I have a 91 F-150, 5.0L, V8, and when it accelerates, I hear a knocking noise which sounds like it is coming from under the cab area of the vehichle. This vehicle has duel tanks, one doesn;t work, the rear one does. Someone suggested that maybe this noise is due to a fuel pum failing. Does this sound familiar to anyone?

 

It could be a lot of things. Your fuel pumps are in your gas tanks, so I wouldn'y worry about them making the noise. It could be an engine with built up carbon in it, or a bearing going out. Try some fuel treatments that claim to clean combustion chambers. If that doesn't do it you need to find someone that knows their stuff to listen to it.

 

Thanks for advice - will try some Techron fuel additive. Would built up carbon be cleaned by the fuel additive? Could it be bad fuel that causes knocking intermittently? Hopefully nothing like a bearing gone bad. Read somewhere that maybe it was a leaf from suspension that was making the noise. Does this sound plausible? Thanks for suggestions.

 

Under acceleration, the engine twists some. More if the mounts get a little softcausing parts directly attached to the engine such as exhaust to move as well. If the noise seems to be coming from under the cab then maybe the exhaust pipe is hitting the frame somewhere.

Worth a check.

Dan

 

That makes great sense, however, last week I had a new muffler and tailpipe installed and the noise is just the same after the fix as it was before. I wish that would have solved the problem, since that repair was cheaper than a few other potential problems. Thanks for the input.

 

I actually had a very similar problem with my F150 and it turned out to be the Exhaust hitting the trans crossmember. Sometimes even the "exhaust guys" miss that fact because all they do is drive it onto a rack and off again. But never have it under acceleration or going over bumps and the like. So I would check the exhaust brackets. They are cheap for universal ones that you just bolt to a near by frame hole.

Mike B

 

Get under it while it's still cold and start grabbing things and give it a good shake (like the exhaust pipe and trans tailshaft). Try to duplicate the noise.

Open the hood and run the engine to see if it makes the noise when revved. When doing this check to see if the motor moves excessively.

 

do this to check for bad motor mounts. do you know how to powerbrake? hold the brake hard put it in gear with the hood up with somebody looking at the motor. then give it some gas enough to make the truck want to go. if the motor moves up or sideways a lot then you have a motor mount problem. do this in reverse also to check the other mount.

 

Thanks for the ideas! I will definitely give them a try this weekend. Hopefully something like a bad motor mount will not cost and arm and a leg. Keep making suggestions, I'll try them all!

 

Knocking under the cab?
Vibration too? If so, may be Ujoints on driveshaft. - or possibly the exhaust hangers. Just a guess.