Spark plug problems? or something else?

[FordsRule01]

Hello All, Have posted here before, but only several times. Been lurking and learning from all the expertise on this site. I am what you would call a shadetree mechanic, so I am not totally clueless, but I am stuck with this one. Approximately 1 month ago, my 1998 4.0L OHV started, Well I called it "bucking" and "missing" during travel. Not only during acceleration, but also during normal highway travel. Initially, no CEL came on or even flashed. This lasted for about 2 days and I initially thought perhaps I got some bad gasoline and tried a different station to fill up and also utilized some fuel injection cleaner. About 2 days later, I got a CEL flashing and the "stumble " getting worse and then the CEL came on and stayed on. I read the code with my reader, and it said "#4 misfire detected". I have, within the past 3 months (3000 miles) replaced all the plugs, wires and the coil pack. After the solid CEl I pulled the #4 plug and found it oil fouled?? so I replaced it with new Motorcraft plug (all were Motorcraft plugs) and the problem was solved. Ran the car for almost a month now and two days ago, it starts doing the same thing again! CEl comes on flashing and the code reader said #3 misfire detected. Okay, I think , I got this . Same thing as last time, I'll replace the plug and I'll be fine until I can figure out what is causing the plugs to misfire, at least I'll be able to drive it. WRONG!! I replaced #3 and got about a mile from the house and the CEL starts flashing again and the stumble starts again!! I went back to the house, read the code and it says #3 misfire detected. I pulled the plug and it looks slightly grey and no oil fouling. The initial change of the #3 plug was the same. Changed the second plug and drove about a mile again and it starts doing the same dang thing!! I am stumped! I hope someone has an idea because I am fresh out. Have also replaced the fuel filter within the last 3000 miles.

 

[markfothebeast]

I haven't worked on our Explorer since spring so I am trying to recall what common problems arise with these.

Just the other day I had the same issue on a friends car and he was jumping to conclusions without actually doing basic tests (fuel pressure, vacuum, spark, compression, etc). It had the same symptoms as you mention but the ECM in this 96 Jaguar had a better tendancy to give a more in depth CEL other than a misfire. Long story short, it came down to an injector that had literally stopped spraying fuel.

Similar issue on our 1999 Explorer SOHC 4.0L V6 I found that multiple injectors were either leaking or had been partially clogged. I tested all 6 outside of the vehicle on a fuel rail system I'd built to clean injectors. It was clearly visible which injectors had a flawed spray pattern or would continue to spray fuel on its own. The fuel pressure was fine when the vehicle was running but when the vehicle was shutoff the pressure would quickly drop down to zero. It should have held the pressure for atleast 5 or 10 minutes at minimum. A trip to the junkyard got me 2 sets of injectors for $10 to play with.

Now if the coil pack had a coil that was failing, I believe there are 3 coils in the one pack and each controls 2 cylinders. That would show a misfire on 2 cylinders if I recall. If that applies to the OHV coil.

You could also clean the EGR valve, clean the mass air flow sensor with mass air flow sensor cleaner in the spray can, and also test the vacuum. I know our SOHC engine has a common vac leak under the intake manifold but I'm not sure about the OHV. I had an older Explorer with the OHV and I would trade our SOHC for it anyday.