I have an 87 F150, 5.0 FI, with 283,000 miles on it. It's run like a champ for all these years. However, recently, it has developed a SEVERE vibration at all RPM's (more so under a load). I thought the worst at first, like a burnt valve, blown rings (compression issue), bearings, etc. I did the usual first, replaced plugs, wires, cap, rotor, looked for any external problems with pulleys, etc., I even did a compression check which showed all 8 cylinders within the same range of 120-125psi (not bad for all those miles). There is no knock or pinging, no smoke, no SES lamp, and the truck seems to run normally otherwise. I can't seem to figure it out. I've heard that it could be the ignition module, (it sort-of acts like a dropped cylineder) but I don't know how to test it or what to look for. Does anyone know it may be or what I can look for?

-Andy Forsythe
Columbia, TN
usscvr@charter.net

 

Could be something as simple as a slipped harmonic balancer.
This is a common occurrence on older engines and an easily over looked item.
I would have it proffesionally diagnosed-you could spend a ton of money "guessing".

 

Dude - thanks so much! It was the front balance/damper! Now I know what is is - that's the good news. Thge bad news is that it destroyed the inside of the balance, and chewed up the crank a bit. I think its fixable. Thanks for your help!