I have a 90 F150 with the 300 I6. Recently (last month), the truck rattles/shakes a lot when accelerating. It's severe too. It gets better at higher speeds (like 65+) but that may just be because it's about to shake apart. I initially thought it was an issue with the rear fuel tank but have ruled that out. The heat shield on the cat is rattling something fierce too but don't think that's related. Clutch is new as of Nov 2015 so doubt it's that. Could it be exhaust related? Doesn't feel like anything front end either because it steers and handles fine. Suggestions?? I'm not wanting to throw money at it to diagnose. I bought it for $1500 and have put $900 into it so I'm close to the buy a new one threshold. It has 144k original miles too.

 

Fuel, spark and air. You have a fuel issue or a spark issue. Sorry to point out the obvious.

How old are the plugs, wires, cap/rotor?

What is the fuel pressure at idle? At idle with the vacuum line removed? Under load?

 

Plugs, etc. are old enough that I don't know. As far as pressure, I'll test it. Thanks.

 

I think randy may have been responding to a different post ?
I would a) inspect engine mounts, b) do something about the heat shield so that you can hear any other problem(s), c) do a good visual inspection of the tires and think about having them balanced.

 

No, replied to this post. Shaking/rattling could be a miss caused by bad spark plugs. Might as well verify fuel too.

 

The tires are brand new as of December. I'll check everything else.

 

...very true ... and while we are on the subject, arty ... might be a good idea to run your engine in the dark to see whether any sparks are escaping from their wires.

 

Just trying to point everyone in the right direction. In neutral/clutch disengaged, how's it rev? smooth, or rough?
Also, if there's vibrations at speed, it may be caused by losing a wheel weight. New tires should have been balanced, and you may have lost a weight since then, due to faulty installation of said weight, or from other reasons ()

 

Harmonic Balancer bad? Had one go out on a 302 engine, shook bad.

Have you split the drive shaft to change the U joints? If you stab the drive shaft back together 90 deg or 180 deg out from original then it will not be balanced anymore, vibrating at lower speeds.