Engine misfire why?

I will change the spark plugs and check the wires in the dark, great help, thanks, runs great now.

I have a skip if I put engine under a load. Doesn't happen all the time. Also skips worse when A/C is on. No engine light on, idles fine and drives fine. But if I step on gas pedal a little harder than normal driving it will skip. This also happens sometimes with normal driving. But definitely when I put a load on engine.

I got a new set of coils that did it, runs great once again.

Truck skipping and popping after plugs and wires changed.

Vehicle was and has ran perfect. Alternator went out, I was selling vehicle and buyer said he wanted it with faulty alternator, put alternator back on and when we first turned the engine over it backfired and smoke came from the engine compartment. Was no fire but when we started engine back up it revs to 2500 rpm(without missing)then when it goes to idle it backfires and misses horribly.(Makes sounds like it is firing in the crankcase)i bought a code reader and replaced the alternator but it still does same thing. It came up with code p0102.I replaced mass air flow sensor with new one and it still does same. When you start it it starts perfectly but as soon as the rpm comes down from 2500 it goes to about 500 and misses, backfires and most of the time stalls.I can give it fuel and it revs back up to 2500 then goes back to 500. It only misses and backfires when it it at idle. The vehicle cant be driven because it wont hold any rpms.I have also cleared codes a couple of times.I lastly unhooked battery and left off for 6 days then went and hooked it back up and vehicle done the same again, so here we are.

I have the same truck and have had the same problem. If you look at the left side of the engine you will find the PCV valve feeds some black vacuum lines and tubing. The line snakes around to the back of the passenger side of the intake manifold into a black 90 degree rubber fitting. It plugs into a 5/8" vacuum nipple on the back of the manifold. You will have to feel for it as you cannot see it from the front. This black elbow will crack and will cause a very large vacuum leak.

In my truck I removed this rear line and capped of the rear vacuum plug. I left the PCV valve feeding to the front line. If you have the original setup you can just get that rear elbow from Ford. I am sure it is a commonly replaced part.

Ed

As noted above, I have a misfire on 4 and 8. When it first happened I replaced all the plugs (60,000km since last plug/wire replacement). Misfire still occurred. I took it to a mechanic (he told me it was 4 and 8 that were misfiring), and suddenly it started running fine and he was unable to diagnose the problem. 1000km later it misfires again, so I replaced the wires, and now it is still misfiring. (A note about those 1000km, they all happened in one day, the day following the testing)

So 3 questions-

1. Where should I look next?
2. Is there an inexpensive method of testing I can use that will allow me to eliminate electronics and look at the fuel injector?
3. How much damage am I doing by running it on misfire (I need it to work)

Thanks in advance for any suggestions or advice you can give.

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.

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

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.

I have a 98 f150 2 wheel drive standard with a 4.6 the engine has a miss and when I put it on the scanner it comes up with a miss on cyl#4 I have replaced the fuel injector the plug the wires and the coils on the engine but it still has a miss. I have checked for vacuum leaks and found none. I am at a lose as to what else it could be. Can you help me please?

I did a coil change out that took care of the problem.