C200 KOMPRESSOR SE Coupe Auto (M271 petrol) ... SEVERE MISFIRE

[johnsco]

Mrs J has just reported that her car is running badly.
I took it for a run.
No problem when cold, but after 5 minutes as it approaches running temperature, a misfire becomes evident and gradually gets much worse.
A scan with my pretty-basic OBD-II scanner tells me this:
P0036 HO2S Bank1 Sensor 2 Heater Circuit
P0171 System Too Lean (Bank 1)
P0136 O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)
P0301 Cylinder 1 Misfire Detected
P0302 Cylinder 2 Misfire Detected
P0303 Cylinder 3 Misfire Detected
P0304 Cylinder 4 Misfire Detected

I could do with a bit of guidance as to what this is telling me.
Obviously, there is a serious misfire when the engine is warm.
Help please.
Thanks.

 

[johnsco]

Hi guys ....
Any help appreciated - Please.
J.

 

[alecmascot]

Replace the O2 sensor ?
Are there two ?

 

[johnsco]

Thanks alecmascot.
I've been using my brain a bit and I am guessing that there are two heated 02 sensors - the first between the exhaust manifold and the cat ... the second downstream from the cat.
The data suggests that the heater circuit of Sensor2 may be open-circuit - or Possibly a break in the wiring to the heater circuit.
I'm going to have a look on the nemigaparts site to search for the location of the Sensor2
I could then get a meter onto the wires and check for the resistance, which I would guess to be between 5 and 10 ohms.
Any thoughts ?
I'll check it out and let you know.
J.

 

[W1ghty]

Thanks alecmascot.
I've been using my brain a bit and I am guessing that there are two heated 02 sensors - the first between the exhaust manifold and the cat ... the second downstream from the cat.
The data suggests that the heater circuit of Sensor2 may be open-circuit - or Possibly a break in the wiring to the heater circuit.
I'm going to have a look on the nemigaparts site to search for the location of the Sensor2
I could then get a meter onto the wires and check for the resistance, which I would guess to be between 5 and 10 ohms.
Any thoughts ?
I'll check it out and let you know.
J.

Any oil in the camshaft magnet’s electrical connections on the top front of the engine ?
Oil can leak and contaminate the front electricals and the O2 sensors as these are the low point .

 

[johnsco]

Hi W1ghty
I can always count on your good advice.
I replaced the two camshaft magnets at the same time as the timing chain/timing gears/tensioner were replaced - about a year ago.
At the same time I installed the two extension pigtails to limit any future oil contamination.
At the time, there was some oil leakage, but it did not appear to have gone very far into the loom and connectors.
So - I'm hoping it's not that.
The misfire is pretty horrendous - Really bad.
Is a failure of the #2 O2 sensor likely to have such a major effect on the engine running ?
When I fire it up, it starts perfectly, and appears to run perfectly for the first 5 minutes until the temperature approaches normal running temperature.
It then quickly starts to get lumpy and quickly deteriorates to the point where it gets close to cutting out when I stop.
I really don't want to drive it like that for fear of damaging something.
Any thoughts appreciated.
J.

 

[W1ghty]

Hi W1ghty
I can always count on your good advice.
I replaced the two camshaft magnets at the same time as the timing chain/timing gears/tensioner were replaced - about a year ago.
At the same time I installed the two extension pigtails to limit any future oil contamination.
At the time, there was some oil leakage, but it did not appear to have gone very far into the loom and connectors.
So - I'm hoping it's not that.
The misfire is pretty horrendous - Really bad.
Is a failure of the #2 O2 sensor likely to have such a major effect on the engine running ?
When I fire it up, it starts perfectly, and appears to run perfectly for the first 5 minutes until the temperature approaches normal running temperature.
It then quickly starts to get lumpy and quickly deteriorates to the point where it gets close to cutting out when I stop.
I really don't want to drive it like that for fear of damaging something.
Any thoughts appreciated.
J.

Thanks buddy , my mechanical knowledge pretty much extends to only what’s gone wrong with my cars over the years .
I would have thought that an O2 sensor issue would be constant rather than after 5 minutes running .
Do you have any breakdown cover who could come out and scan the car , maybe with a better scanner than yours ??

 

[johnsco]

Thanks W1ghty
I'm pretty-much the same - Completely self-taught - Most of my knowledge obtained before cars were computers on wheels.
There's two parts to the O2 sensor:
1. The heater circuit, which appears to be kaput - See post #1 ... P0036
2. The input to the engine management system, which again is showing a fault - As I would expect if the heater circuit was open circuit - See post #1 ... P0136

I think I'm convincing myself that the O2 sensor is the problem.
I'll change it.
If this doesn't fix it, I'll have to get my favourite garage to scan it for me.

Thanks again.
J.

 

[W1ghty]

Thanks W1ghty
I'm pretty-much the same - Completely self-taught - Most of my knowledge obtained before cars were computers on wheels.
There's two parts to the O2 sensor:
1. The heater circuit, which appears to be kaput - See post #1 ... P0036
2. The input to the engine management system, which again is showing a fault - As I would expect if the heater circuit was open circuit - See post #1 ... P0136

I think I'm convincing myself that the O2 sensor is the problem.
I'll change it.
If this doesn't fix it, I'll have to get my favourite garage to scan it for me.

Thanks again.
J.

Might be worth unplugging the large ECU plug on the top right of the engine just to make sure there is no oil in there (from before the camshaft magnet replacement) .
Good luck

 

[johnsco]

Might be worth unplugging the large ECU plug on the top right of the engine just to make sure there is no oil in there (from before the camshaft magnet replacement) .
Good luck

Thanks W1ghty
I'll do that before I throw any more money at it.
Cheers.
J.

 

[RogerSausage]

A lean code and misfires on all cylinders for an M271, could well be a perished breather hose under the airbox.

 

[W1ghty]

A lean code and misfires on all cylinders for an M271, could well be a perished breather hose under the airbox.

Good call !
I replaced mine as a precaution the other year

CLK , changed vacuum breather hose A271 018 1282

This had been on my list for a while as a known M271 engine issue . The hose splits and puts the car into limp mode . I had been waiting for some decent weather to attempt the job . The hose I bought some months ago . It’s a fiddly Job and you need to make sure you have the correct tools to...

forums.mercedesclub.org.uk

 

[Bellow]

Is a failure of the #2 O2 sensor likely to have such a major effect on the engine running ?

No. It is there merely to monitor the cat's efficiency.

.
There's two parts to the O2 sensor:
1. The heater circuit, which appears to be kaput - See post #1 ... P0036

The lack of heater just means it will take longer for meaningful readings from the sensor to be available for the ECU and, when idling said readings can drop away (for the want of adequate temperature).
The heated element will be fused so I'd work from supply side towards the sensor. There should - when you get to checking the sensor - be a measurable resistance across the heating element. I have in mind the first sensor which inputs fuelling. When the sensor cannot provide information the ECU operates as 'closed loop' (pre-set calibrations) until such times as the info is available. Closed loop cannot be so far from what's required as to cause very poor running. I'd expect it to be a touch rich or lean - but not by that much.

2. The input to the engine management system, which again is showing a fault - As I would expect if the heater circuit was open circuit - See post #1 ... P0136

Are you suggesting that that fault is the ECU highlighting a lack of signal from the sensor or that it has identified the failed heating element circuit?
As per above, the want of the heating element wont stop it running - just defaults to closed (or open - depending on terminology used) loop. A lack of signal? - I'd expect to be read by the ECU as the same as merely cold.

When it's running lumpy, what's it like at the tailpipe? Black smoke, smelling of fuel - rich? Or clean - lean?
I'd be wary of the 'lean' diagnosis. Large scale mis-fires will release unburned oxygen into the exhaust - which the sensor can possibly read as lean. Tailpipe will tell you more.

AFAIK, the first O2 sensor influences - but doesn't control - fuelling. It does this by correcting 'fuel trims' - not by directly intervening on fuelling decisions on the fly, reacting to every signal the sensor sends. It should run tolerably well on closed loop - just lacking that last part of control finesse which is about keeping the mixture at stoichiometric for the benefit of the cat as anything else. The second sensor monitors cat activity and I'd expect the EML to illuminate if either it isn't working or sending 'out of bounds' signals.

 

[johnsco]

Might be worth unplugging the large ECU plug on the top right of the engine just to make sure there is no oil in there (from before the camshaft magnet replacement) .
Good luck

Do you mean the top left side of the engine - The Siemens unit - Pt. No: A271 153 91 79 ??
I removed the large multi-pin socket.
There's no oil at all.
Also the cam-shaft magnets that I replaced are not leaking - Good news - They were quite bad when I bought the car.

 

[johnsco]

A lean code and misfires on all cylinders for an M271, could well be a perished breather hose under the airbox.

The breather hose under the airbox was checked about a year ago when the airbox had to come out.

 

[johnsco]

Thanks Bellow for your detailed description.
As I thought - You've confirmed that the two O2 sensors are only there to optimise the air/fuel ratio and would not be the cause of a serious misfire.
I intended to give the car a run today to have a look at the tail-pipe - as suggested.
However - Things took an unexpected turn for the better.
I removed the large connector from the ECU to check for any oil contamination.
Fortunately, I didn't find any.
I took the car for a run, but found that the severe misfire had disappeared and the car ran up to normal temperature with no misfire.
[Thank you God]
The running was OK, but I noticed a slight "hunting" at idle.
Lifting the bonnet, I could hear a hissing.
I traced this to the connector on the top of the air box.
See photo.
This is the pipe that goes to the brake vacuum servo.
The connector that appears to be "snap-on" had popped off.
I pushed it back into place by pushing down.
The hissing stopped.
The hunting stopped.
I checked the tail pipe.
There was no smoke or excessive blackness.
I drove another mile home with no problem at all - Ran perfectly.

How to explain it - I'm not sure.
Removing and replacing the ECU connector was all I did to stop the severe misfire - Maybe a dirty connection.
Maybe the vac pipe connector was moving about and varying the effect.
Anyway - All appears OK - A no-cost repair.
Tomorrow I'll clear the DTC codes and run it for a few days and re-scan.

Thanks guys for your help.
I've learned a lot.

 

[W1ghty]

Do you mean the top left side of the engine - The Siemens unit - Pt. No: A271 153 91 79 ??
I removed the large multi-pin socket.
There's no oil at all.
Also the cam-shaft magnets that I replaced are not leaking - Good news - They were quite bad when I bought the car.

Yep top left as you sit in the car . Good that there is no oil in it .

 

[W1ghty]

Thanks Bellow for your detailed description.
As I thought - You've confirmed that the two O2 sensors are only there to optimise the air/fuel ratio and would not be the cause of a serious misfire.
I intended to give the car a run today to have a look at the tail-pipe - as suggested.
However - Things took an unexpected turn for the better.
I removed the large connector from the ECU to check for any oil contamination.
Fortunately, I didn't find any.
I took the car for a run, but found that the severe misfire had disappeared and the car ran up to normal temperature with no misfire.
[Thank you God]
The running was OK, but I noticed a slight "hunting" at idle.
Lifting the bonnet, I could hear a hissing.
I traced this to the connector on the top of the air box.
See photo.
This is the pipe that goes to the brake vacuum servo.
The connector that appears to be "snap-on" had popped off.
I pushed it back into place by pushing down.
The hissing stopped.
The hunting stopped.
I checked the tail pipe.
There was no smoke or excessive blackness.
I drove another mile home with no problem at all - Ran perfectly.

How to explain it - I'm not sure.
Removing and replacing the ECU connector was all I did to stop the severe misfire - Maybe a dirty connection.
Maybe the vac pipe connector was moving about and varying the effect.
Anyway - All appears OK - A no-cost repair.
Tomorrow I'll clear the DTC codes and run it for a few days and re-scan.

Thanks guys for your help.
I've learned a lot.

View attachment 139319

Hallelujah buddy !