C36 AMG FAILED EMISIONS

BrandHatch1

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Hi

Got a 1997 C36 AMG that's failed its emissions at the MOT

please see reading below.

we have taken it for a good test drive before the test and still the same.

From the readings can anyone advise as the diagnostics are old and the garage computer cant read them.

Is it the CAT?
Air flow meter?
anything else?

notes its just had a full service inc spark plugs.
Mileage is 120k
 

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Jockcl500

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It only seems to be the CO that has failed, I would clean the intakes put some fuel system cleaner in the tank and then give it an Italian style tune,
also you changed the sparkplugs are you certain they are the correct ones? it has just been serviced by an indy? is it running smooth and steady at tick over?
 

Jobsworth

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The lambda figures are good, so the mixture is within tolerance, and the hydrocarbons are good too, so the engine is in good health. The CO is too high. Not by much, but too high. That is catalyst performance. The catalyst is weak. Getting it really hot can sometimes tickle it up enough to pass.
 
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BrandHatch1

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It only seems to be the CO that has failed, I would clean the intakes put some fuel system cleaner in the tank and then give it an Italian style tune,
also you changed the sparkplugs are you certain they are the correct ones? it has just been serviced by an indy? is it running smooth and steady at tick over?
yeah by my regular garage, it had a bit of a misfire but runs fine now after plugs and service.
 
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BrandHatch1

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Sorry what do you mean gutted?

they look like the original ones so not sure if there is much left of them
 

Jockcl500

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yeah by my regular garage, it had a bit of a misfire but runs fine now after plugs and service.
The misfire that it had was probably enough to clog it up a bit, getting it hot and the giving it a revy run 2500-3500 revs for about for 30-40minutes will probably be enough to get it cleared out.
 

zipdip

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Well give it a clean out and do not leave at the garage give it a run and then make sure it has the MOT when it is hot,stay with the car .
 

Petrol Pete

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'Just been serviced...' Is there is a remote chance a vac pipe/electrical connector has been accidentally dis lodged during the work 🤷‍♂️. As said , driving for a while with a misfire could 'clog' the cat's up. A drive with a basic iCarsoft code reader plugged in will show you real time temperatures before and after the cat's .
 

carat 3.6

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As the others have said, make sure the engine is hot when it's re-tested. Ideally give it an "Italian tune up" on the drive down.

The original cats last a long time if the car is looked after, but if they are in need of replacement dont be tempted by cheap pattern ones as you will be replacing them again.
 

markjay

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Sorry what do you mean gutted?

they look like the original ones so not sure if there is much left of them

'Gutting' refers to the act of intentionally removing the innards of the catalytic converters thus rendering them useless, to improve performance (and sound) while fooling the MOT tester into thinking that the car is stock.

From your response I understand that this is not the case with your C36....
 
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BrandHatch1

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so update.

this is a Japanese Spec , and has the Exhaust Temp light on, which come on Japanese Spec vechiles.

Drove it today Italian style and it now passes the Idle CO, but still fails the other too.

After a full service, it still has a misfire under load at around 4000 rpm

Would this be the coil pack ? or the leads? is it running rich?

would this cause the Exhaust temp light to come in if running rich? or does it mean the cats are gone ? but that wont explain the missfire.


any help much appreciated. PS the engine pulls really well, just hesitates under load at that rev range
 

carat 3.6

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so update.

this is a Japanese Spec , and has the Exhaust Temp light on, which come on Japanese Spec vechiles.

Drove it today Italian style and it now passes the Idle CO, but still fails the other too.

After a full service, it still has a misfire under load at around 4000 rpm

Would this be the coil pack ? or the leads? is it running rich?

would this cause the Exhaust temp light to come in if running rich? or does it mean the cats are gone ? but that wont explain the missfire.


any help much appreciated. PS the engine pulls really well, just hesitates under load at that rev range

The exhaust temp light will come on when excessive heat is detected to protect the cats.

When you say a misfire at 4000rpm, will it rev past that or does it cut completely?

There are many things on that setup which can cause a misfire. Coilpacks, HT leads, wrong plugs, wiring harness faulty, throttle body faulty, weak fuel pressure, ignition driver fault. You really need to fix that before the MOT test.
 
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BrandHatch1

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The exhaust temp light will come on when excessive heat is detected to protect the cats.

When you say a misfire at 4000rpm, will it rev past that or does it cut completely?

There are many things on that setup which can cause a misfire. Coilpacks, HT leads, wrong plugs, wiring harness faulty, throttle body faulty, weak fuel pressure, ignition driver fault. You really need to fix that before the MOT test.
thanks for your reply.

It revs past the missfire . it feels more of a hesitation and then pushes past it.

its not got a standard OBD reader either so cant see faults.

what would you change first? its got the right plugs in it
 

carat 3.6

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thanks for your reply.

It revs past the missfire . it feels more of a hesitation and then pushes past it.

its not got a standard OBD reader either so cant see faults.

what would you change first? its got the right plugs in it
You would need the 38 pin adapter to plug a scanner into it, and not one from ebay.

You need to get it diagnosed properly before changing parts, I wouldn't change anything until you've done that.
 

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Do you have rough idle, or is it just the hesitation below 4000rpm?
@Petrol Pete previously mentioned a vac pipe or electrical connector off. When I purchased my C36, it had a service before collection.
I had pretty much the same symptoms as you. It was a vac pipe knocked off that sits just by the oil filter housing. Easy to miss
 

Notwen

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@BrandHatch1 did you get your issue sorted?
 

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