Help!!! Engine Shakes P0300, etc

[ddtham]

Hi everyone,


I'm new here but have been lurking on your forums forever. I decided to post a thread since I'm experiencing weird problems with my 97' C280.


To make my story short. I was driving the other night when the CEL light went on after more than a year of absence.

The next day I brought it to our friendly Autozone, and pulled out:
P0422 (main catalyst below threshold)
P0300 (random cylinders directed misfire)
P0306 and P0304 (cylinders misfire on cylinder #6 and #4)

So I thought I'd bring the car to the exhaust shop to get my cats checked (the cats are still under warranty supposedly)

On my way to the shop, the CEL turned off (to my huge surprise since it was so very close to the shop). I drove some more and I tried accelerating when the CEL went on again (its blinking this time ), and the car stuttered.

I went to the shop and the guy pulled out the same codes, but P0422 is a pending code , so he suggested me to change my sparkplugs.

I bought the sparkplugs and haven't got the chance to change it when I started talking really nicely (literally) to her (the C280) to behave herself and not breaking down (student's budget). MIRACLE!! the CEL turned off and the car was acting well without any weird symptoms whatsoever.

The peace lasted for 2 days until last night I decided to test whats wrong with it and accelerated from dead stop with full pedal. As I reached 4500RPM, the car gave me CEL again! thats not it, the car's engine sounded like boxer engine from Subaru and Porsche!!!

I pulled over and turned the engine off. When I tuned it back on the engine was normal again.

I figured the car was ok when I accelerated from roll at any speed, but started giving me CEL when i accelerated from dead stop.

This is so weird can you guys help me out with any of this please?? I was wondering with FPR, do you guys think FPR might affect these??

Thank you so much everyone for reading my lengthy post. I hope someone out there can help me.


Regards,

[1995c280]

first of all your cats are not under warrenty if they are original. the warrenty is 8 years/80K miles, whichever comes first. as for the misfire codes. i would definatly start by changing the spark plugs and resistors. when you step on the throttle hard...your car is goin into limp mode and misfiring like crazy. under very light loads it will act normal. most of the time it's not the spark plugs that are bad - it is the resistors under the sparks plugs that go bad usually from water/moisture corroding them. but since you'll be changing the resistors, you might as well change the plugs and the wires while your in there. you can buy each resistor serperatly or if you buy the wire set - it comes with all the resistors needed.

by the symptoms you are describing...to me it sounds like your resistors are arching out during heavy loads (fast acceleration). it is common for them to go bad from water because you have a 97' meaning you have a inline 6 engine and water can get in there easy especially when doin engine washes!

i had this exact same problem with mine a year ago and pulled out the resistors and what do you know...white corrosion everywhere. replaced them and problem SOLVED!

make sure you DON'T install platinum spark plugs when changing them.

hope this helps, Jason

[ddtham]

Originally posted by 1995c280
first of all your cats are not under warrenty if they are original. the warrenty is 8 years/80K miles, whichever comes first. as for the misfire codes. i would definatly start by changing the spark plugs and resistors. when you step on the throttle hard...your car is goin into limp mode and misfiring like crazy. under very light loads it will act normal. most of the time it's not the spark plugs that are bad - it is the resistors under the sparks plugs that go bad usually from water/moisture corroding them. but since you'll be changing the resistors, you might as well change the plugs and the wires while your in there. you can buy each resistor serperatly or if you buy the wire set - it comes with all the resistors needed.

by the symptoms you are describing...to me it sounds like your resistors are arching out during heavy loads (fast acceleration). it is common for them to go bad from water because you have a 97' meaning you have a inline 6 engine and water can get in there easy especially when doin engine washes!

i had this exact same problem with mine a year ago and pulled out the resistors and what do you know...white corrosion everywhere. replaced them and problem SOLVED!

hope this helps, Jason


Thanks for the heads up, I replaced my cats last year and the warranty is from the shop.

I never wash any mercedes engine since I know the problems that might occur. I'll definitely check the resistors though.

I checked other thread where there is one guy says something about coil pack causing similar problem. Is this relevant??

[1995c280]

the coil is always a possibility but if it's goin bad it would usually be a constant misfire even at idle (slight to major). you can always swap coils with one in another cylinder and see if the misfire code changes.

PO300 = general misfire code
PO301 = cylinder #1 misfire

the last number indicates the cylinder which is misfiring!

you have PO304 and PO306 and if i remember correctly both those cylinder have a coil above them rather than a spark plug wire. i highly doubt that 2 coils went bad at the same time. it doesn't sound like a bad coil to me but you cannot ever rule it completely out of the picture. try the resistor and plugs before you worry about the cat. when misfiring you are running richer than normal and it could be throwing off your O2 sensors because the voltage is not dropping sufficiently enough from the 1st sensor (pre-cat) to the 2nd sensor (after-cat).

Jason

[Proven Guilty]

disconnect the battery while you get off your lazy ass and change the plugs. reconnect battery, clean the connections inside of the coil to the top of the plugs, check your oil and air filter.

test drive, and if CEL comes back on, pull fresh codes. let us know from there. ;)