OK, this may be a bit lengthy but is an ongoing saga. My 1997 f-150 truck started having a #4 cylinder misfire about a month ago. I couldnt get to the plug, but pulled the wire and re attached it and the problem went away for about 3 weeks. The problem started again so I took my truck to Firestone to see what was up, they found that the air cleaner had worn the plug wires by rubbing them where they were close to the coil pack...easy repair I thought. New wires went in and when the mech test drove it the check engine light started blinking. This time a #7 cylinder misfire. We pulled the plug, it was dry, but worn ( after only 16k miles on it) code kept coming up and the truck was running worse than ever. They did a compression check on #7 and came up with 50psi, added a little oil to it and got 90psi.
They say I need a new engine, my rig doesnt use oil, blow smoke, and the plug wasn't fowled. I don't get it. I'm thinking of changing my coil pack first, and maybe getting a second opinion before I drop $2,500 on a repair. Any ideas?
To do a proper compression test you have to remove all the plugs then test the desired cylinder. To test the cylinder you have to have the throttle plate fully open or you will get a false low reading. FYI With the 1997 4.6 coils packs #4 and #7 cylinder share the same coil pack. They both fire at the same time. Their coil pack happens to be on the driver side. Try swapping coil packs ans see if the misfire switches cylinders.
At this time follow The JMC advice as to the miss fire. On the compression issue, with a cylinder miss fireing because of a non compression related issue, (valves or rings) a false reading can be observed. In your case the rings were probably washed down from the raw gas (unburned) in that cylinder. Repair the probable ignition failure before just jumping into an engine repair.
So I followed the advice, replaced the coil pack. I now have no check engine light on, the truck runs better, but is still a little sluggish on acceleration, I get some hesitation under load ( up a hill, accelerating, etc) I think i've fixed the problem, but what can I do to fine tune it now?
I have 178K on the engine, about 16k on the plugs and fuel filter both. I replaced the egr sensor last year, as well as cleaned the mass air flow. I'm going to replace the air cleaner, and clean the throttle body and replave the pcv valve here next and see where that gets me.