cruiser02

Hey All,

My car is sitting at 44,500 miles and I have a bunch of misfires on the left bank of the engine. Car shakes at idle and can barely be driven. It happened once, I shut the car off and after turning it back on it was fine. Then about 300 miles later same thing. Only this time I could only get rid of the codes by resetting via obd. The car has always been serviced on time, however I do not have records of the spark plugs being done.

The Mercedes dealer diagnosed the spark plugs and two coils (cyl 5 & 6). I had it towed home and will be doing them this weekend, all spark plugs and all coils. However, I have a feeling this might not fix it.. I've never had a car where the plugs have gone at such young mileage.

These are the following codes I have:
P300 - Random misfires
P305 - Misfire cyl 5.
P306 - Misfire cyl 6.
P307 - Misfire cyl 7.
P308 - Misfire cyl 8.

Anyone had this happen? Any other thoughts on what it could be, or am I overreacting?

Thanks!

Shadly1

Are those stored codes or current codes? If two of them are stored and two are stored and current, there's a good chance they swapped the coils to diagnose.

cruiser02

You could be onto something, unfortunately my OBD scanner doesn't say which is which. I did reset them all and they came back though. The first time it happened it was cyl 5 and cyl 7 misfiring.

Shadly1

If you're committed to plugs and coils, that's probably going to fix it. I think I would pop out all the coils on the left bank and look down the tube around the spark plugs with a flashlight. Look for water or oil in there. Also, check the coil boot for any sign of spark escaping.

cruiser02

Will do, thanks a lot! I have a friend that works at a Mercedes shop and he’ll be doing the work this weekend.

BOTUS

dead coil packs on a merc is a design feature

decent diagnostics will tell you exactly which ones are failing - you can then move them around and watch the fault go with it

chassis

Check connectors for cam position sensors and cam magnets for oil. If oil is present check ECU connector for oil. Remediate any oil with contact cleaner then order new magnets and sensors to replace them.

cruiser02

Hoping that’s the case and that the MB dealer did a decent diagnostic. Will be replacing all on Saturday.

cruiser02

I checked them about 4k miles ago. Replaced two that had oil. Will check again after the new plugs and coils. Thanks

cruiser02

Update: My friend came over and installed the new plugs and coils. Old plugs on the misfiring cylinders were BLACK! Coils looked kinda bad too. No fluids in the spark plug holes.

After resetting the codes, we fired up the car. Ran okay for 10 seconds and check engine light started flashing and car was shaking again. Scanned and got the same codes.

We checked the connections of plugs and coils and tried again, without success. No oil on the ECU connections. He’s thinking fuel pump now. I’ll be towing it to his shop next week, where they can work on it. What a pain in my ***

chassis

You have one bank misfiring. Why?

Fuel pump problem would affect all cylinders.

Unlikely that 4 injectors on the same bank are all bad.

Did the cam sensor plate slip? Try replacing cam position sensors and cam magnets?

What retailer sold you the coils?

Bad chain tensioner or chain jumped a tooth or three?

cruiser02

Friend said it could be the fuel pump. Driver side exhaust tip had a strong fuel odor while the passenger did not. It is now throwing misfire codes for cyl 5,6,7.

Haven't looked at cam sensors or magnets yet.

Bought the plugs and coils from fcpeuro.com.

Will have to wait to take it to his shop for further diagnosis. Seeing the black plugs, we both thought it'd fix it (mercedes dealer was 99% sure the plugs were the culprit).

BOTUS

posh diagnostics will give you can cam angles if a long way out should throw errors
for an odd reason they keep throwing in petrol when it knows the cyl isn't firing

blocked CAT this is very common on Mercs ?

cruiser02

Guess this is another reason why dealerships can’t be trusted. The advisor was “99%” sure that it’d be fixed after plugs and coils.

Will update later once the Indy shop looks at it.

chassis

Did the friend explain why the fuel pump would cause problems on one bank?

Codes and black tailpipe confirm what you already know: you have a problem on one bank.

Codes in four cylinders rules out a scored cylinder for now. Focus on camshafts: sensors, magnets and chain drive.

Buy plugs, coils, sensors and magnets from the dealer only.

You have Chinesium coils and plugs and/or a failed cylinder head and/or a messed up chain drive.

Did you do a compression check yet?

If compression is good, replace plugs, coils, sensors and magnets with dealer purchased parts.

cruiser02

Thanks! The fuel pump is just a guess. Won’t know for sure until it’s diagnosed via computer.

The plugs and coils are OE, not even OEM, so they should be good. Compression check was done on cyl 5 only and it passed.

chassis

If coils and plugs were not purchased from the dealer they are Chinesium and should be considered NOK. Not OK.

cruiser02

Original Equipment (OE) means that it is exactly what the dealership has. Fcpeuro also has lifetime warranty on all parts. They’re definitely not “Chinesium.”

JohnLane

OP you need a computer with Xentry.

Get ahold with BenzNinja. He can fix you up.

cruiser02

Yeah, if I would’ve worked on it myself that’d be the first thing to get. What’s the going rate for one anyway? Thanks.

chassis

By this statement are you saying you object to buying a set of plugs and coils from the dealer?

JohnLane

Contact Peter if you haven’t already. Cost of computer and membership are an investment that pays off the first time you don’t have to take the dealer’s word for it. It can be used for all modern Mercedes, and is excellent for jailbreaking features we don’t get here in the USA.

Peter will suggest a used computer and a C4. He will send a 500gig SSD loaded with software. Plug it in and Peter will set the computer up remotely. Then he can assist with diagnosis and change any software in the car you wish for. Your car need not suddenly stop accelerating at 130.

I’ll help.... www.benzninja.com enjoy!

cruiser02

Update: It is the infamous “motor oil in engine wiring harness” that plagues m278 engines. It has leaked enough to reach the oxygen sensors on the left bank, hence the misfire codes.

They have also stated that it has reached the ECU a little bit. They will be attempting a cleaning of all affected areas and see if that fixes it. I have a feeling neither they nor I want to replace the whole harness.

I don’t understand how an engine can be designed like this.

Arrie

Oxygen sensors???

How the heck did oil get there? Oxygen sensor is connected to the ECU that does not have oil in it to leak thru wiring to the oxygen sensors...

I thought the problem is with the cam shaft sensors and magnets. I know the cam shaft sensors as I had that problem already in two cars with this engine. Magnets problem I have not had yet.

cruiser02

To be honest, I don’t know enough about the engine/harness to answer how! I am only posting what was told by the shop.

I am not sure if I’ll be keeping the car much longer though, I bought it for reliability and this isn’t a good sign. It’s not even at 45k miles yet.