MOT FAIL - High Lambda Honda Jazz

yes, an exhaust blow can certainly cause the lambda fail.

Only if the leaks are beyond the sensors.

A lambda reading above 1 shows it's running weak. 1.07 equates to an AFR of 15.7:1 I'd expect other gasses to show up 'wrong' too - can you give all the readings?

Sorry I should have been clearer, both sensors are on the front section of exhaust, just before and just after the cat.

Full Gasses are

Fast Idle Test 1

CO - 0.11% (Limit =

Is that becuase as the "presure wave" of exhaust gass moves down the pipe it would suck in clean air behind each wave this leading to dodgy readings?

Tom

YES - seal the exhaust, lambda over 1 is weak from the extra air. You'll then pass the test. Seen this 1000's of times before...

Tim.

I don't know the mechanism, but I have definitely seen lambda fails cured by only blocking an exhaust blow, literally put in the gungum and watch the readings correct themselves.

Interesting. And this is leaks well downstream of the sensor(s)?

I have a wideband O2 sensor on the old Rover. I recently had a bad leak where the rear box bolts to the pipe - just came loose. Didn't make a scrap of difference to the idle reading.

That should have read before the sensors. ;-)

Low CO in combination with high lambda confirms a weak mixture. The cat if at temperture will tend to alter the HC readings - without, they would likely be high with a weak mixture.

Exhaust replaced from the cat to the rear as it was about knackard anyway.

Hopefully going for a re-test tomorrow so I'll let you know!

Tom

yep, after the sensors, I assume it must be to do with backpressure, but it works, last one was a 1.8 vectra with a little leak just at the exit of the centre exhaust box.

Isn't it because the tester is measuring the O2 at the exhaust exit which is engine exhaust "contaminated with" air (which is 20% O2) drawn in through the exhaust holes. Not really a fail, more an artefact of the measurement technique.

Really, with modern closed loop systems, high O2 measured at the tailpipe isn't really a pollution problem if all other readings are OK. If the ECU is not declaring an O2 sensor problem, is must be OK.

Isn't is about time the MoT included taking readings from the OBD-2 port into account?

Chris K

I'm failing to see an advantage to trusting the cars ecu over getting someone to fix a leaky exhaust.

Agree, still needs fixing, but a slight leak may only need to be an advisory rather than declaring a pollution fault.

CK

emissions are one of the few 'absolutes' in an MoT test. But, badly applied testing can make almost any car fail.

Exactly so....

Tim.

Hi Again

Results of the ReTtest are in,

Emma is delighted to report that "Jerry the Jazz" (Do men ever name cars?) has passed the MOT Exam.

With nothing else changed other than the Middle and Tail Sections of the exhaust (Car was also presented warm rather than hot)

Fast Idle Test:

CO - 0.03% - PASS (Limit =< 0.20% ) HC - 33PPM - PASS (Limit =< 200ppm ) Lambda - 0.997 - Pass (Limit 0.97 - 1.03)

Natural Idle

CO - 0.02% - Pass (Limit =< 0.30% )

Incidentally Last Years figures are identical with the exeption of the HC which last year was 0 ppm. Last year the car had not been serviced for at least 80'000miles had an air filter that was covered in something that looked like treacle and the plugs had a gap you could drive a bus through! The engine contained about a mug full of oil that had the consistancey of snot when warm!

Oh and the brakes bore more resemblance to those used on trains, being metal on metal!

Thanks for all your help

So you'll probably be back at next year's MOT time with failed gas test due to the engine being worn out !!!

Tim.

Yup, but we'll cross that bridge when we come to it....!