W209 CLK200 running lean fault

[Ryannn]

Hi everyone, looking for some advice with my 2005 clk200.

EML came on 6 months ago and threw up P0171 fault code, engine too lean and showed high CO2 at the MOT emissions. After taking it to a specialist who couldn’t diagnose the cause, even with the STAR diagnostics, I started to investigate myself to save some money.

First issue I found was oil in the cam magnets. I’ve changed both of those for new items and added the extra wiring in aswell, not cheap! I think I managed to catch it in time as there was very little oil in the ECU plugs and none in the MAF plug.

Changing the magnets did not fix the issue and I then found that the hose under the air box was split, changed this for the updated part, but I still have the EML.

I the ran the car for a few days with the MAF unplugged, this gave me the running rich fault which lead me to believe it was at fault. However I have swapped this today and the EML came back on after 10 miles or so.

I’m totally pulling my hair out now! The only things left to change are the lambda sensors, are these worth doing? Are there any other common air leaks? When fitting the replacement hose under the air box, I noticed it didn’t have a clip on the end with the least access, the hose itself is very very tight, is this likely to cause a leak which could affect fueling or is it normal?

Thanks guys, sorry for the long post but I’ve tried a lot of things already

 

[alexanderfoti]

When the MAF's go on these, they peg the long term fuel trims to 25%+ and the ECU gets stuck in that state.

After replacing the MAFS (And confirming the Lambda's are functinoing correctly by reading their actual values) you need to carry out default initialization on the engine ECU to get it to reset.

 

[Oldspanners]

Hi everyone, looking for some advice with my 2005 clk200.

EML came on 6 months ago and threw up P0171 fault code, engine too lean and showed high CO2 at the MOT emissions. After taking it to a specialist who couldn’t diagnose the cause, even with the STAR diagnostics, I started to investigate myself to save some money.

First issue I found was oil in the cam magnets. I’ve changed both of those for new items and added the extra wiring in aswell, not cheap! I think I managed to catch it in time as there was very little oil in the ECU plugs and none in the MAF plug.



I the ran the car for a few days with the MAF unplugged, this gave me the running rich fault which lead me to believe it was at fault. However I have swapped this today and the EML came back on after 10 miles or so.

I’m totally pulling my hair out now! The only things left to change are the lambda sensors, are these worth doing? Are there any other common air leaks? When fitting the replacement hose under the air box, I noticed it didn’t have a clip on the end with the least access, the hose itself is very very tight, is this likely to cause a leak which could affect fueling or is it normal?

Thanks guys, sorry for the long post but I’ve tried a lot of things already

I had the same problem with the cam magnets on my C180. Even after changing them, fitting the blockers (expensive for just solid pieces of wire) and cleaning the connectors on the ECU and the after cat CO2 sensor the problem returned. It took several attempts at cleaning the connector at the O2 sensor with contact cleaner and compressed air (oil will collect here as it's the lowest point) over a period of months before it was cleared.
Test the heater circuit on the O2 sensor with a meter there should be two wires the same colour and it should read low ohms don't put the meter across the others. I had to change that sensor first because of that fault a Bosch costs about £40 on ebay.
P.S. If you have a OBD tool that reads live data you could look at the fuel trim.
This video is well worth watching for information on O2 sensors.

 

[Ryannn]

you need to carry out default initialization on the engine ECU to get it to reset.

Can I do this myself? Can’t seem to find anything online.

 

[alexanderfoti]

Can I do this myself? Can’t seem to find anything online.

No, it needs to be done through STAR diagnostics - the MB diag equipment.

 

[Ryannn]

No, it needs to be done through STAR diagnostics - the MB diag equipment.

Another £30 to write off then

Thanks for the help so far, given me some other stuff to look at.

 

[Oldspanners]

When the MAF's go on these, they peg the long term fuel trims to 25%+ and the ECU gets stuck in that state.

After replacing the MAFS (And confirming the Lambda's are functinoing correctly by reading their actual values) you need to carry out default initialization on the engine ECU to get it to reset.

He says he is getting a lean mixture fault.
So he needs to check both the upstream and down stream sensors first?

 

[Ryannn]

No matter how bad the issue... it’s always good to find another one!

I decided to check the oxygen sensors today. Unplugged the down stream one, it was a little damp but no major signs of oil. Gave it a clean and checked the resistance across the heater, 8.8 ohms which I think is okay?

Unplugged the upstream one and it was like I’d unscrewed the oil filter! Loads of oil poured out of it. I’ve given both ends a clean and I’ll run it for a few days before cleaning it again. Looks like I’ll have to replace the upstream sensor though.

 

[Oldspanners]

No matter how bad the issue... it’s always good to find another one!

I decided to check the oxygen sensors today. Unplugged the down stream one, it was a little damp but no major signs of oil. Gave it a clean and checked the resistance across the heater, 8.8 ohms which I think is okay?

Unplugged the upstream one and it was like I’d unscrewed the oil filter! Loads of oil poured out of it. I’ve given both ends a clean and I’ll run it for a few days before cleaning it again. Looks like I’ll have to replace the upstream sensor though.

Keep cleaning both up and down steam sensor connectors and check the plugs on the ECU as well until they are completely clear. I cleaned them with contact cleaner and compressed air and very carefully cleaned around the pins on the ECU with a cotton bud sprayed with cleaner not wanting any ingress of cleaner into the ECU, bending any pins or leaving cotton behind.
You'll need to be clear of oil before replacing anything as this will still give you a fault code and if the oil gets into the new sensor it can destroy it as well. Having said that my down stream sensor survived one time with no problems.
How much oil has wicked into the harness will of course depend on how long it was happening, my car had just over 50K miles.

 

[Ryannn]

Keep cleaning both up and down steam sensor connectors and check the plugs on the ECU as well until they are completely clear. I cleaned them with contact cleaner and compressed air and very carefully cleaned around the pins on the ECU with a cotton bud sprayed with cleaner not wanting any ingress of cleaner into the ECU, bending any pins or leaving cotton behind.
You'll need to be clear of oil before replacing anything as this will still give you a fault code and if the oil gets into the new sensor it can destroy it as well. Having said that my down stream sensor survived one time with no problems.
How much oil has wicked into the harness will of course depend on how long it was happening, my car had just over 50K miles.

I started tracing the oil a few months ago and I think I caught it just in time. There’s no oil in the MAF and a very small amount in one of the ECU plugs. Since I added the extra loom there’s been minimal in the ECU, I think I got lucky! The upstream sensor had the most oil in it by far.

 

[Oldspanners]

I started tracing the oil a few months ago and I think I caught it just in time. There’s no oil in the MAF and a very small amount in one of the ECU plugs. Since I added the extra loom there’s been minimal in the ECU, I think I got lucky! The upstream sensor had the most oil in it by far.

If it's like my C180 the two connectors for the sensors are by the bell housing at the lowest point of the wiring harness at that's where most will collect. A little patient cleaning will most likely clear your problems.
If you have to change the up steam sensor tie a piece of strong string to the end before you pull it out it will make threading the new one around the bell housing a lot easier.
Good luck.