My beloved XJR with 157k miles on the clock has just failed its MoT on emissions. Specifically, it failed the fast idle tests by virtue of producing too much CO.
The car is running like a charm, and is used pretty much daily, and not for short runs. It has a new air filter, and has had a can of magic potion run through the tank in an unsuccessful attempt to resolve the issue.
Indicated average fuel consumption oner the last 2000 miles is 20.2 mpg, which I had reckoned to be slightly worse than my "normal" 20.5/ 21.0 mpg. At the time I put the difference down to different road conditions, but now think this could be a clue.
I connected my scanner to the car, and there are no codes stored or pending.
On looking at the live data, I can see that the fuel system goes quickly to closed loop after startup, and that the short term fuel trim figures fluctuate very similarly, and always settle to virtually identical readings. Similarly, both O2 readings fluctuate in parallel.
The readings which appear odd relate to long term fuel trim, which are both 0 and do not move, and the MAF which shows 0 lbs/s continuously.
Having never had any problems with the car in this area, I have never scanned this data before, and therefore dont know whether my scanner cannot read these values, or they are indeed 0. It is one of the better Autel scanners, so I am currently working on the basis that the readings are correct at 0.
My suspicions have alighted on the MAF, and I have tried disconnecting it with the engine running, which produces no effect whatsoever.
Can anyone help with thoughts on whether the MAF could be the sole culprit, and whether there is anything else I can do to corroborate this diagnosis?

 

As you might remember, mine used to stall. I changed the O2 sensors and the maf and added some bg44k and I've now driven 5000 km without any more problems. I still have some high cO2 readings though but I think it could be related to the crack in my manifold. Have you checked yours?

 

Check the voltage readings at the MAF connector (values are in the Electrical Guides) to confirm it is getting power and is providing (or not) a signal back to the ECU.

 

At 157K miles, the front cats and O2 sensors might have lost their efficiency and may be supplying inaccurate data to the MAF and ECU.

 

The O2 sensors can be removed and soaked in gasoline overnight and may refresh them by removing any deposits on them inside the vent slots . It removed alot on mine . O2 sensor codes are fickle in if they even show a code at all . Mine where cross threaded so be mindful of that . The Green /Pink wire on the MAF should show 1.2 volts at idle . You can take the front cats off ( not fun ) and soak them in oxylic and citric acid to remove the deposits on them . See the similar MAF readings in this example . Notice the numerical values in the table

https://www.jaguarforums.com/forum/x...D184729/page2/

 

Many thanks guys.
I replaced an O2 sensor about 20k miles ago, and am taking comfort from the fact that both O2 readings fluctuate almost in unison. They surely cant both have failed, yet be giving virtually identical readings throughout the range?
My cats were renewed about 40k miles ago, and I am discounting them for now at least.
My exhaust manifolds were replaced about 30k miles ago, and I checked them recently.
That said, there may be a blow from around the downpipe joint, but the MoT didnt pick anything up, so if there is, it is marginal.
Since my initial post, I have checked that there is power at the MAF, and there is, as well as ground. I have yet to check the output from the MAF, and the continuity of the wire from the MAF to the ECU, but that would seem the next logical step.
It is my understanding that in the absence of any input fron the MAF, the ECU uses some default values. Does anyone know whether these are likely to make the car run rich?

 

Engine coolant temp sensor will make it go rich . You can remove it and dunk it into hot water and watch it respond . Mine was coded but gone now with cleaning up the connector with the original sensor . The MAF sensor is tapped into the EVAP valve and you can remove the connector to see any changes . Located behind left headlights item 7 .

 

My XJR runs rich too with high idle as well, despite new injectors, new O2 sensors, new ECU, spark plugs, air filter, fuel pumps, clean-up fuel rails, etc. There aren't any codes but forum members mentioned the need for some sort of calibration with the right tool (VCM and IDS) so the car re-learns the new O2 sensors and new ECU. I have yet to do this pending more work needed to address noises from the supercharger, but calibration may be your answer.

 

Cracked manifolds and bent flairs on the downpipe to manifold donut gasket seat can cause fresh air to be sucked in between exhaust pulses throwing off the O2 sensors . I have both . No codes . O2 sensors are fickle anyway to show codes . To see the flair is a hard job but can be done yourself with some tips . Are the connectors to the O2 sensors hooked up correctly . Easy to miss-connect them . Look for corrosion on them . My idle was running high but is corrected now . There is a Jaguar TSB on a weak throttle return spring but can be accomplished without the new spring by reclocking the the spring on the throttle body to a tighter preload position to bring the throttle butterfly back to idle closed position and the Idle Air Control Valve does it's thing . The connector points up next to the butterfly . Clean the IAC valve poppet for it may be stuck wide open with oil contaminated crankcase breather gasses . Jaguar TSB puts a air / oil seperator into the system to prevent problems . The N/A and the Supercharged throttle body may be different so it may not apply to all . You can also lube the traction control cable if equipped as well as the primary throttle cable , mine moves better now . You can also adjust the throttle cable reach / rig at the bracket before it gets to the butterfly , just don't tighten the stop nuts to tight , mine where way 7 turns loose and out of adjustment .

 

In the absence of a MAF signal the ECU uses Throttle position, Engine Speed and IAT to determine fueling.

Don't know whether that would make it rich or not - but it's probably a moot point. If the MAF isn't working and your emissions are out then the MAF is the first thing to sort out.

 

Agree with the need to sort out the MAF first.
Tested the output on the pink wire. At idle, 0.3v, rises to 0.5 when revved. I did wonder whether the range on my voltmeter was correct, but reads 14v on the supply, which would be correct with the engine running, so I am assuming the readings are accurate.
Next assumption is that my MAF is dying/dead and replacement required?

 

I can't double check on mine for the exhaust is off now . Print says 1.2 volts . Have you seen the reading with the EVAP connector off that may be sucking the signal down ? At least it is responding but may be out of range . The thermistor in the MAF will fail completely and not partially .

 

Thanks Lady P and everyone. I have ordered a new MAF ( which I am delighted to see is available) and will see what happens when it is fitted. Details to follow.

 

There should be a source on the thermister inside as a DIY without buying the whole unit . Just have to drill out the pop rivets and a common screw to go back together . I realize there is a supporting circuit board . There was a class action lawsuit against Volkswagen about this . I have a way to calibrate a thermister ( plus a ELM327 ) as I get around to it to see if one from a source works . I'm a really cheap guy . My MAF reads in the past . Lucas 20AM series = ( ? ) LNA 16200AA supercharged , LNE 1620AA ( Lucas 70390B ) N/A

 

Cracked exhaust manifolds will make the exhaust seem leaner to the O2 sensors than it really is, so they will make the mixture richer. I'd start by examining the manifolds closely.

 

I have both a cracked manifold and a bent flange at the donut gasket that blows dark gasses through the gasket with a short term fuel trim of + 24.2 above baseline 0.0 . Off car now .

 

There is definitely a blow of some sort which is audible when cold. I will have a poke around tomorrow. Thanks guys.

 

The reinstallation of the downpipe is a real pain , I'll get back later with some tips as I did mine earlier but it's back off now .

 

Update:
Had a poke around the manifold and downpipe area.
Manifolds intact, but flange between front manifold and downpipe loose. Now tightened, and hopeful that this and the new MAF will have solved the problem. Will report back when I know for sure, but thanks again for the help.

 

Eureka!
Passed emissions test with flying colours!
Having fixed the exhaust leak upstream of the O2 sensors, my scanner showed the short term fuel trim had gone from enriching the mixture at idle to leaning it, so I was fairly confident it was fixed.
MAF was a complete red herring, and simply couldnt be read by my scanner. A point worth noting is that the XJR MAF wiring is different from the normally aspirated cars. Ask me how I know this.......
Many thanks for all the help guys