Problems after CAT replacement: shaking, loss of power and smoke from engine

[eleven79]

Hi guys,
my name is Ellie, I am new to this forum and to Audi, I bought 10 days ago an Audi A6 quattro 2.8L (petrol/lpg) 2000 and having some problems... :(
Initially the problem was a slight noise coming from the left CAT which was slightly worn and decided to replace it for peace of mind.
I did a diagnostic before that showed O2 sensors malfunctioning both bank 1 and 2.
The garage said it was 1 hour work so I waited there and I saw that it was very difficult for them to connect the cat to the engine, due to the position, in the end the mechanic said that they needed to order an additional part and so I left the car there for the day.
I picked up the car yesterday and noticed straight away that the car was shaking at idle, something that didn't do before (in fact the only problem before was the light noise coming from the cat). There was also some smell and smoke coming from the cat but I thought it could be because it was new. The car seemed to have a bit more power than before though.
The mechanic told me to come back today to check the problem and this morning they checked and couldn't find any problem in their opinion and the computer showed NO ERRORS at all.
However after some driving around to test the car today the problem got worse, now the car shakes badly at idle (not in parking mode but when standing at traffic light) and also shaked at higher speed, it miss power in acceleration, it sobs while driving and it is not responsive in acceleration (like it thinks for 5-6 seconds before going) it is actually quite scary to drive!
It also used £25 pounds of fuel just to drive 10 miles, how is that possible??
Also, after trying one last time tonight, noticed when I stopped a white smoke smelling like plastic burning coming from the back side of the engine (probably below).
Now what can this be? Did the garage damage the car, didn't connect properly the cat or something, or the new cat somehow caused an unbalance of other components?
I don't have much money to spend on the car now and it would maybe be not worthy as it has 172k miles on it, so I would like to know what the problem could be and if it is an easy fix or I should give up and accept that it was a bad buy/bad choice of mechanic.
Any idea and can you recommend me to a good garage in West London?
Thank you in advance!

[eleven79]

I have a doubt, can the garage have placed the wrong cat coverter? or maybe a cat without OBD/ without O2 sensors?
It was quite cheap (£170+VAT incl. labor), it was quoted £375 in another garage... :confused:

[mrnice.]

Hmmmm, that does seem quite cheap, I would take it back again and insist they take another look, state all the problems again and show them, especially the smoke, have you paid them yet?

would be interested to know what the smoke is.

[mike ryan]

I would definately take it back and insist they sorted it out. If it smokes then something is wrong. If it is using so much fuel i would say that it is some how pooring fuel down the exhaust and will probably ruin your new cat. If not that they may have damaged a fuel line pressure regulator etc. Are there any leaks from the car? It should be smooth on tickover as i had one and they are a really smooth engine. If there is a mechanical fault then it will not give a code. Just a possibilty but the cat may be blocked/ collapsed from poor fitting or even poor quality manufacturing. You could try getting them to take the exhaust off pre cat and start it up, it will be loud but should highlight if there are issues with the new cat.

[eleven79]

I would definately take it back and insist they sorted it out. If it smokes then something is wrong. If it is using so much fuel i would say that it is some how pooring fuel down the exhaust and will probably ruin your new cat. If not that they may have damaged a fuel line pressure regulator etc. Are there any leaks from the car? It should be smooth on tickover as i had one and they are a really smooth engine. If there is a mechanical fault then it will not give a code. Just a possibilty but the cat may be blocked/ collapsed from poor fitting or even poor quality manufacturing. You could try getting them to take the exhaust off pre cat and start it up, it will be loud but should highlight if there are issues with the new cat.

Hi Mike,
thanks for the advice. I ended up calling the garage again today and spoke with a different guy that said to bring the car back as it could be that the oxygen sensors are not in the right position.
So we did and while driving behind with the other car, I could smell petrol very heavily and could see the smoke coming from the exhaust where the cat was changed was very rich (not black just very gassy). So yeah it looks like you said that the car is pooring fuel down the exhaust, this explains also the fact that the car has now the same mpg than a Ferrari!
What cause this I don't know yet but the garage will need to sort it out...
I didn't notice any leak. I believe in the poor fitting theory as I could see that they couldn't manage to fit it for a couple of hours and the fact that they has to order an additional part eventually makes me believe that either they damaged something while trying to put it on, or they didn't do it correctly.
Anyway car is back to the garage now and let's see what they will tell me tomorrow... fingers crossed!!! xx

[eleven79]

Ok I would like to post an update: picked up the car yesterday, they told me that there was a leak and they fixed it, I reply I don't believe that the problem was caused by a leak but I would try and if there is any problem I will be right back.
Turned on my Audi, no shake, no noise, nothing, I feel happy! :beerchug:
I thanked God and all the good people that blessed me, but God wasn’t looking at me as the very next traffic light the car start shaking again at idle. :aargh4:
I thought “Oh no, should I bring it back?” but the answer came soon because as soon as I drove a bit more I could see that the car was shaking also at 30-35 mph.
I headed towards the motorway to try it at an higher speed, but it was a long queue to get there because of traffic jam at peak time, so I decided to turn back towards the garage.
Wise choice I must say, as while standing in the queue to get back, the car started smoking again and the smoke smelled again like burnt plastic.
Nonetheless to say that my next pray was to make it to the garage before blowing anything, thank God it was just 500 yards away… :1zhelp:
Get to the garage, start an hysterical scene like only women can do :zx11:, nobody could tell me who worked on the car (it is a big garage) and what they did (one said it was a leak, another that there was something with the wiring…).
The first mechanic that put the CAT on Saturday was there but said that he didn’t work on it because he was off yesterday, but today he will take care of my car and probably will replace the oxygen sensors.
I still think that there could be a problem with the sensors, but cannot explain the smoke (maybe some wires too close to an hot pipe?).
Hope to have good news today…

[mrnice.]

It sounds to me like its over fueling, that would explain the bad performance, poor MPG, fuel smell/smoke from the exhaust (unburned fuel enters the exhaust via the cylinder valves and when it comes into contact with hot exhaust it evaporates and turns into white smelly smoke).

My moneys on the bloke who fitted the new exhaust section either damaging or not correctly re fitting a sensor from the exhaust (Im not sure how these cars work yet as Ive only had it a week and still havent managed to get a workshop manual) do you know if any sensors had to be removed from the exhaust, what new part did the mechanic have to order to finish fitting the exhaust?

Some cars can suffer from lack of back pressure when they have an exhaust leak but I think you would have noticed if your exhaust was leaking that much.

Does the car perform badly under hard acceleration then pick up as you ease your foot off?

99% of the time if you have just had something done to your car and it plays up you can be sure it has something to do with the work thats just been done, it wasnt like that before you took it to the garage - ever, right? If you have any trouble politely remind them about that.

Perhaps if a sensor was not fitted back correctly it would show up fine on the diagnostics (as there is no fault with the sensor) but play up with the ECU? perhaps not?

The smoke from under the bonnet definately sounds dodgy and you should show this to the garage at the soonest opportunity.

Hope you get this sorted as quickly and hassle free as possible, perseverance is the key there are plenty of options if the garage starts to get awkward, I can put you in touch with people who can help out in that situation if the need arises but hopefully it wont get to that.

I could be barking up entirely the wrong tree as mentioned earlier Im very new to this vehicle and still learning.

Hope this helps

All the best.

[eleven79]

Hi MrNice,
even tough you are new to this car, you draw a perfect picture of the situation. Are you a mechanic by any chance? Coz next time I will come to you instead! :p
I picked up the car today and I can say with great relief that it is driving perfectly now, as they replaced the oxygen sensor.
I ended up paying for the Bosch lambda sensor (£62+VAT) but not for the labor.
I know that the reason that they had to replace the sensor was probably due to the fact that the mechanic damaged it putting on the CAT, but they would never admit that and I must say that I did a diagnostic before bringing them the car and it already showed O2 sensors bank 1 and 2 malfunctioning, even tough the car didn't show such problems, so I am ok with that as I would have replaced the sensors soon anyway.
I guess that £280 in total for a new cat and new sensor is still ok, considering that they spent at least 5 hours working on the car (of course nobody is going to pay for my headache and petrol spent to go back and forward to the garage tough...).
Hopefully I can rest easy now and start enjoying my Audi!!
Thanks to everybody for your help and support! :3some:

[mrnice.]

Glad you got it sorted, no im not a mechanic by trade im just into my cars and ok with the spanners (I spend some of my spare time playing/learning about cars plus I own a Landy, you have to learn mechanics pretty quick owning one of those! lol) I was going to say LAMBDA but didnt know if bosch done the electrics for the A6 and Audi used the same terms (Same as in my Disco).

On the mechanics defence they can be a pig to remove as heat, rust and time "weld" them to the exhaust so it could have just been one of those things, for them to charge you only for the part and not the labour is fair play on their behalf and as you say a fault was already showing on the diagnostic, maybe the removal and re fitting just made the problem present itself fully.

Hopefully all will be well now and if so you seem to have found a fair, well priced garage (some charge £40 an hour) that sorts out problems if they go wrong.

Still keep an eye out for that smoke under the bonnet though, thats not a normal thing and needs sorting.

Oh and by the way as your new to the A6 make sure your aware of the blocked gutter fault (near the battery-thread on here somewhere) results in soggy footwells and bad electrics, something Ive just learned as a newbie.