Lambda reading

[P.L]

Gentlemen, I just failed the emission test because of high lambda readings

The readings were as follows:
Fast idle: CO 0.02%, HC 19ppm, Lambda 1.141
Second fast idle: CO 0.01%, HC 1ppm, Lambda 1.15
Natural idle: CO 0.04
The engine idles smoothly and showed 720rpm during the test, the exhaust did not show cracks and did not blow during the test. "Check engine" light off.

Although lambda is definitely too high, HC and CO seem to me surprisingly low.
Any recommendation what to check other than the actual sensor readings?

Thanks,
Peter.

[ptjs1]

Peter,

A high lambda reading isn't necessarily anything to do with the lambda sensors, particularly if the other readings are within limits. Your CO and HC readings don't seem necessarily low for an AJ16 engine in good condition.

As I understand it, a high lambda reading indicates an excess oxygen level. This could relate to a lean mixture in the engine itself, effectively the fuel:air ratio is too lean, but it could also relate to a minor exhaust leak which is allowing air to get pulled into the exhaust.

If you've got some form of diagnostic plugin for your OBDII port you could check the air:fuel ratio as well as the lambda sensors. As a first step, check to see if you've got any exhaust leaks or any inlet manifold leaks (a spray of WD40 around the joints should show up a leak).

Good luck

Paul

[P.L]

Sorry, it took me quite a while but I finally managed to connect a diagnostic tool o the car today.
It offered less options than expected but using the generic OBDII connection enabled me to read some actual values:
Oxygen sensor voltage bank 1: 1,25V
Oxygen sensor voltage bank 2: 0,05V
Swapping the sensor connectors did swap the values as well. 1,25V sounds right for a titania sensor so I conclude the bank 2 sensor (cylinder 4,5,6) is shot.

Checking other available PIDs, I wondered about the long term fuel trim values for bank 1 and bank 2 as both showed values of -85%. I am not sure what this means but a correction of -85% just does not sound right to me.

As always, any comment would be much appreciated.

Thanks,
Peter

[ptjs1]

Peter,

The voltage readings are not necessarily high or low; the issue is more as to how they vary second by second. My XJS sensors seem to drop to 0 at times, so you need to view your OBDII readings real-time and watch how they move. Mine seem to cycle up & down between 0 & 1.5v, the value changing every second or so (although I'm sure that I read somewhere that titanium sensors vary between 1v & 5v?). My (very) limited experience of having monitored the pair of lambda values on all my cars is that they seem to broadly stay in sync with each other.

If your OBDII package can show a graph of the voltage readings of the two sensors real-time (most packages can do this) and you watch them over just a few minutes, then this will be a better indicator of which, if any, of the two sensors is causing the problem.

Good luck

Paul

[P.L]

Thanks for your advice Paul!

I did trace the lambda readings during a short drive and while the bank 1 sensor delivered sensible values, the bank 2 voltage remained at 0V. Both sensor readings are fairly sync since I installed the new sensor.

I got the emissions checked before the official re-test and still do not completely understand what is going on. To recap, I failed with lambda 1.15. Changing the broken sensor followed by a 20 miles run took it straight down to 1.08 - which is still too high. Another 5 miles on high load and it went down to 1.05. The final 5 miles run took it down to 1.017 - which made her pass the test.
This looks to me like some sort of learning/adapting to the new sensor. Is this being reflected in a value accessible to me - or the OBD tool? I thought about the fuel trimming but during all this I never saw the short term fuel trim moving and the LTFT is still -85% for both banks. Do I need someone with a PDU on the case?

Maybe I should just carry on and enjoy the car but I just cannot stop scratching my head...

Thanks again,
Peter

[robv12]

a common mot fail on AJ6, AJ16 and V12 cars, the emmissions test is done with car not fully up to temperature, or taken to the test station and left, then done cold, some of the invoices I have seen to replace lambdas after a fail like this make me shudder

my routine in this on the day of the test, I go the "long way round" to the test station, keep the revs up, lock them in 2nd or 3rd gear, this gets the exhaust gasses flowing well and at a sensible temperature the lambda begins to work correctly, then at arrival get them to do the emissions test straight away,

This trick was taught to me many years ago by David Marks, never had a fail since

also known as the Italian tune up, drive it like you stole it before test time!!