steel decat pipe (price check 06/09/2017 - now £125)
• £800.00 - Genuine BMW replacement part number: 18307788607 (for manual cars only)

Weighing up the positives and negatives I ended up plumping for the Ecotune decat downpipe. It was a darn sight cheaper than the £800 BMW quoted me and, being stainless steel, came with a lifetime guarantee and a 2 year guarantee on the flexi-pipe section, double the guarantee of the pattern copy.






Removal of the original BMW M57 downpipe turned out to be a real pain in the backside, there just isn't enough room for manoeuvre in and around the turbo. The nuts holding the downpipe to the turbo require a 15mm offset spanner to undo, and initially you can only get the smallest of turns due to hand and tool access limitations.




In the end I had to drop the whole exhaust assembly off to fit the new decat downpipe. It was a bit of a faff getting it back on as well. At least I didn't have a lambda sensor to contend with, thankfully all pre 2003 330d models weren't fitted with one. The sensor could have made things very interesting... Here's a shot of the old primary cat downpipe in situ with no sensor, if it had one it would be sticking out somewhere in the middle of this shot.


Anyway, after 5 hours under the car it was all back together and running sweetly. No fumes could be detected whilst stationary with the fans on.

Another plus point of the decat downpipe is that it's freer flowing, which equals a slight performance increase and most notably, more turbo whistle when spooling up - which for me was a massive plus.

EDIT - MOT Pass or Fail?

A lot of people have asked if the car has passed it's MOT with the Ecotune decat pipe fitted. I can confirm I've passed 3 MOT's since fitting it, including MOTs after the new regulations came in (Jan 2012) that stated a car must have the same amount of catalytic converters as it left the factory with. The new regulations in 2012 also reduced the smoke limit by 50% from 3.0 to 1.5.

Here are some quotes from the technical director Stan Ward at Ecotune:

"There is currently no way of testing for the presence of a catalytic converter on a diesel through exhaust emissions"

"A cat installed e46 330d will on average score 0.7 with the cat installed and 0.5 with the decat."

So there you have it, why wouldn't you fit one?

Let me know what you think in the comments below, have you fitted a decat downpipe to your E46? Any tricks or tips for removing the original exhaust?