MOT emission failure MAF/oxygen-sensor/vacuum leak ?

jpaul

jpaul

jpaul

Soldato
Joined
1 Mar 2010
Posts
22,897
MOT had the following emissions fail versus last year
mechanic was suggesting it could be oxygen sensors , or failing cat - any opinions ?



51687322232_fca00955fe_c_d.jpg



Oxygen/lambda sensors are ~£100 each there are 2 on each bank (e: 2 downpipes each 3 cylinders) before cat (bmw M52 i6 ), used for controlling mixture, so I don't want to change them if not necessary.
anlso 2 after cat , apparently just used for EML lights (nothing shown)

I had cleaned the MAF recently and will be checking there is no vacuum leak, if there were a small leak could it be so dramatic ?

some web comments I'd read
- jiggerd 02/Lambda sensor usually shows up as high HC (hydro carbons/unburnt fuel) => so maybe not lambda, it doesn't use much oil either.

- exhaust leak would give a high lambda reading.=> no leaks afaik
- plugs and air filter then a damn good thrashing just before the retest. => haven't yet done/inspected plugs
- Bottle of Catalyst cleaner wouldn’t be wasted either, => I've put in some BP ultimate so far, car had been idle for 3 months, did maybe 10miles before mot to warm it, mpg doesn't seem to have deteriorated in post mot drives.
(posh nail varnish remover or equivilant that burns slightly hotter or burns with lower emissions)
- CO is carbon monoxide not CO2 and is the gas of incompete combustion so it must be running inefficiently, bad plugs, burnt valves etc
 
Last edited:
CO is a sign the engine is running rich

Possible causes:

Air filter? You will be surprised just how cruddy some get and how much it restricts the air intake leading to rich fuel mixes

Faulty 02 sensor - the most likely cause. If it reads too much oxygen by mistake, the ECU will pump more fuel into the chambers leading to rich mixture, high CO

Defective MAP sensor - unlikely in this case i think.

Defective Engine Coolant Temperature (ECT) Sensor - Low engine temperature requires more fuel. When the ECU is unable to determine what the engine's accurate temperature is, it will not adjust fuel delivery properly; resulting in high CO

More often than not it tends to be the O2 sensors.
 
With a bluetooth OBD2 dongle and the Torque app you should be able to check operation of O2 Sensors..

Has it been stood for 3 months? Old petrol wouldn't necessarily be great. Might also be worth giving the battery a proper charge, as low voltage can sometimes cause all manner of random errors with sensors and the like.
 
Greebo said:
Defective Engine Coolant Temperature (ECT) Sensor - Low engine temperature requires more fuel. When the ECU is unable to determine what the engine's accurate temperature is, it will not adjust fuel delivery properly; resulting in high CO

More often than not it tends to be the O2 sensors.
Click to expand...

haven't looked at bentley pdf yet, instrument panel temp has been ok, but maybe that's oil temp, and mixture uses a different sensor. (auxiliary fan has one I guess)
plugs look ok'ish - can't check the gap on this type - and I haven't yet got a new one to compare.

Armageus said:
With a bluetooth OBD2 dongle and the Torque app you should be able to check operation of O2 Sensors..
Click to expand...
yes didn't yet get a dongle after earlier thread - 4 sensors in total ...
and (thinking further) if the pre-cat/mixture ones were faulty, can the post cat (eml) ones be bad too ? and the engine should then tell me it has a problem.
After cleaning plugs, say, I suppose dongle would show if O2 levels change, but inability to measure emissions myself is a pain.

jsmoke said:
What car is it
Click to expand...
bmw 3 05 - M52 i6


51688294376_9b5c8d2dd4_o_d.jpg
 
jpaul said:
plugs look ok'ish - can't check the gap on this type - and I haven't yet got a new one to compare.
Click to expand...
Noticed that the central electrodes are all slightly oval, narrower on side facing outer contacts.
I can't believe that's wear, but, nonetheless, fancy manufacturing.
 
jpaul said:
car had been idle for 3 months, did maybe 10miles before mot to warm it
Click to expand...

Assuming there is no EML/emission lights on your dashboard - this is your problem. It has been driven sedately to the MOT station, where it then sat for 30 minutes on a cool day, then was driven into the MOT test area. The catalytic converter just wasn't hot enough, especially after sitting for so long.

Between the first fast idle test and the second one you can see that the CO level falls by more than half just in this short time of revving the engine a little bit.

The car simply needs a good run where you do some foot down/high revving action for 10 minutes and it'll be absolutely fine.
 
Last edited:
Greebo said:
CO is a sign the engine is running rich
Click to expand...

Wrong.

CO is carbon monoxide which mean its the cat.
Hydrocarbons is to do with non full fuel burn usually running rich/missfire etc. Check sparkplugs if petrol engine.

Get cat up to temp i held mine at 5500rpm for about 3mins till the cat ignited and CO dropped to pass level. Mk3 fiesta not driven much hence
 
Cyber-Mav said:
Wrong.

CO is carbon monoxide which mean its the cat.
Hydrocarbons is to do with non full fuel burn usually running rich/missfire etc. Check sparkplugs if petrol engine.

Get cat up to temp i held mine at 5500rpm for about 3mins till the cat ignited and CO dropped to pass level. Mk3 fiesta not driven much hence
Click to expand...

In my day it was always a sign of a rich burn. But I am used to pre cat cars.
 
I’ve had 3 cars with the M52 engine, both my old e38 728i and e39 528i failed an MOT because,as already said, the cat simply wasn’t warm enough when the car was tested.

Incidentally on e36 328i cleaning my maf sensor broke it :o I used a supposed maf cleaning spray but was told I should have left it alone, had to replace the damn thing!
 
Did the oil&filters, and gave it some 5Krpm action, will put in new plugs when they arrive ... and then schedule re-test,
other than revving it (as proposed), without a nearby motorway, difficult to really warm cat up.

made another mental note - always check the top cap on the waste oil drain pan ... the copper washer as always dropped in, but, minimal splashing.

Scania said:
I used a supposed maf cleaning spray but was told I should have left it alone
Click to expand...
as preventative maintenance I used spray IPA several months back, and drenched it, but, could still be part of my issue.
 
jpaul said:
Did the oil&filters, and gave it some 5Krpm action, will put in new plugs when they arrive ... and then schedule re-test,
other than revving it (as proposed), without a nearby motorway, difficult to really warm cat up.
Click to expand...

You don't need a motorway to warm a car up? I'm approximately 30 miles away from my nearest motorway but dammit I can get my car very warm in just the 5 miles surrounding my town. Just find an A road, and stamp on the throttle and rev out every gear for a bit, it's not hard.

Don't sit there revving the car, that doesn't warm it up properly, just drive it hard for 5-10 minutes on some backroads and you'll be fine - it really is a simple as that.
 
Gaygle said:
but really, the old ones looked absolutely fine.
Click to expand...
ok interesting, I was hoping that might represent significant degradation,
maybe with moden high voltage electronic ignition the spark gap is less important than cars that used to have distributors ?
overall yes the plugs looked the right colour, no glazing or excessive oil, but - the gap represents an unknown, and central electrode wear, becoming an oval surprising.
 
jpaul said:
51687322232_fca00955fe_c_d.jpg
Click to expand...

phew - Passed on the re-test CO dropping 0.83 to 0.17 %, still up on 0.05 last year.

so, although the oil changed (now fresh titan superyn 5-40) had looked a bit coloured, the new plugs seem to have been what did it ?
(not sure why CO limit got changed)


51704776880_4d9cf07cc2_c_d.jpg
 
:p I can almost guarantee that the plugs did the square root of FA. And the oil is meant to turn black, normally within a few miles after changing it, that is it doing it's job

This was only ever about the catalytic converter. Did you do a good warm up this time prior to the MOT?

Ah well, glad you've got it sorted.
 
You must log in or register to reply here.
Back
Top Bottom