Ive been fighting a misfire and its only caused by my dizzy cap and rotor. After a couple days i have to take the cap and rotor off and clean everything by scraping the rotor tip and all the terminals inside the cap with a knife. It will misfire bad but once i do this cleaning procedure then it runs like a champ and i even feel a bit more power afterwards. Before i replace the cap, rotor, wires and plugs i need to stop this first. Also my plugs look great, motorcraft copper. Wires are duralast and same with cap and rotor

 

Also everything except for plugs are over a year old. Replaced plugs a couple months ago thinking that was my problem.

 

When you say misfire can you see it or hear it, as in sparks snapping or arcing somewhere under the hood? (at night is a good time for this, while idling)

Where exactly is the arcing going to ground?

Make sure plugs are gapped correctly. I might measure plug wire resistance for grins, but if they are "factory fresh" they need replacing anyway. You may not be able to "stop this first" before actually replacing those items. They all need to be in serviceable shape to prevent crossfire between wires and arcing to ground, etc. It's part of a good tune-up.

 

I can feel it, it shutters at idle and under load. I can see rpms jumping all over on tachometer

 

Yes, I don't doubt that a bit.

Where is it arcing (sparking) to ground, though? You will be able to see this at night or in a darkened room (don't bring it in the house!)

 

Oooh....i guess i will have to wait till tonight. Will there always be a spark on a misfire

 

Well I'm not there in person, you are the eyes and ears and have only your observations. The arcing to ground is something to check for, at least. It's kind of a process of elimination deal. Given your description it would make sense to go ahead and replace wires, cap, and rotor as well particularly if they are original or old. What year is this thing? That's just basic tune up stuff that needs done regardless.

As a practical matter it's sometimes fun to trace down the offending component, but not very often when it comes to transportation. It sure sounds like you have too much resistance in the secondary ignition somewhere - it just barely works, cleaning the cap and terminals makes it work for a little while.

Usually high resistance in the ignition is going to cause "leaks" somewhere else than the plugs, as electricity seeks the easiest point to ground. So while I'm guessing, based on your description, it's not necessarily a bad guess.

 

Its a 88 f150 5.0 and the cap, rotor and wires were replaced last summer. But i got the stuff from autozone (short on cash). Maybe just inferior quality

 

Hm. They "should" be OK, then. Guess I'd still want to look for arcing to ground, or crossfire, at idle in a dark area. It does misfire at idle, right?

Judging from your description It has to be ignition related, practically speaking.

 

Yes it misfires at idle and under load until i get over 2k rpms. Also after i drive for a good 15 miles it runs good.......

 

You replace the coil?

Put a code reader on it to check for codes?

 

Ive never replaced the coil but i dont know if the PO has. I doubt it. It looks original. The only codes i get are for the egr valve not detecting the valve opening. I put a block plate and for the vacuum solenoids that were removed that are related to the smog pump and egr valve. Egr valve got stuck closed and smog pump seized on me so i removed it and put a smaller belt. Yes i plugged the two holes in the back of the heads

 

Okay, so no codes... I agree with ignition problem...

The only thing left other then spark plug wire routing and checking for arcing would be to try replacing the coil and/or testing the ignition control module in my opinion.

I'm still using the factory ignition control module on mine so not sure what the effects of a bad one are.

 

When you say wire routing do you mean having the wires touching the valve covers and stuff?

 

I should have mentioned that, spark plug routing generally refers to the practice of avoiding particular wires to run parallel with each other for long runs, especially ones that fire consecutively. One wire can induce firing in the other. Not sure this is a problem with modern wire and insulation though. It is good practice to follow factory routing though.