I work at a small camp. We have an 89 F150 with a 4.9L six cylinder. Last year it started running on one cylinder when it was cold or wet. It would run fine after it warmed up. I replaced the plugs, wires, cap, rotor and it was good for a little while. Now it is back to one cylinder (#6). I spray ether in the intake and it does not make much difference, it may speed up a little, but still one cylinder. The fuel pressure is 50 psi when it is running, so it seems like that is not the problem. I drilled a hole in the exhaust to see if the cat is plugged (at my mechanics suggestion) and it made no difference. I put a wire in the coil wire end and held it about 1/4 to 1/2" away from the contact to see the spark, and it ran on all six cylinders. When I moved it closer so it contacted the distributor contact it would go back to one cylinder. I replaced the coil and now it doesn't start at all.

Help

 

Sounds like something down in the distributor

 

I also removed the ignition module from the distributor and had it tested at Autozone and it tested fine.

I could replace the distributor, but I would like a better idea that is the problem before I spend the $60.