|
OK, had the cat replaced by Kwik-Fit (no quibbles at all, so top marks to them for making it easy). Car retested and passed the MOT with flying colours, so clearly the old cat had died. I'd noticied the car had been using more fuel, mentioned this to the mechanic. He thinks this consistent with the dead cat - i.e. exhaust emissions were up, O2 sensor detected this and told the ECU to alter fuel/air mixture to compensate, hence the engine running rich and using more fuel. Sounds plausible, but then I'm not an expert. As the car seems fine now, I guess I won't know if I was just unlucky with the last cat, or if there is something wrong that is killing the cat until I get the next MOT in a year's time!
|
Well done for quickfit for honouring the warranty. My dad has a three (I think) warranty on his new back box they fitted for just £50! Now I know for a fact on my dads car they last 18 months max regardless of the car so it will be interesting to see how long it lasts.
I think these fast 'fit' centres are cleaning up their act.
|
|
|
Could be wrong, but on these surely the lambda sensor is in front of the cat? What the system does is ensure the mixture entering the cat is correct so it can work correctly. Cars of this vintage don't tend to have post-cat sensors (in fact very few cars do even now) so I have doubts about this 'explanation'.
JS
|
John,
You are correct that this car would only have one lambda sensor, and yes it would be fitted before the cat. You are therfore also correct that a dead cat, unless it was blocked, would not give a rise in fuel consumption.
It is however very common for later cars to have a post cat sensor as well. This is how the ECU detects that the cat is functioning, and will then be able to flag a fault code if it detects that the cat is not working as it should.
It may be that there is a fault with the car causing the cat to fail, but I would think it it lightly to be a combination of two things.
First, a cheep cat is cheep because the ammount of precious metals it contains is less that an OE cat. It can therefore not convert the gasses as well, and is less lightly to be able to cope with....
Item 2, the way the car is driven. Lots of short trips, on a "rich" mixture, will cause carbon deposits to form on the lambda sensor, and cat. The ECU will then not be able to "see" a signal from the sensor, and put in more fuel, causing more carbon.
If the CAT becomes fouled with carbon, there will not be contact with the catalitic element, and so conversion will not take place.
A long run, say every three months, for even just 1 hr, will get the sensor and cat nice and hot, and help to burn off some of the carbon.
I would recommend that you get the sensor checked, and possibly replaced, after a trip to try and clean it first.
|
|
|
|
There are a number of things that can cause the cat to fail: if the cat was struck by tools for example it can break up the monolith inside and stop it working, similarly if any particles enter the cat they can damage it - this could happen from exhaust putty being used on any joints in front of the cat and hardened pieces breaking off and being drawn through the exhaust. Poisoning will also destroy a cat - things like silicone, oil, antifreeze getting in the system will definitely kill it, leaded fuel would also damage it. Overheating is a common cause - unburnt fuel entering the cat will burn inside it and cause it to overheat destroying the catalyst.
As you said the O2 sensor plays a part in fuel mixture so definitely worth checking it. If the sensor is sluggish you could be getting high fuel consumption. I'd be checking that and replacing if necessary.
There's some good info here about why cats fail: www.catalyticconverter.org/failure/index.htm
The emissions test results would be a good place to start in determining what's happening in the engine. There's a simple table on this page which shows possible problems for various combinations of CO/HC/CO2/O2 readings. www.omitec.com/us/support/tech-tips-gen/gas-analyz.../
Have you got results sheet from the MOT?
|
Here are the results from the 2 MOT certs (first failed on old cat, second passed with a new cat)
With original cat:
Fast Idle Test
CO: 0.75%vol
HC: 97ppm vol
lambda: 1.014
Natural Idle Test:
CO 0.57%vol
With new cat:
Fast Idle Test
CO: 0.20%vol
HC: 36ppm vol
lambda: 1.003
Natural Idle Test:
CO 0.19%vol
|
The two gas readings are what you would expect for a failed cat. To have all four would have been better, but many MOT gas print outs dont show them. I've never been able to figgure out why, as the analyser is measuring all four.
I would still recommend a run, preferably with the higher grade unleaded fuel in the tank, and then checking the lambda sensor. Or, if you know that the sensor has been in the car for the last five years, it may be better to get a price of a direct fit, (NOT universal) sensor, and replace it anyway.
|
|
|
|
|
