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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53016
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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Just hooked my 2003 e320 to the shops snap-on scanner i got

Customer Question

Hi, just hooked my 2003 e320 to the shops snap-on scanner i got codes (po300) (po304) (po305)(po306). My sympons were while driving the forst time i got a misfire. The vehicle only misfired for about 2 seconds engine light cane on the it ran smooth right after. I let the car sit over the weekend. Monday i turned the car on and no check engine light was on. I took it to my body shop and hook it to the scanner. Scanner codes came up as the ones i explained above. I cleared the codes then filled up te gas tank and added fuel injector teatment. Car ran fine for a week and the it missfired again this time while driving it didnt stop misfireing. I drove the car about 15miles home with the misfire. When i got home i turned the car off then back on snd the misfire went away. I let the car sit saturday and sunday untill today that i could get it back on the scanner @ work and the vehicle ran perfect all the way to work with no misfire. Same codes after i hooked it to the scanner this morning. Any clues on what it coild be. I orded left side (driver side) coil pack incase i need it. Thanks for your help
Submitted: 9 days ago.
Category: Mercedes
Expert:  Marty replied 9 days ago.

Hi Luis, my name is***** first thing I would do is swap the left side coils with the right side coils and see if your misfires move to cylinders 1, 2 and 3. You can do the same with the ignition wires. I doubt you have 3 bad ignition coils or wires but it's possible. It's more likely that your misfires are caused by a bad catalytic converter for cylinders 4, 5 and 6. A restricted converter will cause misfire codes on the left or right bank. Usually the misfire will occur on hard acceleration. You can remove the front oxygen sensor from the exhaust and road test it with the sensor removed and see if it misfires. The computer will turn off the fuel to the misfiring cylinders to prevent further damage to the catalytic converter. When you cycle the key off and on it will reset and run fine until the misfire occurs again. Another possibility is water in the fuel rail. You can drain the rail and take a fuel sample to rule out fuel contamination.

Let me know what you find.

Customer: replied 8 days ago.
Hi Marty, thanks for your reply. I did exactly what you said to do. The coils are switched over with the ignition wires. I also disconnected the O2 sensor and like expected the check engine light came on when you turned on the vehicle. I drove for about five miles with no misfire. So i brought back into the shop and put the scanner back on it. Giving me the O2 sensor code. So i went ahead hooked the sensor back up and cleared the code. Drove it home from work about 18 miles with 2 stops along the way. The car is running perfect with no misfire yet. I will have to wait for the next misfire to further my diagnosis. How can i get in contact with you once I get the next misfire? Come to think of it now. For the past 2 1/2 weeks i have had the car up on the lift because there has been some type of rattle coming from my exhaust on the left front while driving at low speeds. Everytine I had it up on the lift and even today. I have been trying to duplicate the rattle and I can't seem to duplicate the noise. I check all exhaust heat shields to see if they were loose and came up with nothing. Makes me wonder if that rattle is coming from the inside of the catalytic coverter on the driver side where i had the left bank misfire. I will get back to you as soon as get more feed back from the vehicle. I added a pic of the codes. Thanks again Here is my cell#(###) ###-####if you want to get a hold of me.
Expert:  Marty replied 8 days ago.

It does sound like a restricted converter. When you get it up on the lift again try rapping on the converter housing with a rubber mallet and see if you get the rattle. Remember, when testing for a plugged converter you first need to be able to duplicate the misfire on a road test then you need to remove the front oxygen sensor from the exhaust so that the exhaust can escape. If the misfire cannot be created with the sensor removed you have a plugged converter.

Let me know how you make out. Please don't forget to rate my service :-)


Customer: replied 8 days ago.
Got you. I unpluged the O2 sensor insted of removing it. I will try that. I did try rapping on the converter housing with a dead blow hammer but could not get any loose noise. I also had someone inside the vehicle reving the rpm's and low and high revs. I can even get a sound on at slow speeds any more. Seems like what ever was loose in there is now holding tight. I will keep you updated as soon as i get more feed back form the car.
Expert:  Marty replied 8 days ago.

Sounds good.