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Multiple Cylinder Misfire - 2005 Jeep Liberty

ODB2 Code: P0300 - Multiple Miss-fire

I have an issue with my Jeep Liberty. It started idling rough with no error codes. about a week later it through the Multiple misfire code. I started by replacing the spark plugs. The old ones were from 20K miles ago and still looked good. Replaced them anyway. I still had the code. I put K44 fuel injector cleaner into the tank and drove it home (2 miles.) The next morning I started it and went in side because I had forgotten something. I came out to extremely thick black smoke. It went away after about 2 minutes, the computer code cleared, and the engine idled perfectly. About 2000 miles later the car is once again idling poorly. I added K44 again and for that tank of gas it ran fine, no code. Once new gas was added it threw the same error code and idles very roughly.

So here are some basic info pieces
1. 8 tanks of gas from 8 different locations.
2. Adding K44 has shown a direct improvement both times, but more significant the 1st time.
3. New spark plugs
4. New coil packs
5. 2005 Jeep Liberty - 3.7L V6 at 99,872 miles
6. Hopefully unrelated is that the car has a small radiator fluid leak I haven't found. It requires 1/2 gallon every 2K miles or so.

Generally the solutions to this problem seem to be electrical, but that doesn't seem to jive with the K44 helping.

There is no knocking sound, it just sounds like there isn't enough gas. Car responds fine to accelerator. Above 10mph car seems to drive smoothly.

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