Hi Guys,

Been reading these forums for nearly 10 years but this is my first post.

I have a W204 - C200K (M271 engine) with 215000km

I recently could hear an exhaust leak from my first catalytic converter (Cat). I also noticed a gradual decline in performance over the last year were it felt like the car was being 'held back' especially when taking off.

I then deduced that my Cats were probably blocked and since one was leaking I figured it was time to do my first mod and de-cat. So I de-catted yesterday and put in a 'mini cat' which basically fools the ECU so that the CEL does not turn on. The mini cat works like the normal cat but on a very very very small scale just at the post-cat O2 sensor

The car now feels 'free' and more responsive. No CEL on and everything seems fine. I decided to plug in my OBD2 scanner to confirm all's well and I found that my AFR commanded has now changed to 17.4. This used to always be 14.7 (stoich). My O2 sensors are all connected and apparently functioning the same as before the cat delete.

I am worried that this commanded value is too lean. My exhaust cat temperatures don't seem to have changed but my engine temperature was at 94 degrees Celsius this morning despite it being winter. My engine temperature is normally 81 degrees celsius during summer. This temperature increase must definitely be due to the lean mix.

What could have caused the AFR Commanded to change? As I understand, the cat delete should not affect the AFR since the O2 sensor that adjusts these values is before the cat. The post Cat O2 sensor is normally steady and is there to check if the cat is functional.

Hope you guys can point me in the right direction.

Kind Regards

Update:
So my engine temperature is actually fine. I didn't realize that I was at standstill with the car running for 45 minutes so this is expected. When moving, the car is a healthy 70-81 degrees C. There is no detonation/pre-ignition/knocking/pinging or whatever you call it.

I did however graph the precat O2 sensor and found that it is linear on 2volts as opposed to oscillating between 1v and 4v. This may be the reason my ECU is commanding at 17.4. I will check it this weekend with an old used sensor to confirm.

Final Update (Resolved):
So I'm a bit embarrassed....and glad I didn't fork out 6000ZAR for a new oxygen sensor. The ratio of 17.4 seemed vaguely familiar to me from back in my days at university and that's when it struck me...my vehicle profile on my scan tool was mistakenly set for Natural Gas fuel and not petrol. Natural gas stoich is 17.4. Once I changed the fuel type back to Petrol, my AFR was then calculated correctly as 14.7.

This is a lesson learnt regarding reading O2 sensors. The precat is a wideband sensor so its operation is significantly different to the post cat narrowband sensor hence the troubleshooting rules are different. Narrowbands use voltage to measure while Widebands use current which cheap scan tools don't show. However, wideband sensors do show a measured lambda reading. I will never pay attention to calculated AFR again which is based on your fuel input. Reading measured and commanded Lambda values would have pointed this silly mistake out a week ago and saved me a lot of worry:-)

 

Incorrect values are being read from the incorrect cat, you're running lean in closed loop mode. Not good for the engine, revert to the stock setup ASAP

 

Can we clarify this mini cat thing first? Did you put in a high flow cat or just something to fool the electronics? If it's the latter then I would think that's the reason you're getting lean a/f readings. I'm also decatted but my a/f is 14.7 at idle.

 

Do you get much soot, noise and smell from this setup?

 

Agreed on the aftermarket cats. They're nowhere near the quality (or performance) of the OE cats, which, by design, are extremely high performance and also very hard to 'kill'. OE converters are engineered to last the life of the car, and will, unless damaged by outside influence, like bad engine mods, or defective parts in the engine/computer systems.

 

I disagree, there are good and bad cat manufacturers out there. Mil.spec for one is a quality high flow cat manufacturer. Also, OEM cats are generally 400 cell, I hardly call that high performance, it's emissions friendly.

 

Although I'm not sure of the C Class cars, the Germans are leading the way with converter technology. They're making up to 1200 cell cats, which are not only extremely high-flow, but are incredibly efficient.
In general, 400- and up cell cats are what you want for any car that needs to pass emissions. Many high flow cats out there are 200-cell, which are okay for looks, but will most times trigger the MIL (check engine light). They're simply TOO high flow, and don't do their job of burning off excess hydrocarbons (not enough surface area for the catalyst). Often, too, they're not built to a very high standard- not just to be cheap, but the aftermarket parts aren't required to meet the same warranty standards OE units are.
Knightmare, there is (in the US, anyway) no such thing as a 'low flow' cat. They haven't been manufactured for well over twenty years, now. Having spent most of the past fifteen years in the performance exhaust industry (US manufacturer of mufflers, headers and cats), I'm pretty up-to-date on this stuff, and just shake my head over much of what I read on the internet. Most people have opinions, which is fine, but I prefer to look at facts. Anyone reading can take what you want from this; I'm comfortable with my information.

 

I'm comfortable also with the information you've provided you've really been in that particular industry as long as you say you have. Nothing wrong with learning new things. My post is also based off experience but at the same time I can only speak from experience as a consumer. As a consumer, I've come across both good and badly made parts. In regards to cat converters, most trusted brand names are running "high flow" 200 cell cats in place of standard 400 cell because it's a good balance between oem needs and performance needs. I haven't seen any of the cats you've mentioned but if I had known I might have chosen a different route. Unfortunately I didn't and having gone from the oem 400 to de-catted, I can say I gained power but I also understand that the power gained isn't in the areas I really need, specifically my low end which is why I'll be installing a 200 cell unit soon.

 

The OEM cats are 600 cells each, apparently

 

Hi Knightmare, The "mini-cat" is a very small "adapter" that has the same catalyst set up inside it.

Normal set up is your post cat sensor is either screwed into or just after the cat. My set up has the post cat sensor screwed into the "mini cat" which is then screwed into the straight pipe where the cat used to be.

The mini cat has the same catalyst in it as the original cat but is not in the way of the gas flow. The reduction reaction now takes place only for the gas just before the post cat O2 sensor.

My Post Cat O2 sensor is reading the same values as prior to decatting so the mini cat is working fine.

I just realized that my Precat O2 sensor seems to be stuck at 2v. This is the cause of my AFR being commanded at 17.4 despite any throttling. I am going to test this theory this weekend.

 

A ridiculous decision would be to put in OEM cats that are quoted at just under half the current market value of the car.

 

I disagree completely. The Catalyst has a half-life span that would out live the car but the design of the catalyst bed is similar to that of filter mesh...meshes get clogged. My car have been stock until 4 days ago. Every service has had only Mercedes parts and fluids.

Things get old and die...fact of life.

 

The car actually looks, feels, and sounds normal. just a little more responsive and lighter on fuel.

I put in an additional silencer box where the second stage cat used to be to keep the noise down. There's no soot...but this mod should not result in or stop soot.

There is a difference in the smell of the exhaust since its chemistry has now changed. It's exhaust smells like older cars without Cats

 

Doesn't sound right to me. I'm certain the Cat is outside the closed loop. If the ECU was trying to continuously tune according to the post cat sensor reading, the tuning will be relatively fixed since post cat O2 voltage is expected to stay near constant. Constant post cat O2 voltage is expected for a working cat. If it isn't near constant over a specific period it trips the CEL.

This is also why you cannot use an exhaust sniffer to tune cars with cats on them. Tuning a car with a cat installed will need to be done with the sniffer positioned in the exhaust before the cat or with a pre-cat O2 sensor.

 

Fixed...My lambda reading was 1 at idle which made me figure out that my vehicle profile had natural gas as the fuel type. This converted Lambda = 1 to AFR = 17.4 which is stoich for natural gas. Once I change the fuel type back to petrol the correct AFR was calculated. Rookie mistake!!!

 

I know this is an old thread, but I noticed you are in South Africa, I was wondering where you found the 'mini-cat' replacement ? Im also in SA, and my cat is on its way out