1990 f-150 Lariat xlt rwd,auto,58,000 miles. What would cause a cylinder to misfire everyonce in a while.

 

carbon tracking on outside of plug, bad plug wire,weak coil...

 

A badf PCV valve can also mimic (or does it cause) misfiring, especially at higher steady speeds, you will feel a slight lurch.

I like the spark path theory, check that - maybe also get your computer codes read, as they should inform you of misfires, but I guess dirty/water contaminated fuel or a clogged fuel filter are possibilities....but checking distributor cap is one easy thing to look at.

 

new plugs, new wires helped, ignition module helped for a while then went backed to the miss. new cap no diffrence, fuel pressure at 35 lbs .tried new reg coil didnt change, haven't changed pick-up coil ? changed fuel filter. no difference. seames to be worst until it gets hot. even hot does off and on.? Octane booster didn't help. Tried injector cleaner and it made no diffrence. Seem to idle better if i have the selector to defrost, and tuned to off, it idles worse?

 

There is a big clue right there. The EGR valve is controlled by vacuum just like your HVAC. I'll bet a doughnut that the EGR controller is bad. Pull your codes and if there are any EGR related ones, spend the 35 bucks on this...



This is how you can check your codes.

http://fordfuelinjection.com/?p=13

Adrianspeeder

 

Agreed. i have a CEL i pulled it at work. Bad EGR Bad MAF . I am going to get a new EGR and replace it so i can get rid of it. Also i am going to look into upgrading the MAF and throttle body and intake tubing anyone have any idea?

 

I will try that, but i haven't CEL come on, only a few second after start up. But never comes on after that. But when i get a chance, i will try that.

 

Change out the controller first. Half the cost.

Adrianspeeder

 

Well, adrianspeeder, you owe me a doughnut (j/k). I pulled codes for a PFE / EVP? What are they and what do they look like and would any body know costof replacement.