I have a 1998 4.2 with 188,000 miles. Codes reading cylinder 4 and 6 missfire. Engine will missfire upon acceleration and at 2,000 rpm a strong vibration can be felt. It only does it once the engine is warm. I have done: new plugs, wires, coil pack, fuel filter, and ran 3 cans of sea foam through the injectors.
Anyone have any ideas?

 

Check your EGR valve. Apply vacuum with a hand vacuum pump through the vacuum port on top of the EGR valve. If the engine stumbles and returns to previous running condition its ok. If the engine doesn't change or go into a stumble then the EGR is probably stuck open and needs cleaning. Give us a report on what you find.

 

I am having the same problem with my truck and have done just about the same things as you with no success. Did you get the chance to check your EGR like Torkum suggested?

 

Same thing here 1998 f150 4.2L runs like sh*t in the same way. ANY LUCK WITH EGR

 

Edit: Check the rubber pcv hose that looks like a 90 degree elbow. It gets cracks in it and causes problems. Also check the bolts that hold the bottom half of the plastic intake to the aluminum lower intake manifold. The older style bolts have rubber seals built onto them which is what goes bad. Ford has redesigned them. The new bolts will have green rubber seals, and the factory ones are black.

It wouldn't hurt to clean your egr tubes out, and clean the two egr ports in your throttle body. It probably needs it anyway.

 

i had the same problem...at about 30,000 mile is started misfiring and shaking so bad i could bareley drive it.....i had a new coil put in, fuel filter plugs, wires....the truck runs fine, but every now and then when i start it up in the morning it will miss until i get it into about 3rd gear then it stops so...