Got a problem with my son's 1989 F-150/ 305 FI. The other day it sounded as if it had a dead cylinder. Did a compression check and all cylinders were above a 100 lbs except one wich was about 90 lbs. Went ahead and replaced the plugs and wires because it needed it anyway. Started up and it was the same if not worse. Tried to drive it and it got so bad the dash board would shudder and the truck would hardly pull. It's to the point it's not driveable. With the hood up you crank the throttle up and the engine acts as if it wants to do back flips. Checked the wires to make shur they were not crossed; and their OK. Checked the codes with engine off and through the ignition switch and nothing comes up. Asked my son if he had done anything to it reciently and he said he changed the oil the day before and put a quart of Marvel Mistery Oil in it. I know a quart is a bit much but can't see it being enough to cause a problem like this. Truck has 190000 mls and was running good prior. Replace the module, coil, distributer cap less than a year ago. Any ideas please. Mayby skipped timeing (How do you check that?) Colapsed lifters, plugged injectors. Thanks for any help

 

cap/rotor should always be changed when doing plugs/wires. what plugs did you use and what firing order? that motor would use the regular 5.0 firing order not the 5.0HO. Timing should be checked if possible. 10*btdc with spout connector removed.
check out Ford Fuel Injection for instructions on pulling codes, if the computer is working properly it will give a 11 pass code. i would change fuel filter and check fuel pressure.

 

Looks like a GM guy found our site.

When you say the other cylinders were over 100 psi, what were their numbers? And which cylinder was at 90?

 

to really do a proper compression check, you need to yank all the plugs, pull the injector wires, hold the throttle wide open and crank. should get into the 110's 120's then.

check for 7-8 crossfire or a slagged plug.

lastly, replace the TFI. I had a on again/off again/getting worse 'dead cylinder' feel and went looking at injectors as the raytemp said one hole was significantly colder. turns out as the truck warmed up, if I turned it off and it hot soaked, next start it would skip a beat like clockwork.