Ranger hot idle misfire

[Ben Morgan]

Hey Guys,

1999 Ford Ranger XLT, 4.0 OHV, 5 speed, 4×4 w/offroad package. Federal smog, no EGR.

I’m about to pull my hair out. I got this misfire that’s apparent at idle after going up a hill or getting off the freeway. It has chucked a code a couple times, P0306. No other codes. It runs great when revved, got all the power it always had, and starts really well. Doesn’t surge or anything even at idle. Warm idle you can hear occasional misfire, but the hotter I get it the worse it runs (not overheating, at least according to the gauge). I can smell a bit of that classic hot catalytic converter smell when its missin.

What I did first:
Plugs
Wires

*no change*
Coil pack
*no change*

I attempted to do a compression test on that cylinder but my adapter bottoms out on the outside of the head before the threads reach the spark plug hole. So I tried to use my rubber tip adapter and push it in as hard as I could. Couldn’t get the greatest seal, but I did measure 125 psi on that cylinder once, and it was still blowing past the tip some. I might order a better adapter or have a buddy add some clearance to the one I have on a lathe or something. But I don’t think I have a compression issue.

So I tackled replacing the injectors. The injectors that came out looked a bit goopy I’ll admit, I measured 15.5 ohms on all the coils so nothing odd there. Didn’t have any issues getting the new ones in. It started and idled beautifully after this. So I took it and got it hot by running it up a near by mountain a few miles. When I stopped and let it idle down then it ran REALLY bad, missin on multiple cylinders. Pulled out the torque app on the spot and saw my bank 2 sensor 1 O2 sensor voltage flatlined (not spiking like it should) and the short term fuel trim on that side was around -20%. As soon as it cooled some, sensor jumped back to life and it ran much better.

Fast forward a couple days later I get a Motorcraft O2 sensor installed. Took it for the same drive. Result is back to a single cylinder misfire that improves and almost completely goes away as the motor cools down some. Not positive if its the same cylinder, I don’t have enough miles on the drive cycles after clearing the code for it to throw another. This ECM doesn’t seem to have a misfire count output, or at least I haven’t been able to get it to work with Torque. I’m seeing good fuel trims, though bank 1 is showing -5% on the long term so maybe there’s something there?

I’ve also sprayed carb clean all over this thing and yet to find a vacuum leak (went through an entire can). Which I would expect given the misfire seems to be only one cylinder…

Where should I look next? I ordered a vacuum gauge (will have Saturday) to see if maybe my exhaust is restricted slightly and getting worse with heat? Grasping at straws at this point…

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