Ford F-150 vibration problem at 1300 rpm

I have a 1995 Ford F-150 4x4 custom cab automatic with 87000 miles. When I started it 2 weeks ago, I noticed a slight engine vibration. This slight engine vibration occurs at all speeds when I am pushing on the gas. Whenever the engine speed is at 1300 rpm, the engine vibration becomes quite pronounced and shakes the entire pickup. When I let off on the gas, it goes away. That is, when I coast the vibration goes away. When the engine is not under any load, then the vibration mostly stops. When I speed up (above 1300 rpm), it mostly goes away. If I'm at 1300 rpm and I tromp on the gas, it quickly responds and the vibration also mostly goes away. I have 2 gas tanks and the result is the same on either tank. I have replaced all spark plugs and wires 7000 miles ago.

 

Your motor mounts might be worn or you probably have some sort of transmission issue.

 

sounds more like a bad u-joint to me im almost sure if you pull the driveshaft you will find a bad u-joint.

 

I'm going with the bad u joint also. One way to check is to jack up the truck to relieve pressure on the u joints. Grab ahold of the driveshaft and see if there's any play at the joints. Post back and let us know what you find.

 

Put the vehicle in gear with your left foot holding the brake firmly. With your right foot accelerate to 1300 rpm. If it vibrates, it is engine related. If it doesn't, it is most likely the rear u-joint. Adding to what DREGG said, inspect the u-joints for the presents of a powdery rust colored substance around the joints.

 

U-joint first thing I thought of, too. Be sure to post back any findings.

 

a couple, possibly obscure, other thoughts.

You did not truly clarify if the vibration is present when not moving (vehicle in park/neutral).


You also did not say if you have snow and with the 4 wheel drive, if it is with 4 wheel drive engaged or not and if you have a plow.

So with all that said:

1. snow in the rims making the tires unbalanced
2. bad front axle shaft u-joints, generally not noticed unless using 4 wheel drive.
3. if you plow, I have seen a plow driver bend a driveshaft from running over a drift of snow and ice. If minimal, it may show up under a limited speed range

The other guys have hit the more probable reasons, just wanted to toss in a few other possibilities is you don't find it in their suggestions.

 

If you throw it into nuetral at 1300 rpm does it still vibrate ?
Are you leaking oil from your rear differential yoke ?
If you are you may have a bad yoke bearing.
I had the same symptons on an '86 Ford truck.
Drove me crazy, replaced the drive shaft twice but it was the yoke bearing.

 

Usually, a bad u-joint will not rattle during acceleration since it is under load unless the truck has been raised with a kit or big tires which put the u-joint at an extreme angle. I would suggest starting a test by having the truck(drive wheels off the ground) in neutal raising rpm to 1300 or where ever the vibration starts, then put it in 1st, check for vibration rpm etc. continue for each gear, then do 4 wheel. If the vibration is always at 1300rpm it would indicate something engine related if the vibration rpm changes it is most likely with the drive train. Have a nice day. Geo