1996 Ranger 4.0 Misfire
Hello. I have a 1996 Ranger 2WD with a 4.0 OHV with only 62,000 miles. A couple of months ago it slowly started developing a misfire. Initially it would just run rough at idle and by the time I got to work (5 minutes of freeway driving) it would clear up and run fine. After a few weeks it started to miss throughout the driving range except at cruising speed under no load (at least it was not noticeable). Eventually I got a check engine light, P0306 cylinder 6 misfire.
I began by checking the spark plug for cracks or any sign of fouling. Nothing. Even swapped #5 and #6 and still pulled a P0306. Then I checked the plug wire. Nothing. And resistance checked out fine.
I tested the coil pack with a multimeter. The resistance between #6 and #(can't remember which cylinder but I was following instructions) was slightly high, so I replaced it. I could swear it improved slightly, but still pretty bad.
For whatever reason I convinced myself it was the injectors so I pulled off the intake plenum, removed the injectors, tested them, cleaned them, replaced the o-rings and buttoned everything up with new gaskets. All injectors' resistance checked out fine.
In the meantime I had ordered plug wires (these probably needed to be replaced anyways so I don't feel too bad) and they didn't help my case.
Today I checked cranking compression and everything checked out fine (had a hell of a time turning the engine by hand so I skipped the leakdown).
To give you some history, I replaced the water pump and timing chain cover gasket this year and the truck ran beautifully for a couple months after. I took precautions to keep coolant out of the oil and replaced the oil and filter before starting it up. Everything looks pretty well buttoned up. No oil or coolant leaks. (Oil leak was the reason I replaced the timing chain cover gasket).
I'm pretty stuck at this point. The only things I can think that it can be are:
- The fuel filter (this is an easy one I should probably check it).
- The MAF. However voltage readings check out fine. But I don't have the fancy torx bit with a hole in the middle to get the sensor out to clean it.
- The ECM. Really doubt this one but I've pulled code P0306 (cylinder 6 misfire) and P0300 (random cylinder misfire). But when I unplug sensors I don't pull any other codes. I can unplug the MAF sensor and the truck runs under no load (albeit rough) with no code saying the MAF is malfunctioning.
Any help with be appreciated. I have a feeling this is not a major issue, but just something I have not considered or do not have the means to test for. I've tried not to replace too many things in the search for a fix, besides the coil pack and plug wires.
Please offer diagnostics advice opposed to telling me that "component X is bad and I should replace it."
Thanks for your help.