I recently bought a used 1990 F150 2wd Longbed pickup.

The truck runs well, and is smooth up until around 60mph, then it begins to ride very rough, as if driving over bumps. Does anyone have an idea what the problem may be ?

I just had new tires put on and balanced, but this had no effect on the issue.

Thanks for the help

 

5 years ago my 94 F-150 had a similar problem that eventually becamming a wild bucking as it fell out of gear and then went back into gear. I was told online that it was the MLPS (Manual Lever Position Sensor) aka: Neutral safety switch, aka: Park Neutral Start Switch, aka: transmission range sensor. Today I changed it again and she rides smooth at high speeds again. 46.00 at Napa, 39.00 at Checker, 37.00 at Autozone. Very common problem, so the supply is high and the price is low. Good luck man.

 

Oh yeah!
It is located on the drivers side of the transmission where the linkage connects. Pop off the linkage, and remove the nut that holds on the linkage arm. remove the two 5/16" nuts and slide the sensor off. Unplug the wires and plug the new one in. Slide it on, bolt her up, pop on the linkage. And you are smooth driving again. It will even idle better, the computer prefers accurate signals.

 

Ohhhh..

Uhm .. it is a 5 speed manual transmission.

I believe it may be suspension related.

 

it is ONLY speed related? Or can you duplicate the symptoms at lower speeds, but similar RPMs?

 

It is only speed related. Rpm's have no effect on it, I can put it in neutral and it still is pretty shaky as long as I'm still at 55+ mph

 

Sloppy/dry U-joints?
Sloppy Slip jolk?
Driveshaft loose a weight?

If the vibes are still there at speed with the trans in nuetral, all you have left is trans, and driveline. And the only thing that will create such a vibe is a rotational mass.

I would also have the tires re-balanced. And make certain they are doing a high speed balance.

 

First thing you should do is loosen up and retighten your lug nuts. Sounds like you have a wheel hop problem due to a ****ty tire rotation. remember its a star pattern and not in a circle.

 

My truck is a '95 supercab shortbed, and I noticed the exact same thing shortly after buying it. It would start around 60 mph, be worst at 65 mph, and start to disappear at 70 mph. By 75 mph, it was completely gone.

What it turned out to be is a combination of my trucks wheelbase and the length of the cement sections on most of the freeways in So Cal. At 65 mph, the wheels hit the seams in between the sections of cement just right to cause the truck to "buck" quite a bit.

Since then, I've added 3 full toolboxes, which has dampened the effect considerably, but it still happens. I can always tell when I'm near 65 mph just by the way the truck rides.

BTW, I confirmed this was the problem where the 101 freeway switches over to a blacktop surface at the Ventura County/L.A. County line. As soon as I hit blacktop, the problem completely disappears.

Does your issue happen on blacktop as well as freeways?

 

Have you Checked your U-joints?

 

I have the exact same problem with my truck. It's fine at 65, jumps like crazy @ 70, then is fine again @ 75. I rotated the tires, which helped a little bit, but it's still there. I bought the truck around 98,000 miles, and now I have around 153,000 and it's only been happening for the last year or so. I will try having the tires balanced, but I too think it's something with the suspension.