At a cold start, runs good until warm, then runs rough, idles rough and check engine light comes on with p0304 code. If I reset code and restart engine, it idles good for less than a minute, then back to rough idle.
Swapped out ignition coil for cylinder #4 with one from #2 and still same problem. Can't take out spark plug because it is welded in there tight and hate to strip it out, but they were checked about 5,000 miles ago and were good. Any suggestions out there. Car has 75,000 miles. Also I replaced that vacuum hose that caused rough engine a few yrs ago.

 

if it was me i would replace the sprk plugs and see where that leads me. welded spark plugs? ive never heard of that.

 

OK>as you already know, PO304 is "cyl. # 4 misfire". If you switched coils and it still does it, you have to remove the plug. It's in there so tight because the person who "checked" them 5,000 miles ago either did not "check them" and told you they were alright or he checked them and tightened them up WAY too tight and probably without the anti-sieze you should always put on the threads of any plug going into an aluminum head engine. This is well known by all mechanics. Now the plug is probably seized in the thread area and you need to take a spark plug wrench, put it on a half inch breaker bar, get some gloves on, use the shortest extension you possibly can for more torque, and snap that plug loose. Chances are you will break it but who cares at this point. You should have a new set in your hand ready to go anyway. DON"T GAP THEM! They are iridium plugs with a tiny electrode. Just put a nice film of anti-seize on the threads and just snug them down. No need to crank them in so hard. Spend the money on the iridium BOSCH plugs or whatever brand you can get them in. Your mileage will increase a bit and the car should run great. At 100K miles, my '05 C230K plugs were gonzo. Made a big difference. Now....having said that...if it is NOT the plugs and the code comes up again, you will have to be sure the coil packs are all good and go from there, but at least you will know it's not the plugs and they will be new. You're going to want them at 100K anyway. OK? I now have plug replacement down to about 20 minutes on my car. Oh yeah, I almost forgot. Put a little di-electric lube on the part that slides down over the porcelain on the plug so it will go on easier and not stick next time. Good luck.

 

Thanks for the advice mrcoyote.

 

Misfire caused by cylinder #4 only having 105psi compared to 150-160 on the other cylinders. Stealership charged me $2,000 out the door, for a new cylinder head.
Valve guides were worned out at 75,000miles. They said it normally would cost over $7,000 but factory & stealership would help out, yeah right. 6yrs/75,000 miles needing engine work for a MBZ, I'll never buy one again. Getting rid of this soon before something else goes wrong.