My car just failed the UK MOT - (annual roadworthiness test which includes emissions tests.)
The Lambda reading on both "idle tests" is too high. Hydrocarbons are very low which is good, and CO levels also low - maybe too low ??


I'm not getting any DTCs or engine malfunction light.


I have got Jaguar software on my laptop but I'm not very experienced at using it (yet).


Any thoughts from the experts here on where to start looking. I don't want to start throwing money at new parts if it's not necessary. Mileage is 130k, but engine was replaced by Jaguar at 60k miles, but I assume that catalytic convertors, MAF sensor, Lambda sensors are all still original.





All advice as always, gratefully received.
Cheers,
David.

 

Well, I'm no help, but I'd be curious to know what the lambda is ? My guess would be something to do with the oxygen sensors - maybe the swing between high and low ?

 

Yes- it is connected to the Oxygen sensors but a high lambda reading doesn't mean a faulty sensor.
The oxygen (Lambda) sensors tell the ECU what to do to regulate the air/fuel mixture for optimum economy, emissions, performance etc. I am fairly sure that on early UK cars, there's only one O2 sensor on each side, whereas on US cars, there's two sensors on each bank - one before, and one after, the catalytic converter. If the CO2 sensors detect a fault, they trigger the EML (Engine Management Light) on the dashboard.


In the UK test, a probe is inserted into the exhaust pipe, which measures what actually comes out. One implication is therefore that there's a hole in the exhaust pipe somewhere AFTER the CO2 sensor, which is what's causing the problem. However, I'm pretty sure my exhaust is 100% OK.
Or - one (both) of the catalytic convertors has failed - in which case I need to start saving up now.


Just wondered if anyone here with more knowledge and experience than me could point me in the right direction (or what to look for on WDS ?)

 

David,
Out of curiosity, do the two fast idle measurements relate to a reading of the gas 'sniffer' from each exhaust?

Thanks,
Mike

 

Mike.


No - I think it's just that the Technician doing the testing is allowed two attempts at getting a figure which would pass the test. The two banks join together in a central muffler box under the car before splitting again, so I assume (unless there was a leak in one of the rear boxes or pipes) that what comes out of one pipe is the same as what comes out of the other.
A previous Pass test sheet shows only one fast-idle test.

 

For those not familiar with MoT: it's an annual very detailed inspection (brakes, suspension, corrosion, lights, ... and emissions).

Fast Idle test is a quick emissions test at highish revs and at idle. If it passes then that's it, a pass and no more emissions testing.

If it fails, more is done in case the car is marginal e.g. cats are not hot enough and are not efficient enough.

Things that stand out:
HC - way too high for a decent modern car (*)
lambda - also out

So... why?

Things involved: fuel pumps, injectors, O2 sensors, cats, MAF, IAT, plugs, ... (Also, the PCM, but it's not faulty here.)

As a guide OnBoardDiagnostics.com - Engine Basics

Worth cleaning the MAF (cheap and may be to blame).

Probably not plugs unless overdue.

Sadly could be the cats but be reluctant to assume so as they're not cheap.

On some jags there are 2 separate exhaust (tail) pipes and then you can check each. If both bad then you're looking at a cause common to both engine banks. If not, a one bank issue.

I don't know the piping on this model! If the banks join and mix thoroughly that's a nuisance.

You can do some DIY work using OBD live data (elm327 if they work on these as I hope and expect). You can't see HC but fuel trims will be out if HC is (and of course you can in effect see lambda as that is from the trims). LTFTs on this car pretty much must be off and signalling rich.

Trims are from O2 sensors which will be before any exhaust joins.

You might also try some fuel cleaner (in US BG44K but is it available here?) - fairly cheap and easy. Don't overdo it.

With live data, with a cold car look at those sensors you can grasp (IAT, ECT, any others?) for plausibility then watch them as the car warms up to see if they stay plausible. Hot ECT is near boiling. Idle IAT then tends to be well above ambient air if you're parked, due to engine heat (aka engine soak).

(*) I've roughly the same engine, 130K miles, last HC was 3.

 

JagV8 Thanks very much for that. Yes - I can't be sure but there's one central muffler box under the car, so I assume that both banks mix before going backwards in the two separate tail-pipes.
I have got WDS (and OBD) so will look at live data, although I'm not sure I will understand what I see !!
The garage I use for my MoT are very good and very honest, but they are not Jaguar experts and would admit they don't have the knowledge and/or expertise to do sophisticated diagnosis on this engine. I have another guy a bit further away who is an Independent Jaguar Specialist so before I start chucking money at stuff I don't fully understand, I may well get him to have a look for me.


I will also look at the MAF but on these early engines it's not as straightforward as later cars as the MAF is sort-of built in to the housing just after the air filter, so not as easy to clean.


Will update as and when there's anything to report but all ideas gratefully received.
Thanks again.
Cheers,
David.

 

Even the cheap (but great) elm327 will let you look at LTFTs (and STFTs).

Hot engine, park, idle. LTFTs (2 figures, 1 per bank). Rev to about 2500 and see what the numbers do.

Post 'em if it's easier

You can use WDS but it's kinda overkill for this.

You can usually get at a MAF then spray MAF cleaner. Do not touch the tiny wires at all.