Ok guys I’ve got an’86 F150 4.9l and all of the sudden it’s developed a miss. I replaced all the spark plugs because that’s the easiest thing to do. That didn’t work so what direction should I go next? I guess this could be a fuel or electrical problem. So do I go distributor next or fuel filter and pump?

 

Is the miss at idle? Does it still miss at higher rpm? Is it a steady miss at steady rpm? Or more of a random miss not related to rpm?

 

Yes need to know a little more information.
Now before throwing more parts at it lets do a compression test.
Pull all the plugs when you do this. The throttle should also be open all the way but I never have.
You are looking to see if all holes are with in think it was 10% of each other.
If you have 1 that is really low that is your miss but why?

If you have a way to put air into that hole get the hole at TDC put a little air in and give a listen at the carb (filter off), tail pipe, oil fill cap.
If you have air coming out of any of them that points to why the low compression, carb intake valve, tail pipe bad exh valve, oil cap bad rings or hole in piston.

Also a fast way to see what hole is not firing is to wet the exh manifold at the head. start the motor and only let it run for a short time or better yet look at the manifold.
The water will evaporate off the manifold on the holes doing the work faster than the one(s) not doing the work.
OR
With the motor running pull 1 plug wire off at a time. If it is doing any work the motor RPM will change (slow down) no slow down no work being done.
Wear heavy gloves, pliers with rubber grips (they make pliers just for this) and if you can just place the wires on the plug so you don't have to yank them it helps.
Dave ----

 

It doesn’t miss fire at ide. It’s at higher RPM and random. It’s fairly consistent but not at any rhythm.

 

That could be caused by a plugged fuel filter or perhaps the advance mechanism is not operating efficiently. Doesn't sound like a cylinder related issue. But Dave's suggestions could help eliminate those concerns. Do you have vacuum advance? Possible vacuum leak?

 

I'd be willing to gamble your hard-earned money on a new distributor cap and rotor before digging too deeply elsewhere. These two parts are normal wear and tear items, intended to be replaced on a regular basis. Call me Mr. Vegas, but that's how I roll.

Make sure you get a quality name brand, not a white box special from a discount chain. If no help, you've got spares that you will need at some point in the foreseeable future.

Next thing I'd suggest is a set of plug wires. Same deal, they don't last forever.

 

Don't forget this is not an ordinary engine setup. It has the computer control controlling all the timing advance and trimming the fueling. with lots of sensors, vacuum lines and wiring. If the sparkplugs are good, I would check the timing. There is a special procedure for setting the timing on these type of engine control systems. It's spelled out briefly on the radiator sticker. Write back in if you need more help checking and setting the timing.

 

If you are missing at higher RPM you need to narrow it down is full load or just RPM dependent. Generally, high RPM misfires are ignition related, if you are getting a no-load high RPM misfire it is ignition.

First place to look is cap and wires. When were they replaced last? Since basically no shop has an engine analyser any longer you and people don;y do yearly tunes ups any longer (as was the norm back in the day) things end way worn out. So to check your ignition wires give them and the cap a spray down with some water (a spray bottle works for this) while the truck is running somewhere dark (outside at night in the garage with lights off etc) any weak wires will show up with arcing.

When was the last time the fuel filter was replaced? If more than a year replace it.

 

I recommend doing a tune up every 10,000 miles. Less mileage if the truck is being worked hard. Cap, rotor, plugs. I always buy high quality wires and they usually last two tune ups. Inspect the cap and rotor if you recently obtained the vehicle and don't know the tune up history.

I like the idea of spraying water on the wires in the dark. For added ambiance during the test, get out the old lava lamp!

 

Yup the water trick is one of the oldest tricks in the book and what was done before the advent of engine analyzer oscilloscopes that could display the spark pulse and isolate faulty secondary components.

 

Spray bottle on a dark night works wonders. If you have any doubt on the condition of cap/rotor/plug wires or they are over 5-10 years old, do yourself a favor and replace them with good quality parts.
Test coil with ohm meter and observe if laminations are coming apart.

High RPM miss could be a faulty coil, ICM, bad plug wire.

Pull codes too, that will really help you. All you need is a paper clip and $5 test light.