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2002 E430
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53 Posts
Discussion Starter · #1 ·
Hello, sorry if this has been covered, just haven't seen the same situation as I am having. My 16 y/o daughter's 1997 c230 acted up on her yesterday. She calls to tell me the car is shaking real bad and the CEL came on. She stated it also had very little power, hard to get from 30mph to 40mph. Weak acceleration. Tis is all I have to go on.

I come home and drive it (it sat for a few hours) and it runs fine, but CEL is on. I check the codes and get a P0300. I reset the code and drive it again....no problems, the CEL did not come on. I can't replicate the issue she had. I know the 0300 code can be several things, just don't know where to start, but I don't want her having issues on the road. Should I take it to my indie, would their code reader more accurately pinpoint the cause? Any help appreciated. Thanks
 

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1999 C280, 2015 C300
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411 Posts
P0300 is usually due to misfire. I had similar situation but in my case one of the cats got clogged. See my other post on catalytic converter replacement. It could be also due to bad spark plugs, coil or spark plug wires.
 

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2005 C230 Kompressor
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4 Posts
I have had a similar issue with my 05 C230. You have a random misfire and mine was specific to cylinder 3. Check engine light came on with a shudder and shake of the engine, cleared the code, it ran fine for awhile then the code re appeared in a month or two. I would suggest as a starting point to pull the spark plugs and taking a look. Any indication of fouled spark plugs will give you more direction. Hope this helps!
 

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2002 E430
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53 Posts
Discussion Starter · #4 ·
Thanks for the replies....

Ok, so today she drove it and it was fine, then it wasn't. After getting off the interstate, she said it wouldn't accelerate good, and was "shaking like crazy" when she came to a light. Of course I drive it and it is normal. Go figure.

I did buy new plugs today (OEM), I guess I will start there. IF that doesn't solve the problem, I will try wires and coils. Does the symptoms sound like plugs? When my Suburban had bad plugs, it ran rough all of the time, like on 7 cylinders.

I watched a video of the plug replacement, it doesn't look bad at all. Any other input is really appreciated.
 

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2002 E430
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53 Posts
Discussion Starter · #6 ·
I changed the plugs today with OEM Bosch Coppers. The old plugs were platinum and looked pretty rough. Wires looked good. The throttle body was dirty, cleaned it as best I could. So far, no CEL and it's running good. Hopefully this will help.

Thanks for the replies and i'll follow up if the CEL returns and it starts running rough.
 

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1999 C280, 2015 C300
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411 Posts
You cannot tell if a spark plug wire is bad by just looking at it. My wires looked pretty good from outside but one of them was completely dead when I tested with an Ohm meter. if the car is running fine after changing the spark plugs, then I wouldn't worry about it
 

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2005 C230 Kompressor
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4 Posts
I believe MBMedic.com is a helpful website in information and troubleshooting for the DIY. Keep in mind the engine will need compression, spark(plugs and coil), fuel and air to run. Spark plugs are good place to start. If you believe the coils may be a problem you may be able to carefully pull the coil off the spark plug with the engine running and run it next to the engine body. If there is spark jumping from out of the side of the boot you have found a problem. I prefer to do this with an in line spark tester and test light hooked to battery ground.

In the case of my 05 c230 the electrode was completely gone. The car was still running but the check engine light would come on more frequently after start up. Turned out, two of the injectors were plugged (dripping and not spraying).

In your case your spark plugs look much better than mine!
 

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1995 C220
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Typically by either taking it into a good dealership and running diagnostics on their multimillion dollar stuff...

For the typical garage guy, eliminating everything else.

Air, fuel, spark, compression.....I dare say sensors too in the modern age. Your stored codes give you a starting point, eliminate the rest systematically. Plugs are an easy to check indicator for fuel.

Pic showed my initial inspection of the plugs on a m111.... #4 plug was fouled, rest looked ok, but slightly worn. 8 bucks replaced them all... stored codes showed #3 and #4 were misfiring, and multi cylinder misfire occurring. Checked resistance of the plug wires and # 4 wire was off the charts...like 9k plus. So replaced plug wires also.

Now that did not explain the #3 cylinder....but the coil pack ran them both...so I replaced coils (both since they were the originals and a big job to do). And they were inexpensive.

Cleared codes and rechecked...no misfires, yet I now had a before undetectable stumble in the idle, acceleration, and the car was running rich. (Could feel and hear a pulse in the exhaust and actually smell fuel and see white smoke on start up from the exhaust.). Compression test showed great overall compression, so No bLow by on the rings or worn cylinder walls to blame. Also eliminated valve seal issues and head gasket as the compression was even and close to factory new.

Vacuum leaks were fixed, and noticed fuel pumping in from the pressure regulator straight into the intake manifold...this allowed raw fuel to run straight down to the #4 cylinder and puddle making each intake stoke excessively rich on #4 and to a much lesser extent #3 also. Replaced regulator and that was the last piece of the puzzle. Had that not solved the issue, the only thing left would be the injector...

Sadly without expensive diagnostic tools, it is a game of trial and error, visual clues, and deductive reasoning.
 

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1996 C220 2007 ML320 CDI
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If you get a mechanics' stethoscope you can hear faulty injectors. But usually the injectors are fine. If the fuel pressure regulator is leaking, the expected problem would indeed be a #4 spark plug fouling from excessive fuel, since the vacuum port for the fuel pressure regulator is directly above #4 intake runner. Similarly, if there is evidence of oil fouling on the #3 spark plug (with white smoke coming from exhaust), it's a sign the transmission vacuum modulator is leaking ATF right up to the #3 intake runner where the vacuum port is.
 
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