Misfires are tough to find and but not impossible if you put some time into it. A misfire can be caused by a ignition problem, fuel problem or a compression problem. You have already narrow it down to cylinder #4 and #8 . You also have elimated the plugs and wires so far. I would also at least inspect the cap and rotor. If the car is equipped with coil packs test the coils resistance for #4 and # 8 . Other causes can be fuel related, a vacuum leak or a compression problem. Next check the car for a vacuum leak with a valcuum gauge. It should be between 17-22 steady. Dirty or clogged injectors can cause a lean mixture and cause a misfire. A weak or fautly injector can also cause the same thing. You can add fuel injector cleaner and see if the problem goes away. You can test the suspected cylinders by checking the resistance of the injectors with a ohm meter and see if they are with in specs. And the last thing is compression. Check the engine with a compression gauge and make sure the readings are with in specs. :) Good luck

Answered by backyardmechanic
3 questions asked

If 4 and 8 share a coil pack replace it. Ive seen many coils and injectors go bad in those trucks.But if they share a coil it should take care of the problem

Answered by 2CarPros Jack (expert)
12,033 answers provided

Great, thank you guys. :)

Does it make sense that occasionally it would run ok if the problem were a coil pack, or an electrical problem in general? The other day it ran great for a few km and then out again.

Editing the post to say I just checked for spark, and I have spark on all 8 cylinders (method I used - insert old spark plug into wire, ground to the engine and turn over). Does this mean I should look into the fuel system as the problem, or should I start to check the resitance at the coil pack (Learning all this by reading the Haynes manual), or replace the coil pack altogether?

Lots of questions, I know. Sorry.

Thanks for your time.

Response from ChrisK
1 question asked

Yescoils can intermittently short out.I have seen both coils and injectors create misses in Fords. The odds of two injectors on opposing sides going out is slim.If 4 nad 8 share a coil try that first. Alot of what we do is trial and error, seen this many times.

Answered by 2CarPros Jack (expert)
12,033 answers provided

Thank you. I will try that this weekend, then. :)

Response from ChrisK
1 question asked

So, in the end, reluctant to spend any more $$ on an unsure bet, I bought a Ohmmeter and consulted my Haynes manual.

I tested both my coils - they were fine.

I tested all my injectors, and one was bad, so I replaced it, and now the truck runs fine.

I'm somewhat impressed that I was able to fix this problem, and also somewhat surprised that, after educating myself, it took less than 30 minutes for me to diagnose the problem.

I'm more than a little dissappointed in the garage that charged me 3 hours of labour in an attempt to diagnose the problem. If it took me 30 minutes to figure it out once I knew how to look for a problem, at the least it should have taken them the same amount of time?

I can't be certain of the original diagnosis - they said I had a misfire on 4 and 8, and they checked it after I changed the plugs. I replaced the injector on #8, so that part of the diagnosis was correct, and I can only assume that when I changed the wires after they looked at it, that doing so fixed #4.

Well anyway, I fixed it, and I did it myself. Not bad for a carpenter. Thanks again for your help - I'll be sure to recommend this site to my friends.

Response from ChrisK
1 question asked

Glad to see you fiix it yourself. :D

Answered by backyardmechanic
3 questions asked