Looking for your expert advie! Newbie here... I Keep getting P0304 error on my 2006 MB E350... Runs fine and then after a while, boom feel misfire and CEL comes on. Feels shaky at stops and lower RPMS (at least more noticeable). Also, if I get on her, this will happen too.

I will pull over, shut it off and start back up. Sometimes CEL stays on, sometimes it goes out and runs fine again. I swapped coil and replaced plug in Cylinder 4 still happens! Can't be a valve or piston/ring, because it runs fine again after sometimes for a long period of time, but...it always comes back.

Any ideas at all? Wire shorting? ECU? Vacuum leak that ONLY affects cyl-4? What would cause only Cylinder 4 to misfire randomly?

It is currently at shop, and the guy doing the work replicated the problem, he said he finally felt misfire at stop light.. Doesn't seem to be noticeable at higher RPMS. Cleaned injectors, and now he's as stunned as I... Pressure test is fine. I am baffled.

Car was sitting around for a while as I was using my other 98 E320 with for sale sign on her (wish now I would have kept it). Regardless, it was my daily driver and then sat for a few weeks before I started driving again. Had rear right bearing replaced, but this started happening a few times before that was done..

PLEASE.... any help would be appreciated. My baby runs great, 192k and looks as good as the day I got it... Anyone ever experience anything like this? No other misfires on any other Cylinder.... Save the whales? How about SAVE MY BENZ! lol

 

What do you mean by cleaned injectors? Why not just swap it? Checked wires going to coil pack? Use correct replacement plug? (once this is all over may want to replace all of them depending on age)

Does the shop you are using have a decent diagnostic computer or just a crap reader?

 

samaritrey, Thank you for the quick reply and thoughts... I'm sorry if I sound a little naive, but I'm just trying to state the conversation with the mechanic as I remembered it. The ONLY cylinder I'm having a misfire on is 4. The first thing I did was put a new spark plug (Bosch, the one MB suggest in my owners manual to use) in cylinder 4 to make sure it was the plug, although the old plug looked alright. I swapped several other coil packs to make sure it wasn't that.

This was before I took the car to him. As for the tools he uses or diagnostic equipment, I am sure they are top notch because he was 1) Highly recommended by my long time friend who is GM at a huge auto mall in Hemet where I live, who also takes his BMW to this guy, and he swears by him (I know, so what?..lol) 2) Tony's import automotive (name of co.) makes me hope this isn't his first rodeo, although it could be his first rodeo for this problem. 3) I'm not a rich guy and can't afford the Steelership..

I don't know, at this point my frustration is getting the best of me brother, and I am just "dazed and confused" over it... As for the injectors, all he told me on the phone was that he cleaned them or cleaned them out and they appear to be okay. My car is spending the night there, so hopefully, I'll have more info for you tomorrow.

What do you suggest I ask the mechanic as I am lost...
Than you much for all your help. Have you ever heard of such a thing? It happens (misfire P0304) randomly. I clear the code out and it will come back sooner or later. The CEL blinks first, and then stays steady. I can shut the car off and turn it right back on and although the light remains on, the problem disappears (shaking and misfire). Sometimes, the light will go off... Go figure..IDK... no clue...I'm the one with a crap reader....lol

 

The specific question you have to ask the mechanic is if he has a star diagnostic machine. Probably not, but there are some indys that have them. That would probably tell you more than just the CEL light you're getting. You might have to take it to the dealership and have them diagnose it for you, they'll probably charge you an hour for labor and it'll probably be in the $150 range. Or find an indy that has a star diagnostic machine, I search once in a while in my local craigslist and I see them charging anywhere from $50-$75 an hour. You might also search through the forums and maybe you can find a member near you that has a system.

 

cetialpha5,
Thank you my brother, I sure will ask him about the Star Diagnostic Machine. I'm pretty sure he does as he specializes in BMW and Merc's.

I've been reading every single thing on this I can find... So far, I am finding more common than not (and for some reason more on Cylinder 4 than any other) that this issue is related to the fuel injector for this Cylinder.

The more I read, the more I see this. My mechanic said that he "cleaned them" but I don't think he pulled them and tested them, probably why he's kept the car there overnight...

There was someone who posted he had this issue (just like mine) and after two weeks of Fuel System Treatment (whatever that is), he no longer gets the intermittent and random misfire on cylinder 4..

Any thoughts?

 

I found that most independent shops in my area don't have or won't admit to having Star Diagnostics. I think if they do have it, it's likely one of the illegal versions from China/Greece/etc.

I've never found fuel system treatments to do much good. Sometimes induction cleaning (top engine cleaning) helps other issues but that won't do anything for the injectors. Also this didn't work well for my car (E550) as there is some sort of baffle/butterfly valve in the intake that doesn't allow all the fluid to make it into the combustion chamber without a load on the engine (driving at heavy throttle), and then it all dumps in at once and could cause issues.

I have a friend that pulls the injectors, and powers them up with a battery
pack and blows compressed air through them. That seems to work and doesn't cost anything to try.

Did he do a compression test? If so what were the results?

 

About a year ago, I had your exact symptoms and cylinder #4. This happened right after I had an INDY change my spark plugs while he was replacing the engine cover gaskets. I ended up changing the plug wires which did not resolve my problem. I replaced the coil which did not resolve my problem. Eventually, as I decided to closely inspect the work, I found that spark plug #4 was cracked, as the idiot had over torqued it. Once I changed that plug, all was fine.


So, I'd recommend that you check the coil, the wire and the plug going to that cylinder.

 

Changed with several plugs I knew were good. Same with coil. After extensive research, I'm leaning towards number 4 Fuel injector... Seems to be pretty common. Exact same symptoms I am having...

 

[QUOTE=rtbrjason;6791281]I found that most independent shops in my area don't have or won't admit to having Star Diagnostics. I think if they do have it, it's likely one of the illegal versions from China/Greece/etc.

I've never found fuel system treatments to do much good. Sometimes induction cleaning (top engine cleaning) helps other issues but that won't do anything for the injectors. Also this didn't work well for my car (E550) as there is some sort of baffle/butterfly valve in the intake that doesn't allow all the fluid to make it into the combustion chamber without a load on the engine (driving at heavy throttle), and then it all dumps in at once and could cause issues.

I have a friend that pulls the injectors, and powers them up with a battery
pack and blows compressed air through them. That seems to work and doesn't cost anything to try.


You were RIGHT! He does NOT have Star Diagnostic! Just tried fuel injector cleaning. NOT GOOD, won't be using him again. Asked if he pulled the injector, he said, no, it's kinda hard to get to! I am going to try your method. Where is your friend located? THANKS for all your help!

 

Okay, picked up car from mechanic, said he took for a drive and everything was working fine. He stated he had done a fuel system treatment (big word for "added Chevron Techroline" fuel injection cleaner additive). and said it was running great. BS! I asked if he did a compression check on the cylinder in question. He said he didn't have to because, the problem P0304 goes away after simply restarting the car, that the problem is intermittent/random.

The SOB left my car full of oil stains and a mess and $100 dollars later (at least he had receipt for the $10 dollars of Chevron gas he added) I was driving home (5 miles from his shop) and it happened again! I was stepping on it, it flew, but when I came to idol, car started shaking and CEL came on... Really tough to get it going to speed after that. So when I was near my home, I threw it in neutral turned it off (as I was driving) and qucikly turned it on, CEL went out and car drove home again no misfire... WTH?

Has to be a fuel injector on #4... So I can see I'm going to spend a ton of $$$ on crapy mechanics, so I'm going to buy a new Fuel injector and do it myself... I'm losing fait and money in anybody else... Will keep you all posted as I take off on this new endeavor..

 

I'm located in the Rochester NY area. I wasn't recommending you bring the car to my friend, just that i've seen him do his compressed air cleaning on several other cars fuel injectors over the years and you can see junk coming out with the compressed air.

One re-man fuel injector for the E550 is about $45 on Amazon. Not sure if those are the same injectors in yours or not but for that price it's worth buying one to test.

 

Thanks rtbrjason... Can't go wrong with a remanufactured one at this point. Can drive it until it goes bad or who knows? Might last another 192k...lol...Either way, an inexpensive alternative as I've already lost money trying to have other "qualified" mechanics do it...

 

Did you end up finding the problem? I have the same exact problem started happening a couple of weeks ago, changed all spark plugs, moved the coils around to see if i'll get a code on a different cylinder but it was still the same issue, i ended up changing #4 injector and it was good for like a week and yesterday I went on a 100 mile round trip and it started doing it all over again. I'm running out of ideas. please let me know if you figured out what's wrong with the car. thank you.

 

alsheikh, Yes it stopped all together. After spending a couple hundred dollars for indy shops including Autohause (so called MB specialists) here in San Jacinto CA, I just did a Lucas gas treatment, and the problem vanished. Can't say that's what did it 100% but it had to be more than a coincidence. Every shop confirmed the problem, but nobody had any advice on how to fix it, just said it won't hold the error code . But it's been months since I originally posted this and it hasn't happened again...Try a very aggressive gas treatment and see if it does the same for you. My theory is, that since the P0304 error was only on Cylinder four, at the end of the fuel rail, the fuel rail was clogged with something before it got to the last injector on that side of the rail... Keep us posted