M271 2006 CLK P0101 & P0172 & P2029-002

[hofnerpres1]

Hello again. Following a fun time and replacement of the Seimens ECM the car is running well but every run eventually throws up a MIL telling me the mixture is too rich at Idle - Merc fault code P2029-002. This is confirmed by a high fuel condition reading on the downstream of the Cat Lambda Sensor - steady at around 900mv at tickover - should be nearer 500mv I believe. Naturally this causes the Fuel Trims to cut back on the injection times up to over 25% and start logging fault codes. Given the car is 16 years and has had the benefit of an oil filled engine harness, both Lambda sensors have been replaced with Bosch OEM parts - 22mm quality spanner and plenty of plus-gas. ( Yes, I've replaced the $12 hose, checked for air leaks and fitted new Cam Sensors and wiring oil blockers). The rear sensor readings are steadier but the rich condition at idle remains. Looking at the OBD readings the figure that jumps out is 18 g/s air flow from the MAF. WIS data states this should be 5-10 g/s at tickover so I'm thinking an over reading signal is fooling the ECM and asking it for the equivalent excessive fuel supply. The Lambda sensors are seeing this rich mixture and struggling to correct it via Fuel Trimming the injector times. This air flow reading is the ECMs processed figure , not the raw output so a good spare ECM was quickly substituted and the figure remained the same. I know that an over reading MAF is rare and also when it was disconnected the fuel trims and downstream sensors remained unchanged which confuses my poor brain. Are we good to call " Bad MAF" ?

Attached are a few screen shots from the iCarsoft CR Max. Can some kind member tell me what that Self Adaption screen is trying to tell us ?

 

[LostKiwi]

Can you reset the fuel trims?

 

[hofnerpres1]

Thanks Lost Kiwi. Yes - had that good thought. Sadly my iCarsoft scanner tells me it can't do that on this system but that might be my lack of know how. However, the battery has been disconnected a couple of times for other matters and I did the system drain trick through a 10 ohm resistor. Also nothing changed when I tried the spare ECM which has been sitting on the bench for two months.

 

[hofnerpres1]

Update. Leaving the battery disconnected overnight seemed to do the trick. Also removed and cleaned up the oily Throttle Valve to make sure this wasn't confusing things. Left the MAF disconnected - turned ignition on and the fuel trims confirmed they were at nearly zero. Car started and ran very smoothly. Watched Live Data via OBD and all looked good - trims looked excellent - both less than 2%. They stayed that way as the system went into closed loop - both reacting to throttle increases but staying well down as controlled by the default fueling map. The only oddity was that post- Cat Lambda Sensor started at 0.445 then settled at 800mV - maybe that's OK since the Long Term Fuel Trim which it sets was a very happy bunny running at less than 2% at idle and less than 7% at higher revs.

As soon as the MAF was connected it showed a crazy 20 g/s at idle and the STFT went to minus 25% so I think we have uncovered a "Weapon of MAF Destruction" However, nothing is for sure with these complex modern cars so we will get a replacement on order and keep you all posted. ( Yes, I have tried cleaning it but this type is difficult to access and the EGR fumes tend to degrade the delicate parts beyond a simple solvent blast - Plus it's 16 years old and not Japaneses.... )

 

[hofnerpres1]

Well the MAF change was a good call. The FEBI replacement is reading a steady 11 g/s at idle which is almost half the previous figure and is where it should be. Sadly, the underlying fault is still there. The canary in the coal mine is the scanner screen showing that " Self Adaption Enabled - NO " The long term fuel trim seems to be frozen at 1.6% and fails to budge whatever revs are thrown in. If you just look at o2 sensor voltages it's not much use - the downstream one starts at 0.442 v in open loop then quickly goes to around 0.850v when hot and controlled. It will vary sharply when snap throttled so we have a connection to the computer. However. this is just a carrier voltage for the oscillating voltage signal and that's what communicates with the fuel injector adaption circuits - we need a scope to see the full picture. It' a possibility that there is enough oil around to distort the signal enough to prevent acceptance by the control circuit. Interestingly before this post cat unit was changed it faulted as a short to positive - this fault has gone away but it may be the dreaded oil contaminated harness is playing tricks again. When the two "conveniently" placed o2 sensor connectors were cleaned up, the front sensor plug was soaking but working (self draining !) but the rear sensor plug was bone dry which seems odd. Looking at the construction it seems watertight - ie not porous so that any oil coming down the wiring will back up behind it. So - undertrays off again and we start with getting that investigated and maybe running a new o2 signal test wire to the ECU etc etc. I'm trying to avoid stripping out the engine harness and throwing it the the washing machine. I wonder what settings Hotpoint would suggest ? Is this an Oil or Nothing strategy ?

 

[LostKiwi]

Are you allowing the engine to get fully warmed up? The engine will run open loop until warm at which point it goes closed loop and will adapt fuel trims.

 

[hofnerpres1]

Hi LostKiwi. Yes - running up to 20 minutes per time and she settles out at 80 deg C. Starts in open loop and goes closed loop very quickly after a few minutes from cold. All readings looking normal now but Long Term Fuel Trim not moving. See screen shots attached. The fuel adaption system has never moved from "NO" and this is triggered by the post cat sensor so something is not getting through to the ECU despite it showing voltage readings. Looks like we are connected but not communicating. Will investigate more tomorrow.

 

[hofnerpres1]

Well, tried the old ECM again and good result. Once warmed up the LTFT and STFTs both reacted nicely and settled down at an almost perfect + or - 1%. Lambda spot on. The "Self Adaption Enabled" occasionally went to YES so everything was communicating cleanly. The only fly in the ointment was despite the FEBI Air Flow Sensor ( MAF ) giving a consistent 11 g/s at idle the system was still flagging a pending fault stating the signal was out of range. The original unit is a Siemens which might be the Mercedes dealer catalogue item at £445 - ie also "out of range". It remains to be seen whether this will settle down or cause a CEL. Will report here if the other electrical gremlins ever let the car run long enough.....

 

[hofnerpres1]

So after much labour we now have the car starting reliably ( see other posts ) but despite all the actual fuel values and trims reading text book perfect the Siemens ECU is working with the information but tells me it's not happy. We get a "Two Event" CELight. "Signal from the MAF not compatible" The Febi MAF is certainly built differently to the Siemens original - looks the same as the Chinese cheapies. I'm tempted to go for the Pierburg unit which has the same sensor layout as the Siemens OEM. Anybody had similar problems before we throw more money at yet another mystery ?

 

[hofnerpres1]

PROBLEM SOLVED. I found a new posh Pierburg MAF at a great price so went for it. Same result - far too much air going through the intake. So once again I've been fooled into changing parts due to inexperience and stupidity. Nothing wrong with the MAFs. Now fate steps in and it gets funny. I went for a test drive and it's a good idea to do this with the bonnet closed. It's also a good idea to check the bonnet catch is working properly ( the cable had to be disconnected to do the ESL removal). Idiot. Jack car up - remove RHF wheel - remove front corner plastic cover and the inner mudguard liner - this will give enough access to the cables - a strong pull on the junction box may work but better to release the cable in the cabin and reconnect the junction box end under the bonnet. ( LHD models are on the other side ). Whilst trying plan A and removing the undertray (totally pointless) my gaze was met by a blown joint in the intercooler piping where it goes into the large plastic muffler thing in front of the multi vee belt. Idiot me had clearly left off the spring clip. The day before this I had realised that my understanding of the air flow system and the supercharger system on this car was wrong. There are no flow diagrams available on the web but the penny finally dropped in that all those pipes are under boost pressure. Air goes into the MAF and is then immediately pressurised by the supercharger and runs across the engine, through the intercooler radiator and back into the throttle valve. Any bad joints or splits in these hoses will mean the supercharger sees less resistance and the air flow through the MAF will quickly increase and exceed the engine computers parameters. Fault codes will start to appear even though the engine sounds OK. So lesson is - HIGH AIR FLOW READINGS - PROBABLE AIR LEAK IN THE PIPING. Even at idle there is around 5 psi going over 10 psi when revving so it will leak. The now sealed system is showing an air flow of less than 4 g/s ( 14 kg/hr ) at idle. If you're seeing 20 g/s - there is a pressure leak somewhere. Your local garage are used to chasing vacuum leaks but probably not familiar with these Kompressors and like me throw a new part in before the penny drops. I will condense this ramble into a new post in order to highlight this trap. Also a few screen shots of what normal live OBD data should look like on a healthy M271 would have been very useful - I'll post these separately - would make a good sticky. Happy postscript - The MoT garage has just called - passed with honours. Amen and hallelujah.