Failed Emissions MOT test

[nullogik]

I have 1993 MB 190E 2.0 litre engine. The previous owner took in for MOT and it failed the "closed loop cat test".

The problems where specifically the "fast idle" CO and HC tests where it scored 6.3% (max was 0.3%) and 206ppm (max was 200ppm).

It also failed the second fast idle test with similar results. However, the car appears to have been retested and passed - quite what the previous owner did to make it pass I don't know.

I'm just trying to narrow down what the problem could be.

The car has a slight rough idle (but the RPM needle doesn't move). It will surge and then relax for 1 sec intervals in a cycle and seems to improve as the car will warm up. Could this cause it to fail this part of the emissions test?

Also the car has a very tiny oil leak, but it doesn't burn any oil, has no white gunk in the coolant, there is no blue/white smoke on cold start and it doesn't smell of burning oil. Its suspected to be a head gasket but the problem has existed for 3 years and hasn't worsened. Could this cause a failure of emissions?

Or is it the dreaded CAT that is on its way out, though I'm not hearing any CAT rattle.

 

[normanr]

I expect the owner gave it an Italian service to get the emissions down.

What that is take it on the motorway and give the engine a good blasting does not cure the problem though.

 

[Bazzle]

Here we get engine up to hot running temp before testing is performed. Cold/Warm engine may be running rich due to MBs cat warm up cycle.

Bazzle

 

[Dieselman]

Here we get engine up to hot running temp before testing is performed.

We do here as well.
At 6% Co that is waaaaay too high even for a non catalysed car. At 10% a proper misfire will set in.

The engine should be able to run at about 1% Co with no cat an d the Lambda sensor then holds this figure so the Cat can clean up the downstream emissions.
The base setting for your engine is just way too rich, so it's either blockage of air or an intake problem, injection system problem or failed Lambda sensor.

You need to get this sorted otherwise it will poison the Cat.

#
It's just possible the injection system temperature sensor is faulty so holding the system 'on choke'.

 

[columb]

try changing oil before doing MOT. I had simillar problem ages ago in other car - and I was advised to change oil...

Cheers
Chris

 

[AndyS]

They got my old Carina through by putting the hose on the exhaust of the car next to it!

 

[evdok]

They got my old Carina through by putting the hose on the exhaust of the car next to it!

I'd rather have my own car running how it should!

 

[moakesr]

They got my old Carina through by putting the hose on the exhaust of the car next to it!

That kind of thing happens more than you would think!

 

[AndyS]

I'd rather have my own car running how it should!

Me too but it involved buying a new cat for more than the car was worth!

 

[recycled]

brilliant, imagine not having to fork out for a new maf/ cat at close to a grand.

fast idle co used to be 0.2% now .3%
If the lambda reading is ok <1.03 then you should check your fuel injection system

 

[nullogik]

We do here as well.
At 6% Co that is waaaaay too high even for a non catalysed car. At 10% a proper misfire will set in.

The engine should be able to run at about 1% Co with no cat an d the Lambda sensor then holds this figure so the Cat can clean up the downstream emissions.
The base setting for your engine is just way too rich, so it's either blockage of air or an intake problem, injection system problem or failed Lambda sensor.

You need to get this sorted otherwise it will poison the Cat.

#
It's just possible the injection system temperature sensor is faulty so holding the system 'on choke'.

Thanks for that info Dieselman & recycled

The car seems to run fine but it does have a very slight rough idle could this be related?

I gather late 190E's have a diagnostic port - is it possible to identify from this what sensors/systems that could be playing up. I don't have much cash after just buying the car and insuring it, so I'm reluctant to take it in for a diagnosis at the moment.

If its the fuel injection system, might chucking in some injector cleaner help things out - or is it a stage beyond this now?

 

[recycled]

the head gasket will not cause it to have high emissions.Mine has been leaking for nearly a year now. if it running rich it would. what was the lambda reading? was it okay. if that is too high, then the mixture needs looking at.
it could be as simple as replacing spark plugs, but we need to know that lamda sensor is good.

 

[Dieselman]

Plugs won't cause high Co readings, They could cause high Hc but not this time.
This is a fueling problem.

 

[columb]

I've suggested oil change because ages ago I've failed MOT as well. Bloke told me that if I've have fuel in oil (for whatever reason, can remember now - it was like 10 years ago) emissions are up by 1000...00 times Changing oil has sorted my problem and my car passed MOT.

(actually, it was in different part of Europe, but same idea)

Cheers
Chris

 

[nullogik]

what was the lambda reading? was it okay. if that is too high, then the mixture needs looking at.
it could be as simple as replacing spark plugs, but we need to know that lamda sensor is good.

Funny you should mention the Lambda readings recycled, I checked the MOT test documents this morning and the Lambda readings are as follows:

Car Reading 1: 0.83 (Test requirements, min = 0.97, max = 1.03)
Car Reading 2: 0.81

So it would seem that the Lambda sensor is too low. Consequently the car failed the Lambda test.

 

[Dieselman]

That's because the mixture setting is too rich. A richer mixture makes the Lambda reading go lower and vice versa.
The cycling at 1 sec intervals is the Lambda attempting to compensate.

Ge the base setting right and all will be Ok.

What's the AFR reading?

 

[nullogik]

Thanks Dieselman, I really appreciate your help on this matter

What is an AFR reading? I'm just guessing but does AFR stand for "air fuel ratio"? How might I get this reading?

I see from my Haynes manual that adjusting the richness is quite straightforward, I think there is a screw or Allen screw, than can be turned to adjust richness. Without equipment, is it possible to "do this by ear" i.e. listen to how the engine responds as I turn the screw?

 

[Dieselman]

Thanks Dieselman, I really appreciate your help on this matter

What is an AFR reading? I'm just guessing but does AFR stand for "air fuel ratio"? How might I get this reading?

I see from my Haynes manual that adjusting the richness is quite straightforward, I think there is a screw or Allen screw, than can be turned to adjust richness. Without equipment, is it possible to "do this by ear" i.e. listen to how the engine responds as I turn the screw?

Absolutely correct, on all counts.
A vacuum gauge would help but you should detect the change as the engine smoothes out and picks up a bit. If you go too far it will quickly start hunting.
Another gauge would be to check the plug colour after a run when hot. Black is rich, white is too lean, brown is good.

I'd be checking all vacuum and auto choke components before adjusting anything though.
I wondered whether the AFR reading might be on the MOT slip.

 

[nullogik]

Absolutely correct, on all counts.
A vacuum gauge would help but you should detect the change as the engine smoothes out and picks up a bit. If you go too far it will quickly start hunting.
Another gauge would be to check the plug colour after a run when hot. Black is rich, white is too lean, brown is good.

I'd be checking all vacuum and auto choke components before adjusting anything though.
I wondered whether the AFR reading might be on the MOT slip.

I don't think there is an AFR reading on the MOT slip from memory, but I'll check it again when I go home.

Thanks, you've given me a lot to think about - I'll see what I can do this weekend.

 

[Dieselman]

By 'auto choke components' I mean mixture enrichment devices, such as the thermo switch for the cold start injector.

Something has made it go rich, unless it's been like that for a while and been masked..