W209 fluctuating o2 sensor values

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aliharry

Active Member
Joined
Jul 24, 2020
Messages
81
Location
surrey
Car
Mercedes W202 C230k
Hi All,

My 2006 CLK 500 has a rough idle and a multiple cylinder misfire (usually cylinder 3 alone but sometimes multiple) however I’ve replaced all the spark plugs and the coil pack + Leads on cylinder 3 to no avail. I’ve noticed my o2 sensors are fluctuating hugely in Voltage when the car is idling and wondered whether that indicated a clogged cat or other issue. I’ll put the results below but i have read that big fluctuations can mean a clogged cat.

These are the lowest and highest values recorded while sitting idling over around 30 seconds.

B= bank
S = sensor
Oxygen sensor b1-s1 ( 0.100-0.710)

Oxygen sensor b1-s2 (0.2-0.560)

Oxygen sensor b2-s1 ( 0.095-0.715)

Oxygen sensor b2-s2 (0.695-0.700)

Could anyone give some expertise on what this could mean and whether this is normal, as shown b2-s2 does not seem to fluctuate much and seems correct. Any help would be appreciated

thank you
 
As you have probably worked out, the values for B1-S1, B2-S1 and B2-S2 all look to be good. The S1 maximum values are slightly lower than the ones I see on my CLK 500 - I get around 0.750 to 0.800 but I wouldn't worry.

The problem is B1-S2. This sensor is still switching rich/lean. This could be a duff bank 1 cat, but it could also be that bank 1 of the engine is running very rich, especially as the bank 1 s2 sensor is still trying to switch lean. Can your diagnostic reader show the long & short term fuel trim values? A large disaprity between B1 and B2 might give a clue. Excessive richness on one bank could be due to a faulty injector or an air leak into the exhaust manifold.
Ian.
PS. Could always be a duff B1-S2 sensor or wiring fault as well as something else!
 
As you have probably worked out, the values for B1-S1, B2-S1 and B2-S2 all look to be good. The S1 maximum values are slightly lower than the ones I see on my CLK 500 - I get around 0.750 to 0.800 but I wouldn't worry.

The problem is B1-S2. This sensor is still switching rich/lean. This could be a duff bank 1 cat, but it could also be that bank 1 of the engine is running very rich, especially as the bank 1 s2 sensor is still trying to switch lean. Can your diagnostic reader show the long & short term fuel trim values? A large disaprity between B1 and B2 might give a clue. Excessive richness on one bank could be due to a faulty injector or an air leak into the exhaust manifold.
Ian.
PS. Could always be a duff B1-S2 sensor or wiring fault as well as something else!
Hi there, thanks for the help so far I’ll add my fuel trim values below

Short term due trim 0.8% - 1.6%

Long term trim -7

Short term fuel trim b2 0 - 2.6

Long term fuel trim 4.7
 
I was going to compare your fuel trim values with mine, but I can't find the values on my iCarSoft 980. I remember seeing them, but I can't remember which menu shows them.
Since the problem is apparent on bank 1, and not on bank 2, I suggest that you use the time-honoured mechanism for finding or eliminating faulty components - swapping items between the left & right banks.
You've eliminated coil pack+leads+both plugs on cylinder 3, so I suggest that you swap injector #3 with one on the opposite bank, disconnect the battery for 10 minutes (to clear any current values) then go for a test drive (or idle until up to temperature if the misfire is too bad to drive). Then re-check the oxygen sensor & fuel trim values to see if the fault stayed or moved.
If that proved inconclusive, the next swap would be the upstream (B1-S1 and B2-S1) oxygen sensors and repeat testing.
After that, I'm a bit stumped - time to call in the professionals??? Replacing the bank 1 cat(s) without being certain of their failure could be a waste of money. I doubt that they're cheap.

As for the misfires reported both on cylinder 3 and multiple cylinders: I saw exactly the same problem when a coil pack failed on mine. The coil pack on cylinder 6 had failed, but I kept getting 'multiple misfires' listed as well which threw me off the scent. Replacing #6 coil pack fixed it, but only after I'd replaced all the leads as well (which didn't fix the problem). In the end, it was the 'smooth running' values on the iCarSoft that consistently identified cylinder 6.

Ian.
 
I was going to compare your fuel trim values with mine, but I can't find the values on my iCarSoft 980. I remember seeing them, but I can't remember which menu shows them.
Since the problem is apparent on bank 1, and not on bank 2, I suggest that you use the time-honoured mechanism for finding or eliminating faulty components - swapping items between the left & right banks.
You've eliminated coil pack+leads+both plugs on cylinder 3, so I suggest that you swap injector #3 with one on the opposite bank, disconnect the battery for 10 minutes (to clear any current values) then go for a test drive (or idle until up to temperature if the misfire is too bad to drive). Then re-check the oxygen sensor & fuel trim values to see if the fault stayed or moved.
If that proved inconclusive, the next swap would be the upstream (B1-S1 and B2-S1) oxygen sensors and repeat testing.
After that, I'm a bit stumped - time to call in the professionals??? Replacing the bank 1 cat(s) without being certain of their failure could be a waste of money. I doubt that they're cheap.

As for the misfires reported both on cylinder 3 and multiple cylinders: I saw exactly the same problem when a coil pack failed on mine. The coil pack on cylinder 6 had failed, but I kept getting 'multiple misfires' listed as well which threw me off the scent. Replacing #6 coil pack fixed it, but only after I'd replaced all the leads as well (which didn't fix the problem). In the end, it was the 'smooth running' values on the iCarSoft that consistently identified cylinder 6.

Ian.
How can I use the smooth running values to identify a cylinder being problematic, I’ll look at getting the coil packs replaced on that cylinder
 
To identify the misfiring cylinder, go into the 'smooth engine running' menu on the diagnostic tool (it will be under the engine management or motor electronics menus -> 'Actual values'), check that the pre-conditions are met (engine up to temperature, air-con OFF), then make a note of the smooth running value for each of the 8 cylinders. These show the contribution made by each cylinder to the engine idle speed. A small variation per cylinder is normal, but misfiring cylinders show a value significantly different to the others.
Once you've found the misfiring cylinder, the fault could be due to spark plug, lead, coil pack, injector, low compression. Assuming the test confirms that it really is cylinder 3, you've already changed the coil pack, plugs and leads, so the next step would be checking the injector (by swapping with one from a good cylinder). If the misfire stays with cylinder 3, then a compression test would be a good next step.
If the misfire remains, but none of the cylinders show an unusual smooth engine running value, then I'd start to look at things common to all the cylinders - fuel pressure/delivery, MAF, air filter, valve timing, CPS.

While you have the diagnostic tool handy, it would be sensible to check the other engine related components for stored fault codes - any faults might shed a bit more light on the problem.

Ian.
 
I found the LTFTs on the OBD reader:
Bank 1: +3.1%
Bank 2: +4.7%

So, well within tolerance and not that far from the values you're getting.

Ian.
 
Hi
I found the LTFTs on the OBD reader:
Bank 1: +3.1%
Bank 2: +4.7%

So, well within tolerance and not that far from the values you're getting.

Ian.
I did the tests again today all my smooth running conditions are fine and everything is steady

Short term fuel trim B1 -0.8 - 1.6%

Long term b1 -4.7

Short term b2 0-3.1

Long term b2 -3.9

Intake pressure 30kpa

Ignition timing advance -5-15

O2 sensor b1 s1 0.09-0.685

Short term b1s1 -0.8 - 1.6

O2 b1-s2 0.715-0.720

Short term fuel trim b1 s2 = 99.2

B2-s1 0.08-0.685

Short term fuel b2s1 -0.8 - 0

O2 b2s2 = 0.690

Short term b2s2 99.2%

So from this I’ve noticed bank 1 sensor 1 and bank 1 sensor 2 both vary from 0.08 to around 0.685 they should only vary about 5% from 0.650-0.700 these two are varying a good %80 which tells me it’s something to do with bank 1, either a bad cat or something else. I think I’ll get the cat checked since I’m getting an exhaust system anyway. And then if not the cat I’ll have to check the fuel delivery system next likely
 
So from this I’ve noticed bank 1 sensor 1 and bank 1 sensor 2 both vary from 0.08 to around 0.685
?? Not with the figures you give above - bank1 sensor 2 is 0.715-0.720, a more-or-less steady value. +/- 5 millivolts can be ignored on the oxygen sensors - there're not that accurate.

The oxygen sensors S1 (on both bank 1 and bank 2) are upstream of the catalytic converter, and are supposed to switch between roughly 0.1 volt and 0.8 volt. These are the sensors used to derive the short term fuel trims for each bank. From the figures that you've given, they look fine.
The oxygen sensors S2 (again on both bank 1 and 2) are downstream of the cats, and once the cats have warmed up, should show a steady reading, which according to the values you give above, is the case for both banks. The S2 sensors are used to check the operation of the S1 sensors and the cat. If the cat is faulty, then the S2 values follow the S1 values, which is not the case here.

The long term fuel trim values for both banks are less than 5%, well within the tolerance of a properly running engine. Curiously, the same numbers as I get, but in the opposite direction!!!

Are you still getting the misfire on this most recent test? The O2 sensor & fuel trim values look quite good.

Ian.
 
?? Not with the figures you give above - bank1 sensor 2 is 0.715-0.720, a more-or-less steady value. +/- 5 millivolts can be ignored on the oxygen sensors - there're not that accurate.

The oxygen sensors S1 (on both bank 1 and bank 2) are upstream of the catalytic converter, and are supposed to switch between roughly 0.1 volt and 0.8 volt. These are the sensors used to derive the short term fuel trims for each bank. From the figures that you've given, they look fine.
The oxygen sensors S2 (again on both bank 1 and 2) are downstream of the cats, and once the cats have warmed up, should show a steady reading, which according to the values you give above, is the case for both banks. The S2 sensors are used to check the operation of the S1 sensors and the cat. If the cat is faulty, then the S2 values follow the S1 values, which is not the case here.

The long term fuel trim values for both banks are less than 5%, well within the tolerance of a properly running engine. Curiously, the same numbers as I get, but in the opposite direction!!!

Are you still getting the misfire on this most recent test? The O2 sensor & fuel trim values look quite good.

Ian.
Hi there sorry I meant b2 s1 and b1 s1 we’re both varying hugely however if you say that is correct then maybe I should look elsewhere, and on the most recent test I had been driving it with the rough idle still however I was able to do kickdown with full acceleration without an engine light coming on, so I’m not too sure because it’s intermittent and sometimes will allow me to fully rev out to redline however sometimes it will only allow me to do a little acceleration before triggering cylinder 3 or multiple cylinder misfire. I’m not sure then might have to start checking components
 
?? Not with the figures you give above - bank1 sensor 2 is 0.715-0.720, a more-or-less steady value. +/- 5 millivolts can be ignored on the oxygen sensors - there're not that accurate.

The oxygen sensors S1 (on both bank 1 and bank 2) are upstream of the catalytic converter, and are supposed to switch between roughly 0.1 volt and 0.8 volt. These are the sensors used to derive the short term fuel trims for each bank. From the figures that you've given, they look fine.
The oxygen sensors S2 (again on both bank 1 and 2) are downstream of the cats, and once the cats have warmed up, should show a steady reading, which according to the values you give above, is the case for both banks. The S2 sensors are used to check the operation of the S1 sensors and the cat. If the cat is faulty, then the S2 values follow the S1 values, which is not the case here.

The long term fuel trim values for both banks are less than 5%, well within the tolerance of a properly running engine. Curiously, the same numbers as I get, but in the opposite direction!!!

Are you still getting the misfire on this most recent test? The O2 sensor & fuel trim values look quite good.

Ian.
I should also add there’s a rotten egg smell the whole time the car is running which also leads me to believe the cat is bad
 
The rotten egg smell does point to a duff cat. The CLK500 has 2 cats on each bank, so this could get very expensive - I hope that the people checking the cat can tell you if both (or even all 4) are working or defective.
Now that you've replaced the coil pack, plugs & leads, will that be enough to stop the replacement cat(s) being damaged? It wouldn't be sensible to get new cats without fixing whatever caused their failure in the first place.
Ian.
 
The rotten egg smell does point to a duff cat. The CLK500 has 2 cats on each bank, so this could get very expensive - I hope that the people checking the cat can tell you if both (or even all 4) are working or defective.
Now that you've replaced the coil pack, plugs & leads, will that be enough to stop the replacement cat(s) being damaged? It wouldn't be sensible to get new cats without fixing whatever caused their failure in the first place.
Ian.
I’ll be visiting an exhaust place soon I think my best bet is to replace the cat and do the coil packs and leads as preventive maintenance
 

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