CLK200 Emissions - MOT Failure - Running Rich

[TNG]

Anyone come across this before?

1998 CLK200 Auto with 68,000 on the clock has just failed its MOT on emissions. CO and HC are way high and Lambda is low. Chap doing the MOT said it needed a new cat but I'm struggling to see why the cat would cause low lambda.

I've pulled the plugs and all 4 look very similar and are typical of an engine that's running normally - none are black, wet, etc. and all electrodes look good. The air-cleaner looks reasonably new. Overall, the car runs pretty well with decent fuel consumption. The only noticeable issues are a slight roughness (but maybe that comes with the 4 cyl engine?) and a low idle speed when the engine is partway warm.

I've done a quick check on the lambda sensor with a multimeter and it's showing about 1 ohm heater resistance and signal fluctuating between about 55 mV and 850 mV. From what I can see this indicates the sensor is working OK and correctly indicating a rich mixture.

I'm in the wilderness that is East Lancs so there don't seem to be any good independents really close. Currently waiting for an OBD adapter so I can connect and read the faults.

Any wisdom would be greatly appreciated - it's my wife's car so you can imagine I'm under a certain amount of pressure to come up with a fix!

 

[Howard]

Does your wife just potter about and go to the shops ?

Perhaps an Italian tune up on the way to the mot station will help get it through.

 

[television]

This will be the 111 engine, could the MAF be failing.

 

[Steve@Avantgarde]

I would say it has oil in the loom from the cam magnets leaking and its knackered the 02 sensor and the MAF or has residue in the charge system and its knocking the MAF about.

 

[TNG]

Update

Thanks for the feedback. MAF sensor makes sense. I've read about oil in the loom and that might well be contributing but I can't see any evidence. Think I'll try putting in a new MAF and see what it does for me.

 

[TNG]

Fixed!!!

Got a 38 pin to OBD adapter from Amazon and connected to my cheapo ELM bluetooth OBD adapter running with Torque Pro app on my phone.

Showed MAF sensor fault and warnings for engine, O2 sensor and O2 snesor heating circuit. Ran a test on the O2 sensor using Torque Pro and it looked OK.

Got a new MAF off Ebay from sjrautoparts and fitted it. Cleared the faults. Gave it an "Italian tune-up" for 15 minutes and then took it off to be re-tested.

Now the proud owner of a shiny new MOT certification and the emissions results showed everything well within limits with Lambda running at 1.000.

Total spend £45 on bits plus the re-test fee. RESULT!!!

 

[FELLOW]

Congratulations on the self diagnostics

Congratulations on the diagnostics and fix.

I run an old CLK too. (1999 320)

Please could you post a link to your diagnostic kit as I've found it difficult to find one that actually works with old adaptor plug?

Many thanks