I just bought an XK8 that failed its MOT on, amongst other things, it's emissions. The (trusted) garage doing the testing told the seller they thought it needed two new cats. I agreed to buy it as a project as I'm an enthusiastic amateur mechanic/diagnostician and like a challenge . Having now had a chance to look through the emissions results a few days before it arrives I disagree with the garage.

• I do not have the car yet so I have no codes and no fuel trims ONLY the emissions results.
• The seller (trusted) says the car runs smoothly

Image of emissions:

I'll post my theory shortly but what would you say and why?

 

Wight,

Nice to see you sharing this little preemptive problem. I can see why you posted your previous question regarding the cross over silencer/muffler.

Firstly the car is running rich/overfuelling. There’s the usual probable culprits ie MAF, injectors etc. But....

Next question is why one side of the exhaust is so much different from the other. Hence your cross over question! hmmmm.....

Firstly, have any exhaust mods been done ie fitting straight through pipes is common to increase car noise? Next the O2 sensors are on the CATs before the central balancing muffler so one bank is probably telling the car that side is lean so hence the over fuelling.

Once you have the car, the DTCs and fuel trims will isolate the problem. As you say, it may not be bad CATs.

 

My take on it : the RH bank CO is 6.5%, Lambda is 0.8 meaning that the bank is running really, really rich - a cat just does not work with the Air/Fuel Ratio that far out- no wonder the HC were twice what they should be.

As to a clue as to what the cause is, well first off the car (apparently) sounds lovely and runs smooth - so that likely rules out a misfire (failed spark/injector/compression) that would trick the O2 sensor into dumping more fuel in (- hey look lots of left over oxygen - must need more fuel!)
LH bank is also slightly rich but nowhere near as much so that rules out things that would affect both banks such as a blocked air filter, coolant temp/air temp sensor (brrr... it's cold lets add more fuel for cold running), the MAF sensor, fuel pressure regulator, EVAP jammed open...

Not having the car I can't see the Fuel trims or read codes but my best guess at the moment is that I will see elevated +25% short and long term fuel trims on the RH bank, as for cause I think most likely a faulty O2 sensor . If the fuel trims are negative then in my mind that would point to the VVT system on that bank.

LH bank is running slightly rich (Lambda 0.965) , this is reflected in the CO content at 1.14% but the HC is a pass.
Two thoughts on this one, either there is a separate problem partly affecting both banks (blocked air filter for example) so I would expect to see negative fuel trims on the LH bank as the ECU tries to cope with it. Or as there is an exhaust crossover the other bank is contaminating it and throwing off the reading on the analyser in which case it is fine and Fuel trims should be nominal.

 

Wight8,

Yep, that’s a possible theory. Well thought out. Let’s see what the DTCs say when you get the car. Replacing the O2 sensors will be a cheap fix, but I would also do a full oil/air filter service. Check the breather pipes too as you might see more contamination than usual from excessive blow by of fuel.

Trouble about over fuelling long term is potential damage to the CATs. As these are expensive items maybe worth a few DIY checks there as well.

 

Picked up the cat this morning.


So first thing knocked off the list - window drop on opening, was all over the place. Easy reset procedure done.
No ECU codes stored, a bunch of other random stuff from the BCM that I suspect are generated by a low battery.

Fuel trims at idle.
Bank 1 RH/OS)
LTFT = 15%, STFT = 19.6%

Bank 2 LH/NS
LTFT = -3.9%, STFT = +3.9%

So so far that supports the theory,. On the iCarSoft I have lots of PIDS but can not see an O2 voltage. However I have to say that I think I am feeling a misfire - but that of course might be just because it is running so rich!
Will try the scan in generic mode and also my cheapo bluetooth+torque. Otherwise off to the oscilloscope.
It will also be a chance to try out a home made exhaust pulse sensor.

 

Exhaust pulse sensor + scope confirmed 8 cylinders firing equally.
Found O2 PID (under H for Heated O2 sensor .. doh!) confirmed Bank 1 dead at 0.1v. Replaced and all fuel trims are nominal. Will get a gas analyser on it to check cat function but so far so good.

 

Well done....