[986s]

Hi Folks,

Had a check engine light appear- reduced power- in handbook mentioned this was related to emissions. Took the car to my local garage who replaced the spark plugs and a coil pack. They have since came back to say that the front catalytic converters is gone due to too much fuel reaching it (in fairness there was a strong fuel smell from the exhaust). They have ordered a front catalytic converters and will check o2 sensors once this is fitted. My question is what are my options as this is working out expensive? Can I run the car without the front catalytic converter-I see people referring to removing the primary cats and leaving the secondary cats in place?
My plan is to replace both turbo's shortly with some upgraded ones - the option people mention is catless downpipes once the turbo's are out so I guess am I wasting my money now replacing the front cat converter when I plan to do this? Anything else maintenance wise I should do if upgrading turbo's (don't want massive power gains- just stop the wastegate rattling)- ? Walnut blast, ? Upgraded PCV valve.
Any advice would be greatly appreciated

[wallsmk2]

Did they read the codes? If they did they would know if it's the spark plugs, cats, O2 sensors etc. It seems like you're paying for their lack of equipment. Take it to a garage that can read the codes.

[986s]

Yes they read the codes- can't remember them exactly- codes were related to misfires on a number of the cylinders. They seem capable enough just don't know if I'm wasting money now replacing the front cat converter if I'll be going catless dp's when I upgrade the turbo's. Guess I don't know enough about the exhaust system to know what's what.

[therealdb1]

Unless you get the over-fuelling sorted first you are wasting your money because the new cats will go the same way as the old ones.

Why are you getting the misfires and exhaust that smells of unburnt fuel?

[986s]

My initial thoughts were the coil packs and injectors- they replaced the spark plugs (no oil around them as I thought it could be a leaking valve cover gasket). One coil pack was bad- they replaced that, they reckon that the injectors are ok. They are still getting bad lambda readings so they recommend replacing the front cat converter first and then checking o2 sensors. Any other things that could cause over fueling that I'm missing?
Many thanks for the advice

[wallsmk2]

I'm not an expert but a failed catalytic converter can be diagnosed without clutching at straws. And if there's any doubt it may not be the cats and may be the O2 sensor, replace the cheapest first. It still sounds like they aren't diagnosing the problem if they are still considering the coils.

[therealdb1]

I'm starting to agree with the reply that suggested the garage does not know what it is doing and they are randomly changing bits at your expense.

A cat whether it is working or not will not give you a bad lambda value. Their only job is to reduce the carbon monoxide and hydrocarbon emissions from the tailpipe.

Lambda values result from unburnt oxygen in the exhaust gas and is measured by the O2 sensors. Since these need to be upstream of tha cats it cannot be the cats' fault that lambda is not stoichiometric.

Most likely culprit is the O2 sensors causing overfuelling.

Of course it could be leaky injectors, how were these tested if they are being given a clean bill of health?

It could also be an ignition problem creating the misfire and therefore unburnt fuel.

Do not replace the cats until you source the lambda issue.

I would at this point go somewhere else who has access to something like INPA diagnostics where you can see what the sensors are doing in real time and you will probably find that the fault is obvious.

[986s]

Many thanks for the replies,

Beginning to make sense of it now- will go back to garage with above suggestions. Question is can I run the car without the front cat converter? A faulty o2 sensor could be the culprit for the over-fueling and if that is replaced can I run the car until I get to a different garage (with a bit more experience of dealing with 335i's although it's a bit of a trip to it). I hear of running the car without the primary cats just leaving the secondary cats in place to pass an MOT- is this applicable to the above situation? Apologies for my lack of knowledge on this- I have a rough grasping of it but obviously nowhere 100%.

Many thanks again

[therealdb1]

In theory you could run the car with no cats whatsoever!

As far as the car is concerned they are irrelevant and have only become necessary because some tree-hugging law maker has made vehicle emissions such a prominent topic.

Our issue is that as each generation of engine management comes along and improves fuelling and ignition a suit in an office that cycles to work tightens up the emissions tolerance so we still have to bolt a cat into the exhaust.

It is likely that once you get your current problem resolved the cat system under the car will be enough for your emissions to pass an MOT and you will not need the downpipes to be 'catted'.

I suspect that the reason that multiple cats are fitted is more to do with packaging the things under the car whilst not making the exhaust too restrictive or noisy rather than needing more than one to reduce CO and HC emissions.

You do have to consider that BMW sell these cars worldwide and some countries, especially USA which is a huge market for Beemers have even tighter emissions rules than we do. In fact I read somewhere that the origins of OBDII that we all know and love so that we can plug our mini computers in to grab fault codes and reprogramme angel eyes is soley down to the fact that Florida wanted zero emissions vehicles years before it was trendy so the manufacturers needed a way of communicating with the vehicle electronics.

[therealdb1]

How did the garage respond with your newfound knowledge?