High Lambda MOT failure ??????

[MarkJunedd]

Evening all, i've been off here too long and now typically i come back with a problem ! I havent driven the merc for about four months (lost it to my wife )but took it for test today as i forgot about it and it has failed on emissions.
Readings are

CO 0.041 (max 0.30)
HC 94 (max 200
Lambda 1.084 (max 1.030)
In fairness to the tester he tried it three times but couldnt get the lambda down ( i suspected the MAF to be honest but tried with it disconnected which then brought the lambda down but threw the other two to ridiculous levels so i assume the maf is working properly.

Also it has code P0306 (misfire cyl 6 ) which is odd as i replaced all 12 plugs and leads along with a coil last year. It also hits a hiccup at 4000 rpm . Gutted is not the word as I love this car and so does my wife but i have to be sensible with what it is worth before i throw a wedge at it. Theres an indy about 5 miles from me (SK Automotive) but that would risk driving it with no ticket on. Any help would be very much appreciated.

 

[Howard]

Has anyone driven it at all in the last four months ?

You might find an italian tune up on the way to the MOT station helps.

 

[LTD]

You could legally take your car to another garage for the work to be carried out on. You cannot drive it around though - straight to the other garage. You will then have to pay the full retest fee however if you do take it away.

The question is ....... who do you trust to do the repair ??????

Howard's advice is pertty solid - I always take the cars for a thrash down to local motorway to 'clear their lungs'

 

[JohnnyW202]

Doesn't the 3.2 V6 in the 320 have individual pencil coils shared by the dual spark plugs in the cylinder head?

I know my C200 has two coils firing two spark plugs at the same time (Wasted Spark) Much like my Mondeo did.

I had a 540i that failed it's MOT in a similar way... on these oil leaks from the rocker covers into the spark plug wells covering the plugs and getting under the rubber boots of the pencil coils.

Does it run rough at idle, or only begin to misfire at a certain RPM frequency.

Could be Camshaft Position Sensor, Crank Position Sensor or Indeed another Coil Pack, since you said it has HT leads. I'd assume it's just one coil? Although I didn't think the newer Mercedes particularly the Twin Spark V6 still used leads, I thought it was direct ignition, in which case a new coil on your Cylinder 6, a couple of new spark plugs, or clean them up carefully with a wire brush and some parts cleaner, some redex and a good booting... should decarb the engine, hopefully solve your misfire and get it through it's MOT, especially if the car has been pootling at low revs around town (nothing against female drivers)

 

[stevesey]

High Lamba = excess Oxygen - could be as simple as an exhaust leak allowing extra Oxygen before the tailpipe probe (and after the Lamda sensor).

 

[Dieselman]

You could legally take your car to another garage for the work to be carried out on. You cannot drive it around though - straight to the other garage.

If it's still within the original MOT certificate date you can just keep driving until the certificate runs out as the car still has a valid MOT certificate.

 

[MarkJunedd]

Has anyone driven it at all in the last four months ?

Yes Howard my wife uses the car regularly,but as long as the aircon and make up mirror are working she's happy. I have another car hence i havent driven the merc at all. I just tried swapping coil 6 for coil 3 but it still weirdly shows the same fault code P0306 ?????? Could i have an ecu fault ? I seem to remember a thread about ecu output transistors or similar but not sure if my car is susceptible to this.

 

[MarkJunedd]

Doesn't the 3.2 V6 in the 320 have individual pencil coils shared by the dual spark plugs in the cylinder head?

I know my C200 has two coils firing two spark plugs at the same time (Wasted Spark) Much like my Mondeo did.

I had a 540i that failed it's MOT in a similar way... on these oil leaks from the rocker covers into the spark plug wells covering the plugs and getting under the rubber boots of the pencil coils.

Does it run rough at idle, or only begin to misfire at a certain RPM frequency.

Could be Camshaft Position Sensor, Crank Position Sensor or Indeed another Coil Pack, since you said it has HT leads. I'd assume it's just one coil? Although I didn't think the newer Mercedes particularly the Twin Spark V6 still used leads, I thought it was direct ignition, in which case a new coil on your Cylinder 6, a couple of new spark plugs, or clean them up carefully with a wire brush and some parts cleaner, some redex and a good booting... should decarb the engine, hopefully solve your misfire and get it through it's MOT, especially if the car has been pootling at low revs around town (nothing against female drivers)

Pootling around ?? She drives it like the STIG !!!

Its has 6 coils each feeding two plugs, no oil leaks at all . Its lumpy at idle with the engine visible shaking. As the revs rise the miss is still present but less audible. Driving it above 2000 rpm its smooth but then it splutters at 4k and then away again like a bullet.

 

[stevesey]

Yes Howard my wife uses the car regularly,but as long as the aircon and make up mirror are working she's happy. I have another car hence i havent driven the merc at all. I just tried swapping coil 6 for coil 3 but it still weirdly shows the same fault code P0306 ?????? Could i have an ecu fault ? I seem to remember a thread about ecu output transistors or similar but not sure if my car is susceptible to this.

Apologies for sounding patronising - but you did clear the codes after swapping the coil packs didn't you.

If you have a code reader you should be able to see the lamda sensor reading (both of them) and so on. So are both reporting correctly and similar on both banks?

Don't forget to check for exahust leaks.

EDIT: Just read you update about lumpy idle - might be worth checking all the vaccum pipes as well for an inlet leak, which would throw the mixture out.

 

[JohnnyW202]

Pootling around ?? She drives it like the STIG !!!

Its has 6 coils each feeding two plugs, no oil leaks at all . Its lumpy at idle with the engine visible shaking. As the revs rise the miss is still present but less audible. Driving it above 2000 rpm its smooth but then it splutters at 4k and then away again like a bullet.

It does indeed sound like a Sensor problem.

Prehaps it's the actual HEGO sensor it self. You'll either have 4 two on each back pre and post cat. or 2 before the cats, since I'm not familar with the car know my M62 V8 had four... Once I'd cured the leaking oil and coils it still ran a little bit rough despite new ignition components.

It didn't hunt on idle, it just ran badly, you saw the engine "shake" as you describe at idle.

a New MAF cleared it right up for me. Although I tried a generic one that made it worse, it really has to be a proper Bosch/MB part... the others don't have a temperature sensor in and use a fixed resistor value, which either makes you box start changing harshly or lose a significant amount of MPG.

 

[horgantrevor]

also check that the MOT VALVE under water bottle passenger side is NOT stuck open it will make the car run like this the valve should open and close and be closed when car off

 

[stevesey]

also check that the MOT VALVE under water bottle passenger side is NOT stuck open it will make the car run like this the valve should open and close and be closed when car off

Yep that will create a vaccum leak - should remain closed for the first 2 minutes after a start and then tick as it is pulsed (if not audible you should feel it by holding the valve.)

 

[MarkJunedd]

Yep that will create a vaccum leak - should remain closed for the first 2 minutes after a start and then tick as it is pulsed (if not audible you should feel it by holding the valve.)

Firstly what is a MOT valve ?? Never heard of that before. Although i could hear something ticking on the passenger side while i was using the reader. Though surely a vacuum leak in this area would affect all cylinders not just no6 ?

 

[stevesey]

Firstly what is a MOT valve ?? Never heard of that before. Although i could hear something ticking on the passenger side while i was using the reader. Though surely a vacuum leak in this area would affect all cylinders not just no6 ?

Valve is used to vent the charcol fuel tank vapor canister Onboard refueling vapor recovery - Wikipedia, the free encyclopedia

I wasn't assuming the bad Lamda etc was specifically realted to the misfire code. Using the code reader on to look at the lamda sensor and fuel trim readings for each bank - might help confirm things.

 

[MarkJunedd]

Valve is used to vent the charcol fuel tank vapor canister .I wasn't assuming the bad Lamda etc was specifically realted to the misfire code. Using the code reader on to look at the lamda sensor and fuel trim readings for each bank - might help confirm things.

OK Stevesey i'll hook the reader back up tomorrow and see what it throws for the lambdas.

 

[rajinder_1]

Hi,

Do the simple things first,
as howard mentioned itallian tune up with a bottle of Forte gas treatment. and really give it a thrashing red lining it, may be crap in the CATS.
if that dont work
change the plugs on the faulty cylinder or swap them around.
check the MAF as that is what caused my issues on the 320 and the lumpy engine at those revs.
all of what is mentioned in above posts..
if that dont work then start diagnosing and replacing parts....

That would be my advice for what its worth

Raj

 

[MarkJunedd]

Seriuosly thinking i have an ecu fault with the car. Swapped a few things around tonight, plugs, leads, coil pack, whatever i did i'm still showing up the code P0306 ,misfire cyl 6. Even if i clear the codes and then do a rescan without starting the engine it will still bring up the fault code. The only logical thing i can think of is something amiss in the ecu. To add the plugs on the other cylinders were a nice light brown whereas no6 were damp with petrol so that rules out the injector i suppose. Anybody had any dealings with ecu repairers coz if i cant sort this relatively cheaply it will be bye bye coupe

 

[BlackC55]

Right. You need to get it on a Star firstly. Your fault code reader will tell you not enough info.

I would also do a compression test on all cylinders and compare the values against cylinder 6

 

[BlackC55]

Also, check the wiring to that number 6 coil back to the engine control unit.

 

[MarkJunedd]

Should of mentioned i have done a comp test, all within 5 psi of each other so all good. Thinking ecu because of the fact that it has a fault showing without even starting it.