Strange Vibration

[caseyj512]

I have a 97 F-250 4x4 7.3L Auto. I recently noticed a new vibration in the engine with the RPM between 1950-2100. Above 2100, the vibration dissapears. Its a fine vibration that is present anytime the engine is between those RPMs; In park, Neutral or shifting up. I don't hear any squeaks or grinding when the vibration is present, or when its at idle with the hood up. I had one sticky idler pulley bearing thats been replaced.

I'm looking for any suggestions.

Thanks,

Casey

[forddieseldoctor]

pull the belt off and see if it still vibrates.

[caseyj512]

I pulled the belt off and noticed a definite decrease in the noise and less vibration. With the belt off, when i spin the fan blade, I hear a distinct medium pitch groan coming from the water pump. If I push/pull on an edge of the fan blade there is noticible play in the pump. Im guessing these are not normal and should replace the water pump. Thoughts?

[forddieseldoctor]

sounds like you got a winner

[caseyj512]

Well, after all that (changing the water pump), the vibration decreased, but is still there. The PS pump seems to turn smoothly but has a decent amount of resistance. The PS pump has about 3/16 play in and out, but no play up and down. Think it needs to go also?

[forddieseldoctor]

nope that's normal to have that kind of play.

[caseyj512]

The alternator doesn't have any play up/down, in/out but squeaks a bit when I turn it by hand. The only things left are the balancer and the vacuum? pump near the PS pump. Any recommendations? I realize this vibration probably isn't a huge deal, but its driving me nuts. Thanks for the info so far!

[forddieseldoctor]

is the vibration totally gone when the belt is off?

[caseyj512]

The vibration is definetly present with the belt on or off. I noticed today that the RPM range is increasing and I feel a series of different vibrations on the floor under the brake pedal as the RPM increases/decreases. I'm going to crawl under there and see if something is loose and touching the floor or frame or if a mount is broken.

[caseyj512]

I wasn't able to see anything rubbing on the cab or frame, and the mounts don't have any obvious fractures.

When I run the engine up in neutral to 2000 rpm, the rpm will drop by 300-400 without any change in throttle position on my part. I have to depress the throttle again to get the rpm back up to 2000. I got the same drop when trying 1500 rpm and 2500 rpm.

I changed the fuel filter recently BTW.