Hi All. I've searched both Engine & XK8 forums for an answer, but haven't found anything similar to my problem. I have a 1997 naturally aspirated XK8 with an AJ26 engine, at 116,000 miles. The car has failed its roadworthiness test (MOT) solely on high CO (0.9 for a maximum here of 0.3). A small local Jag indi has run a diagnostic and tell's me that bank A is running rich and has probably burned the cat. The possible causes, he says, are a faulty VVT or slack chains causing the cam to be slightly out; or low compression due to Nicasil wear. The O2 sensors are, apparently, OK.The indi is recommending a replacement engine, which puts the car beyond economical repair. So, your thoughts would be most welcome.
Some history:
My car was a COVID lock-down project, which I put back o the road late 2024.
The car failed its MOT in 2004 for the same reason. I checked for air leaks, cleaned the MAF and replaced the plugs, which were very badly worn - no improvement. New UK manufactured aftermarket cats were fitted, which allowed the car to just pass the emissions test.
Twelve months, and six hundred miles later, the car again failed the MOT on emissions, due to high and variable CO. I rechecked for leaks and changed both the part load and full load breathers, which were split, and gave the car an Italian tune-up before the retest. This gave some improvement, but we couldn't get below 0.9.
Off to the small local indie, who gave me the good news. However, a well known supplier of used engines here in the UK has never come across a problem like this, and suspects that the aftermarket cats are the culprit. This doesn't explain though, why I needed new cats in thge first place.
I'm a competent home mechanic, but don't have access to diagnostic gear that will read this car. What are your thoughts please?

 

I should add that the engine has no misfires, runs smoothly, and is showing no fault codes (important information)

 

Get a OBD dongle for your car and the Torque app - you can then start looking at live data such as the fuel trims, what the car thinks the O2 sensors are reporting and so on.

Have the timing chain tensioners ever been changed? Did your mechanic do a compression test before diagnosing a new engine is required?

 

I have an OBD dongle and app - haven't tried these though because i expected my iCarSoft 960 to be better. The top tensioners have been changed, but not the bottom ones, and there is some chain slap from the bottom chain when cold.
No, we haven't done a compression test, but I can do perhaps later in the week.

 

Were the O2 sensors replaced when the cats were replaced?

 

The O2 sensors were not changed, but the diagnostic showed that they were OK.

 

Well have a look at the live data to see what is going on . Your iCarSoft should read everything you need, so I'm not sure why you say you haven't got the right equipment? Is it no good? You need to be looking at the fuel trims and lambda values from your O2 sensors.

It does sound like you maybe the chains/tensioners are now worn to the point of needing to be replaced. Definitely worth checking before scrapping the car.

 

I bought the iCarsoft second hand and it doesn't seem to read live data, only fault codes, so I may have bought a dud. I'm planning to whip the cam covers off tomorrow to check that the cams are timed correctly, so I can look at the chains and tensioners at the same time.

 

Definitely use OBD.

Check you have (pending) P1111. If not, you'll have P1000 and in that case check which of the OBD monitors is not set (aka complete). Each unset one points to problems and also means many codes cannot flag.

Check fuel trims - fully warmed up engine i.e.hot cats - at idle.

Do not clear codes! You'll cause P1000 and quite likely grief till you get P1111.

 

So did mine, but I was getting loads of oddball rich mixture problems and the CAT monitor refused to set. All of which were fixed by a new air filter and new upstream O2 sensors.

 

Thank you both. JagV8, I'm not familiar with the setup of OBD monitors, so what am I looking for and where do I check this please?
I'll see what I can find out and will report back. Otherwise, I'm booked into Swallows Jaguar on Friday for a second opinion on the dioagnostics.

 

You want all the supported ones set, shown by P1111.
https://www.obdautodoctor.com/tutori...ors-explained/

 

If your icarsoft won't do it, then your OBD dongle and app (such as Torque) should give you live data and read all the standard codes - i.e. all the basic stuff you need at this point.

 

How do you know the O2 sensors were ok? If the readings look ok, but the mixture is off, that strongly points to them not being ok. I'm always concerned when a garage suggests a new engine but hasn't actually diagnosed the problem. That should ring alarm bells - and have you looking for another mechanic.

 

@Jagv8, thanks for the link to the tutorial, that was helpful. I had the battery disconnected to prevent it draining, so OBD monitors may not be complete nut, as the car has failed its MOT I can't do a drive cycle. I've reserved tomorrow morning to try my iCarsoft again and to play with my dongle
@bladerunner919 I only have the technician's word tyo go on, but I agree with you. I specifically asked him whether the O2 sensors could be the cause - and he told me they were fine. If Swallows Jaguar tells me otherwise I'll be asking for my money back!

 

I assume the mechanic compared the lambda readings for the O2 sensors with the lambda at the tailpipes to see if they match. I think I would go back and ask the mechanic how he knows.

 

FWIW department: My icarsoft v1 is useless in the OBD-II option, I always use the Diagnostics selection in the top left corner of the display. Then follow the menu with the appropriate choices of year and model being testing . It also helps to let the device use the automatic mode instead of picking the manual selection.

Hope this helps, feel free to PM me if you
have more difficulties with the unit. I’ve got mine pretty well tamed and can have success with most investigations.

Z

 

Battery off causes all monitors to be cleared (unset) so you start from ground zero.

Also clears learned values such as fuel trims.

 

@zray, thank you for your kind offer, I've tried the iCarsoft again this morning and it's giving me no live data, and is picking up no DTCs.
Using my ELM emulator and "Car Scanner" free version, I have:
Bank 1:
STFT 19.53%
LTFT 14.84%
O2 sensor 0v
O2 sensor STFT 19.53%

Bank 2:
STFT 0%
LTFT 0.78%
O2 sensor 0.58v
O2 sensor STFT 0%

The ELM is finding no DTCs.
This confirms what I was told, that Bank 1 is overfueling, but no clues as to why.

 

Do the tailpipes actually equate to one per bank? I know that there is a single centre box, but are there separate tracts inside for the left and right bank?