Still a misfire on cylinder 3
02-19-2019, 08:36 PM
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Still a misfire on cylinder 3
So I am at loss here . I have replaced all the coils, spark plugs , and air mass flow sensor. I don’t have a check engine light but when I went to my mechanic he connected the scanner and is gives an error p0303 cylinder 3. He checked the spacing of the spark plug and also swapped the coils. He told me after swapping the coils to drive it for a bit and see if the code poops again. I really don’t know what else to look for. You can hear the misfire as it has a bit of a rough idle. The mechanic said that it’s not enough to trigger an check engine light
Last edited by clb00196; 02-20-2019 at 02:40 PM.
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vettebk (11-04-2022)
02-19-2019, 09:52 PM
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'15 S550, '10 E350 P1/P2, '02 ML320
It is just one cylinder, so might just have it checked. Injectors can be tested to verify their spray pattern. Is the car being diagnosed just on codes alone or is your mechanic using the Mercedes SDS? I don't like to just replace stuff because it can get expensive quickly!
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vettebk (11-04-2022)
02-20-2019, 03:18 PM
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2014 E350 Wagon, 2007 S600, 2009 SL550 with ABC removed, 2011 S400 with Hybrid removed
Replacing parts is not the way to go (been there, done that, it's a hit and miss, mostly a miss). For the next steps I would go with injector testings (all the voltages etc, using a scope if needed, there are procedures posted). I once had a similar case, after replacing all the ignition parts, replacing the injector on the offending cylinder etc it turned out the problem was that one REPLACEMENT electrical part was bad. So the best investment is finding a mechanic who has spent years troubleshooting MB misfire and really has a clue, or devote time and equipment money to become such an expert yourself :-). There is a story on one of the forums where a dealer replaced everything, and went to disassembling/replacing internal engine components and did not find the root problem in case of a V12 engine - after the engine was put together it was still misfiring (rare, but I assume true). Let us know how it gets resolved
Last edited by George993; 02-20-2019 at 03:22 PM.
02-20-2019, 07:59 PM
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George,
my mechanic is very good with Mercedes . He said that it could be the ecu . He has owned over 100 cars and is straight from Germany . I am hoping it’s something stupid.
my mechanic is very good with Mercedes . He said that it could be the ecu . He has owned over 100 cars and is straight from Germany . I am hoping it’s something stupid.
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Relevation (11-04-2022)
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02-20-2019, 08:00 PM
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The reason I did the coils, plugs, and maf is that I had many misfires. At first and then afterwards they all went away. Like I said there isn’t an check light on
02-21-2019, 01:28 AM
#9
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So there isn't a check engine light on? Does the engine shake when running? Idle? cruising? S550? Mileage? Could be a worn out intake or exhaust valve seat.
02-21-2019, 05:22 AM
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2007 Mercedes s550, 150k. The engine doesn’t shake at all. You can hear n feel it when the car is idling. It’s very faint sound n feel. The valve gasket was replaced along with the other parts that I listed above. Including engine mounts The p303 is pending and according to my mechanic the misfire is sovmittor that it doesn’t trigger a check engine light.
02-21-2019, 09:37 AM
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S550 w140, 07 S550
2007 Mercedes s550, 150k. The engine doesn’t shake at all. You can hear n feel it when the car is idling. It’s very faint sound n feel. The valve gasket was replaced along with the other parts that I listed above. Including engine mounts The p303 is pending and according to my mechanic the misfire is sovmittor that it doesn’t trigger a check engine light.
I had cylinder 8 misfiring bad. I swapped coils from another cylinder and the code still showed cylinder 8 as misfiring. Then I turned off the car, delete the code and turn it back on. 1 minute after it started showing cylinder 7 (as it had the bad coil in it from cylinder 8). So i had a bad coil which I replaced and all is good.
I hope your mechanic delete the code when doing a coil test. On that note, assuming he did, I would see if you can swap the injectors. It is rare for the ECU to go bad and misfire cylinders on these cars. At least I haven't heard of such issues with the M273..
02-24-2019, 12:14 PM
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2007 MB S550 4Matic
The valve seat was my problem. Just a suggestion before taking it all apart, do a compression check first, then if your mechanic has a bore scope stick in the sparkplug hole and turn it over and see if the valves are seating. I had a valve that had carbon build up on the seat and it was throwing the cod,e.
