Long shot - Can any armchair mechanics diagnose an MOT emissions fail?

UsernameAlreadyExists

My car failed it's MOT a week or so back with shockingly bad figures. It's an old 2001 SLK 200K (R170). The garage changed both lambda sensors and allowed me to give it an Italian tune-up with some additive and it's made next to no difference.

I've since changed the plugs, air filter, fuel filter(!), cleaned the throttle body, the MAF and it's a little better but nowhere near enough.

The figures are now:
Oil temp 20c, CO 6.98%, CO2 11%, O2 0.06%, HC 140ppm, Lambda 0.816.

I think next an ECU reset is in order, but after that I am really struggling for things to look at. The car "seems" to run perfectly fine in every respect (if a little underwhelming performance wise - but then I have nothing to compare it to) but there's clearly something more sinister at fault.

I've had it suggested that it might be in permanent cold enrichment mode to explain the over fuelling but don't know how to check this.

Any ideas please?

Slowly_does_it..

Oil temp 20 degrees C? That would cause the ECU to do your cold enrichment issue. Does the temp needle move to the middle under normal driving conditions?
The engine might be getting up to temp, but the ECU might not be knowing this, and thus keeping it over-fuelled, which would also add to your underwhelming performance.

Just an idea!

UsernameAlreadyExists

Until you've just said that, I'd never really considered it - how embarrassing! 20 degrees C seems quite mad. The temp gauge on the dash gets up to 80-90 (middle of the dial) when running, but that's the water temp isn't it? I think I'll be looking for oil temp sender units next. Thank you


I forgot to mention that the fuel consumption seems outrageously high too so this is all beginning to make sense now

BeenThroughItAll

UsernameAlreadyExists wrote: »

Until you've just said that, I'd never really considered it - how embarrassing! 20 degrees C seems quite mad. The temp gauge on the dash gets up to 80-90 (middle of the dial) when running, but that's the water temp isn't it? I think I'll be looking for oil temp sender units next. Thank you


I forgot to mention that the fuel consumption seems outrageously high too so this is all beginning to make sense now

I doubt that'll make a difference, as the oil temp reading isn't coming from the car, it's coming from the emissions tester. The probe is often not used by testers; they make a judgement on whether the vehicle is at operating temperature, so the probe is quite often left hanging in ambient air.

EdGasket

You need to take it to a Merc indy and get the codes read on STAR and take it from there. There's no point just guessing and replacing things at random.

onomatopoeia99

Have you tried the "drive it for 10 minutes in 3rd at 60mph before the test to get the cat nice and hot" trick? You can smell when it's properly hot.

My car needs quit a bit of "engine pre-heating" (or whatever the instructions on the testing computer call it) before it will pass the MOT. My garage is understanding of this and will hold it at 2500rpm for some time and repeat the test until it works.

UsernameAlreadyExists

onomatopoeia99 wrote: »

Have you tried the "drive it for 10 minutes in 3rd at 60mph before the test to get the cat nice and hot" trick? You can smell when it's properly hot.

Yes, I have. That's what I referred to as the Italian Tune up

UsernameAlreadyExists

EdGasket wrote: »

You need to take it to a Merc indy and get the codes read on STAR and take it from there. There's no point just guessing and replacing things at random.

I've just bought a code reader from ebay to see what I can discover myself first. During the first MOT the codes were P203B, P2031, P2036, P203A and P2039. Two of these were the lambda sensors, and another was air intake related. I don't have to hand the other meanings at the moment. The garage that MOT'd it have been scratching their heads and said the next port of call would be to the Merc dealer as they couldn't diagnose anything specific from the codes. They think it's probably a "mechanical" issue somewhere as none of the sensors seem to be indicating anything obvious.

I am treating this as a learning experience and trying to understand the problem before I just pass it off to someone else to fix! All the things I've replaced so far haven't been particularly expensive and have already made minor improvements. When I finally solve it, it should be a decent runner and I'll probably end up keeping it for longer than I'd intended to

When I've got the reader and can check the ECU coolant temp reading it'll determine where I go next. A dodgy fuel regulator is another possibility I've since read about.

mrmot

The low lambda indicates that it's running rich. it's either going to be running rich because it's getting too much fuel or not enough air.

What I've read here: http://www.briskoda.net/forums/topic/316488-2001-mb-slk-200k-failing-mot-on-emissions/
assuming it's your car, it's down on performance, so is the supercharger working correctly? are the sensors calculating the correct amount of air that's going into the cylinders? are there any induction air leaks?

That's the lambda reading, the high CO could be down to the Cat now been useless as it's been running rich for so long.

I did see similar CO readings on a Audi TT, that was down to an unplugged fuel temperature sensor.

The oil temp at 20 deg C, is a red herring as the tester would not have used it and it would have been measuring the ambient temperature in the MOT bay.

UsernameAlreadyExists

Yes, that's my car! There are other similar posts around on Merc forums too.


Too much fuel I can totally understand - hence cold enrichment, or a malfunctioning pressure regulator. I can't see how it's not getting enough air though, as I've replaced the filter with a new OEM (Mahle)

With regard to the supercharger actually working... it's hard to tell. It certainly whines (you can clearly hear it with the airbox open) and is spinning all the time as it doesn't have a clutch, but there's a bypass valve in place which appears to be operating correctly when blipping the throttle to over 2.2K revs. In terms of can you actually feel anything happen? Again, I don't know as I've never driven another one, they're supposed to be smooth and not kick you up the !!! like my other turbo'd car does. I still think it's pretty lame compared to my partner's Clio 172 which has very similar book specs but this could be because of the over-fuelling.


For induction air leaks, the garage guy told me to spray WD40 all around the intake manifold side and see if it affected the running. It didn't.


I still haven't ruled out a cat failure, but again the garage guy didn't seem to suspect this himself, and I am not entirely sure why

londonTiger

if an italian tune up cant fix it i'm afriad you need to take it to a merc specialist.