Sudden drop in MPG

Yes but unlikely. The mech could have left a carrier bearing loose allowing the driveshaft to bind, or harmed(bent/broken/moved you ride guage to cause a bind in the driveline. A drop that big would most likely feel like a brake hamging though so I would think you would feel the difference if any of those are the cases. Does it feel harder to start out empty?

I would check to make sure the driveline is secure, and properly aligned.

And which U joint was it. If he backed of a pinion nut and forgot to retighten it?????

 

5 days for a u-joint what kind of u-joint do that truck have. my mechanic got a drive shaft slip joint assy u-joint and had it welded up within a few hours. But for that amount of drop in fuel mileage it might be a combination of things.

 

If it's out you will see new wear lines at the slip joints, and /or knicks on the u joints.

Best would be to have someone forward and back( maybe two three feet) and watch the driveline for movement. If somethings out of line you'll see it in the difference of torque between forward and back.

I would call and talk with the mechanic that effected your repair with your concerns. A reputable one will think through anything they could have done to effect it. Honestly I believe you would already noticed the drag if that was the problem.


I just saw the 5 days part. Did they replace the mid-shaft between axles or something? Man a ujoint should have been in stock or a couple hours at most..


And this mileage number is that a pump/mile figure or an ECM number?

 

If it had anything to do with the U-joint you'd notice a vibration of some sorts. Even if the carrier bearing was off it wouldn't cause a loss like this in fuel. You most likely wouldn't even know a difference until it wore out another U-joint.

Your problem lies somewhere else. Does this mechanic have access to a computer and any programs to hook to your truck and change anything in the ECM? If so, you may want to consider stopping somewhere and seeing it they can tell when the last time someone got into it.

 

yes, i agree with Les2 have the ECM checked, restricted air intake can cause terrible fuel loss but that dosent happen overnight. meaning you park truck had good fuel milage then next day your getting 2 gal less a mile. my guess is the moron was under your tractor. check breather line (bad pcv filter in car + bad fuel milage) truck has crank case breather i,ve seen these kinked back causes fuel loss and excess oil consumption. check those first if nothing apparent then have ecm checked.

 

Is this your truck or their truck?

 

Have you thought that maybe the mechanic helped himself to some of your fuel? Were the readings you made including fuel that was in the truck while being repaired, or did you fill up right after and then calculate from the next fill up?