My father's 1997 F150 with the 4.6 two wheel drive, has me stumped again. I have a lot of mechanic knowledge but this one has got me wondering whats going on. Here is the run down. When you are driving down the road, it rides just fine and makes no noises from 0-20 MPH, after about 25 to 35 mph it makes like a metal on metal noise that is coming from the rear, and forces the truck to slow down, when you try to accelerate through this problem it has a bad bang and it wont allow you to speed up anymore, it does not seem like a brake issue (but I could be wrong) since it does not actually pull to one side or the other, it seems like an equal force trying to stop the truck. I have heard rear ends that were going out and they make clunk noises or whirring noises etc, but its still some what driveable. I have checked the rear end oil and it does have oil in it, and its probably about an inch from the full level (but that shouldnt cause this issue im sure). My father wants a definite answer before we start tearing this thing apart to try and fix it. Since its mostly used to take trash off, and also get the kids down the dirt road to the bus stop. Any help would be greatly appreciated, and if you have any questions that could help me out. Please ask, I will be here. Thanks y'all....

 

Welcome to FTE. You might want to put the truck up on jackstands and turn the wheels and listen for grinding or other abnormal sounds. Might even pull the cover off the diff. and look for metal shavings or other bits laying in there. My 92 did something like yours and it was the outer bearing and seal. Diff had got water in due to the bad seal and caused all kinds of havoc. I'm not a mech. anymore, but that's where I would start. Good luck.

 

Well I dont want to pull the cover off until I have someone say it could be something in the rear end. As far as the outer bearings, would it all of a sudden do this? there has never been an issue and one day it just started. I checked the gear oil, and it looks almost brand new, not much of a discoloration (which would be weird since it has almost 400,000 miles on it. After a Motor and transmission replacement the gear oil is still almost perfect? Wow.... but it is definitely coming from the rear of the truck. Thanks for all the help.

 

Hmmm. Do you have any vibration when the bang occurs? Did you remove the driveshaft from the diff. when you did the engine/tranny swap? They are supposed to go back in the same position. Did it throw any codes? Try driving it till it does that and feel the rims to see if they are hot, i/e brake issues.

 

Both you and father are not being reasonable by wanting someone else to guess what's wrong before you take any action..
If you need that, guess yourselves. It won't be any worse.
What would slow a truck down like you experience?
Think something in the drive train binding?
Make some effort to try to identify the source.
Drive the truck to bind then come back and begin to eliminate what isn't the cause.
Check wheel rotation, rear rotation etc. until it narrows down to a suspect.
Jack it up in the rear and spin the wheels in drive. Apply brakes etc anything to provoke the problem.
If you took the truck to a shop, what would you expect that shop to do?
Good luck.

 

1997 with 400,000 miles driven over dirt roads and you don't want to pull the diff cover? U-joints checked? brake inspection? Dude you have some work to do. No clear cut internet fix on this one.

 

I am not a mechanic, but I would think it would start all of a sudden one day. Knowing the effect of adding the first little drop of oil on metal contacts, I would assume the reverse would happen as well. When the last drop of oil dries off, friction will rise sharply.