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.

Given the coolant loss I wouldn’t do another thing until I checked the compression. If its found to be low you’d want a leakdown test the see about the head gasket. Quicker/easier checks might be a block tester and/or a vacuum gauge.

Aside from that I saw no reference to new wires to go with those new plugs & coil packs.

You also need to check out the fuel system - fuel pressure & the fuel pressure regulator at minimum.

Wires and then ignition module then injectors…Check fuel pressure first…

One thing to try is obtain a aerosol can of MAF sensor cleaner and remove and clean the MAF sensor.

The MAF sensor is a primary input into the computer. And as it heats up and there’s debris on the wire elements it doesn’t measure the proper amount of air entering the engine. This then causes multiple/random misfires and codes. This is about the cheapest thing to try other than connecting a professional scanner to determine if the MAF sensor is the problem. I’ve expeirienced many vehicles that display multiple/random misfire codes but yet no MAF sensor code, tho the MAF sensor was either contaminated or defective.


Vacuum leak (intake manifold gasket?) is another possibility.

you might need intake seals?

I think this vehicle uses a MAP sensor, not a MAF.

I have seen this odd misfire problem on more than one Liberty before. There is a problem with the intake valves sealing and there is a bulletin to repair it.

Before replacing the valves I would try GM top engine cleaner. I use this on Lexus direct injection cars with dirty valves/misfire with success.

The fuel pump bleeding down after shut down is a posibility but usually causes a misfire on the front cylinders for about 15 seconds.

No plug wires (3.7L), ignition module or mass air flow sensor on a Liberty.