Hi guys, my '06 SL65 has a strange misfire on cylinder 1. The misfire was detected about 2 months back, but has been intermittent ever since.

How it happens: On cold start up, the car never misfires. After driving for about 15-20km the car starts misfiring and engine check light comes on. If I turn the car off and start again, the misfire still remains. I usually leave the car to cool down for 15-20min, and start up and the car drives perfect. I had the plugs changed and the problem persisted.

I had the plugs pulled out again today and noticed that one red rubber on cylinder 1 was soaked in oil, which I suspect that it happened due to Mercedes removing and reinserting the same plug when they diagnosed the vehicle.I purchased a new set of plugs and rubbers for cylinder 1 and replaced them. The problem still persists.

What I have noticed and tried:
I tried a few different fuels available and somehow, I almost always got better luck of not getting a misfire for a longer period with one brand of fuel, but eventually the misfire returns.
I also noticed that the fuel tank is building up a lot of pressure - a high amount of gas releases when opening the fuel tank.
I fitted original Mercedes benz Plugs and I tried Bosch plugs.
I managed to drive the car for close to 100km, at under 3000rpm without any problems by leaving the engine covers off and removing the fuel filler cap. I drove the car hard for about 20km and I didn't have any problem. After leaving the car for about half an hour, and starting again, the misfire returned immediately.
I did not do fuel filters or air filters.
I use Shell 5W-40 oil.
The misfire is persistent on cylinder No.1.

I do suspect that it could be the coilpack, but I always read of a random misfire or misfires on multiple cylinders in the same line when a coilpack is bad.
The strangest thing is, the car drives well for a while, and then suddenly starts misfiring. And the problem goes away, and returns. It occasionally goes away for long enough for the CEL to turn off on its own, and then returns. Sometimes the misfire is rough, and sometimes it's very light.

Please suggest recommendations as I would not want to replace coilpack only to find out that the problem remains.

Regards

 

Single misfires can be isolated to that cylinder. Not likely to be the gas tank or fuel type causing the issue. A few things I would look at to resolve this...

Coil. If a single coil buy a used one on eBay and try it. If a coil pack, this is more costly.
Plug wires. Swap them and see if the misfire moves with it.
Injector. Run injector cleaner first, then pull it and clean it, or replace it.

You have already removed the plug as the problem. Oil on the seal could be from valve covers.

 

You need 3 things to ignite a cylinder:

- Ignition
- Fuel
- Compression

Based on what you're describing, you're likely to have a bad injector or coil.