I have a 2001 ranger 4.0l AT that I've been messing with as a covid project and I have a crazy intermittent issue that nobody seems to be able to figure out. Truck shows no codes, but the number 3 cylinder has an intermittent misfire. No real pattern to it, but it does seem like revving up to 2700 and dropping the RPMs back to idle sometimes causes it to occur. If you let the truck idle it'll also happen every so often on its own. I have a scan tool and can see that on the cylinder balance test the graph is basically straight but then #3 will start to dip a little and drop down to -25 -50 from the average before straightening back out. Can feel the truck start to shake a bit but it doesnt stall or set any codes. LT + ST fuel trims are <+10 and closer to +5 with bank 1 being a little higher.

I've replaced and tested (compression, vac leak, fuel pressure, exhaust leak) just about every part you can name (plugs, wires, coil, injectors, intake seals, all the sensors) with no results. Took it to a shop that looked at it and suggested the problem might be the pass side head and to wanted to rebuild the motor. So I picked up a used low mile motor out of a wrecked truck that I know was running fine and swapped that in. Guess what.. problem still the same on the same cylinder. At this point the only parts I haven't replaced are the plastic intake manifold itself, the wiring, and the PCM. I'm out of ideas. Would appreciate any suggestions you can offer before I set this thing on fire.

 

May have found a pattern to my misfire or at least a way to reliably reproduce the issue. While in park if I slowly raise the RPMs up to around 1300-1500 RPMs cylinder 3 stumbles just about every time. Still no codes though. Anyone know what the computer does around that RPM that might cause this?

 

Hi yolner, it's obviously something electronic since the swapped junk yard motor is doing the same thing. Maybe the ECU? What about the ignition coil? I've had a bad mass air sensor do some wierd things without codes that could be something to check.

 

+1 on check the ignition coil
Once ​​​​​​I accidentally crushed the rubber washer in the coil plug trying to plug it in. It's back up against the firewall and hard to see. It caused 2 of the terminals to arc across the gasket and caused misfires and early ignitions.

 

Still haven't figured out my issue but did a little more testing.

Tested compression just to make sure the junkyard motor was as advertised. All cylinders are within a couple PSI of 190. Fuel trims are hovering close to 0. Double checked all the sensor connections and cleaned grounds. Swapped in a used set of plugs and wires off the junkyard motor and that did nothing. Swapped my aftermarket new coil for the old coil off the junkyard motor. That made the stumble on #3 a little less noticeable, but didn't completely eliminate it. Then my radiator started leaking at the plastic tank seam. Starting to wonder if the previous owner ran over a voodoo priest in this thing.

Plan for right now is to replace the radiator, get an OEM motorcraft coil, and see if I can find a PCM somewhere. Seems like that and the wiring are the two parts still original to the truck.

EDIT: Anyone know how to take the black plastic cover off the big PCM connector to take a closer look at the wires? Having a hard time seeing how it comes apart since the bolt in the middle goes through the connector and into the PCM.

 

Another update. Waiting for a PCM to come in. Replaced the radiator and installed an OEM coil. No real change. Still perfect LTFT and STFT and still the intermittent cylinder 3 issue. When running the KOER test via the scanner I can also feel a stumble when it idles the motor around 1300RPM, but it never comes up with any codes. Couple of pictures of the power balance test. One is "normal" and one with the cylinder 3 drop. At this point I'm pretty much convinced that it has to be the PCM thought I don't see how a PCM could go bad even in a 20 year old truck. Can't think of anything else that could do this considering that everything has been swapped around or replaced.

 

Decided to do a test drive around the block to see if I could set off any codes while waiting for the PCM. When driving the symptoms look more like a multiple cylinder misfire not just #3. The truck stumbles/misfires at light/medium throttle, but clears up when I floor it. Still no codes.

Think I might try to get a MAF from the junkyard to see if that has any effect. My MAF is aftermarket from advance auto and I've seen some posts online describing similar behavior caused by either aftermarket MAFs or a bad OEM MAF. If not, then we're back to the PCM.

 

You ever figure this out?

 

Have your fuel injectors tested or maybe see if the trims change when it misses.