Cross-posting here (was in performance) in case there are more people here -

I recently picked up a really nice '98 C43 AMG. Car has 98K. Runs perfectly except that if you really floor it, it throws a P0303. I swapped coil packs, stayed on cyl 3. I looked closer at the wires and found the shorter one wasn't clipping into the coil pack properly. I pulled it, repaired clip and moved that wire to cyl #1. Still firing the 303 code. Can a plug cause this or should I be looking at the harness? I wasn't at a place where I had the socket to pull the plug so I haven't seen it. However, I am thinking that if it's been doing this a long time, the plug is prob fouled.

Other symptoms - when the 303 get's tripped, the car idles terrible like a cyl isn't firing. Simply shutting the car off/restarting clears the rough idle condition. Does the computer have a way to lock out a cyl on misfire detection? Also, car high idles (about 1400 RPM) for a little bit on startup. I found a vacuum line off coming off the back of the manifold and it heads over towards the passenger side corner of the engine compartment then disappears, presumably into the car. Reconnecting it has had no affect.

Other than that, car runs beautifully and I am averaging 23MPG which I think is decent for this car.

Thoughts?

BTW - this is my first non-diesel mercedes so even 1/3 throttle is way more than I ever had before LOL, but I'd like to figure out what's going on.

Causes include, spark plug, spark plug wires, injector, vacuum leak, & ignition coil(s).

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Check to see if there is an ECU software problem or upgrade to your 1998 C43.

I had a similar problem with my 2005 G55K.
Dealer replaced a whole bunch of coils, wires, plugs and when floored it still pulled a P0304 code.
Dealer finally discovered that the ECU had to be reflashed with a later version.

What indicates this to me is the fact that if you shut down and restart, it runs properly with no "misfire" ( G55K did the exact same thing )

I have a 1999 C43 and never encountered what the OP has.

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1971 280SL ROADSTER
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1994 E320 CABRIOLET
1999 C43 AMG
2005 G55K AMG
2008 CLK63 AMG BLACK SERIES