Error code P0300

[jinglis]

I have a W 212 E 350 2011, with 109,000 miles on it. Started idling rough out of the blue, was on my way to buy some fuel injector cleaner when the engine light went on. Friend works at a Chevrolet dealership and I plugged the car into the diagnostic device and it said misfire on #2 and #5 coil. So bought 2 new coils and installed them, still had bad readings on both and a rough idle. Swapped the coils around and its not them. (should have done that first) Removed the plugs, they only had 11,000 miles on them and the gap was off, so changed them to 0.8 reinstalled them and it ran smooth and the readings were fine on each cylinder. About an hour later the engine light went back on and it starting idling rough again. So back to square one, guess I will take it back and have anther diagnostic check, but I have a feeling it is the fuel injectors. I did add a can of Lucas injector cleaner but have not driven it much since then. Any advice would help, have been able to fix everything so far on this car myself without going to the stealership.
Jim

 

[Doug1234]

You made it run smooth by cleaning plugs so put a new set in and go from there.

 

[jinglis]

Thanks, I will go back and get another diagnostic check and if it says the same thing will get new plugs and try that, just weird I replaced them 11,000 miles ago. Bosch too.

 

[mattkh]

....... the gap was off,
Jim

Was it wider than you expected..?
What was the colour on the tips..?

 

[jinglis]

Yes the gap was way off, much wider, tips were grey and worn down. Replaced all six with iridium plugs and that stopped the rough idle and lag in pedal response. But the engine light is still on, will try to reset the code today and see if that's all it needed. Thanks.

 

[mattkh]

..... tips worn down.

The colour is OK but the worn tips..??
Its not pinking is it..?

 

[jinglis]

The new plugs solved the problem, engine light went off and the car runs smooth. Got the error code two coils were bad #2 and #5, so thought they needed to be changed not a big deal. But it was not them it was the plugs, finally took the plugs and switched them from #2, to #1 and got a bad reading for #1 so knew then it was the plug.
But since I had changed the plugs 11,000 miles ago I never thought it would be them. All solved, but can only imagine what the dealer would have charged me for this!!

 

[mattkh]

Well done.
Might be an idea to check one of the plugs after the next 10000 miles.