Car model and year: 2012 Benz E550 with 70k miles
car starts to stumble
Check engine light comes on. Check engine light flashes at idle, but goes solid while driving and seems normal minus the light under light load.
misfire codes cyl 5,6,7,8 but nothing else. Clear codes.
Drive car around trying to repeat issue but Cars running flawless. Wtf? 3 days goes by, no issues, non cel. Driving home today, boom. Light comes on again.
Benz scanner to use:
Don’t start changing parts until you verify source of misfiring, may not necessarily be coils or even ignition.
First, check all cam phases/sensor connectors for oil.
The codes are telling you it’s a series of misfires so my first suspect is a connector or wire that may cause intermittent voltage drop to a component or components. Fuel issues can also cause misfires so at this point don’t rule out anything. Agree that you don’t get 4 coils going out at the same time or 4 bad plugs. However there may be an intermittent drop of source or trigger voltage to the coils. But again, a series or random misfires can initially point to several things.
So next questions:
– does your scanner capture freeze frame data ? It’s data the pcm captures at the time any codes are set.
– can you also see real time run data at idle and approx 2500 rpm?
– do you have any wiring diagrams?
Scan data is needed to see what other components are reporting to the pcm. Can provide valuable clues as to source issue.
Action and question:
Ok started with the phaser plugs. One plug had oil inside and on the plug itself. Other one had a little oil inside. Question. There is a similar style plug just on top of those that had quite a bit of oil on it. Not sure what it is or if that’s normal. I’ll clean plugs and see what happens before i move on.
You will see that there are essentially 2 plugs for each cam, one for the cam phaser (adjuster) and one for the cam sensor. So 4 plugs per bank, 8 plugs total for the engine pertaining to the cams. All of these need to be clean, if there is oil, the leaking component (sensor and/or phaser) needs to be replaced. Oil is not supposed to be in there and can wick into the harness, often resulting in misfires. Hopefully the oil has not penetrated into the harness too far, but you absolutely need to clean it as good as you can. Meaning very clean! Emphasizing this a lot, but oil in that harness can damage O2 sensors, pcm, all kinds of things. Worst case means a harness and/or component $$$$ replacement, so immediate attention is needed if you are finding oil. You can verify extent of oil by opening up the harness at a further point if accessible to see how far it’s gotten.
Arrie, yes I had mentioned fuel delivery issue as a possibility, but I’m thinking for now that the oil may possibly be the cause of the misfiring. Just have to see, but without any freeze frame or run data it’s really hard to tell anything else about where to look and internet diagnosis aren’t very accurate.
There’s a fuel rail for each bank, feeding the injectors for that bank, connected to a single line that usually has a fuel pressure regulator on it. Scantool (one that can do obd modes 1-10) can see the fuel pressure to verify, or most rails have a schraeder type valve that you can connect a pressure gauge to. That doesn’t tell the story about injector pulse, but that’s ok, that doesn’t seem to be an issue since it’s random misfiring. Again, it’s not really feasible that several coils or injectors have suddenly gone bad.
Pics can help to better understand what I’m looking at, you can see both phasers unplugged and then this plug is just on top of valve cover:
Yes those are the cam phaser/sensor connectors. Not good at all to have any oil inside of them. They are fairly straightforward to replace and I believe places like Pelicanparts, FCPEuro, etc. sell them as kits.
What to do next and problem to meet:
I pulled all 4. 3 have oil on them. Last oil change i used an extractor and sucked it out until it was dry (or so I thought). Added the correct amount and didn’t think about it. Drove around for a couple days and though I better check the oil. It was over filled by a quart (reasons i don’t love the extractor) and drained it out until it was good. Didn’t think much of anything but now seeing this I didn’t come away unscathed. I’ll toss in 4 new ones and chalk it up to a lesson learned. Glad I caught if after just a couple days.
My Left bank map sensor was reading .01 voltage. Right side was 4.19. Figured it was an easy fix. Not so fast lol swapped sensors side to side and voltage still read .01 on left so both sensors are good. Getting 5v on both plugs and ground is .03. I’m guessing it’s the signal wire giving me fits. I’m not sure how to diagnose it from here though.
You did the smart thing by simply swapping sensors to verify operation.
Just to ensure we are talking about the same thing, are these 4 wire connectors?
You really need a wiring diagram for your vehicle/engine to be able to trace wiring and connectors from the sensor back to the assigned ECU terminal.
I’ve got the diagram for my V6, so I’m not sure if any of this applies, but the process may help:
On left bank sensor (identified as Hot Film MAF sensors on the WIS diagram):
Term 1 Rd/Gn wire, I believe this is + feed (but I don’t see battery voltage identified), goes to term 16 of ECU
Term 2 Bl/Bn wire, shows as ground, goes to term 18 of ECU
Term 3 Bl/Ye wire, shows as 5v ref to sensor, goes to term 68 of ECU
Term 4 Bl/Gn wire, shows as ref signal from sensor, goes to term 70 of ECU
If I was looking at this, I would use a backprobe at the pins at the ECU to verify continuity and voltage readings from each terminal of the sensor itself. Using a correct backprobe kit will help ensure the connector weathersleeves are not damaged. Not an expensive item.
I think you were seeing some residual voltage at the ground, check from connector to body bolt (ground) for continuity. This will verify that the wire all the way to the ECU (and thru the ECU) is properly grounded.
You saw 5v reference, need to ensure that the 5v reference remains with both engine off and engine running,
The return voltage is supposed to vary, depending on engine run factors. Check the corresponding ECU terminal to see if that same 1.8 voltage is present and if changes at all with engine running.
Since the sensors were swapped, it’s a good assumption that they are ok. I would verify that Term 1 wire voltage. Check the right side sensor to see what the voltage reading may be, Term 1 on that one too. As you already had started to do, look at a known good sensor readings to compare.
Again, this is the diagram for my car and may (probably is) incorrect for yours, but the idea is to verify that all voltage/grounds present at the ECU terminals are continuous all the way to the actual sensor.
There may be a possibility that the cam sensor oil leak may have done some damage to the wiring but lets hope that it might be a corroded or poor connection somewhere.
My car has 3 wire map sensors. I’m Hoping the oil wicking isn’t the issue and wouldn’t know where to begin on that. If i can get a diagram i can find what pin it is i could atleast eliminate that.
Getting a WIS/EPC is less than $20 and would be useful to you.
You can also simply trace the wiring color back to the ecu. If you think it’s the right wire first do a simple continuity check, ecu to sensor connector. If correct wire you can then check voltage. 3 wire is pretty straightforward, should be ref volts, signal return and ground, as you found. Do this for both sides so you can compare, should be the same or very very close.
I like using my scope for that, as the voltage waveform is easily seen and compared, plus it can be reviewed in milliseconds as the sensor works. However I think a digital voltmeter should at least be able to capture voltage window for comparison.
Picked up a foxwell NT520 scanner. Did some live data and watched the front o2 sensors. Both were right around 1.0/1.2 when all of a sudden the left bank jumped really lean like 1.28/1.3. Just after that started the car started to misfire. Looks like it’s my o2 sensor after all.
Credits to @ Garagebuilt and @ Mud who are super members of https://mbworld.org/forums/
Thanks to Laurance collection.