Engine still slamming after motor mounts replaced

Have you made sure your 5th wheel jaws are adjusted tight enough? Sometimes you can feel the trailer slide forward when braking and backward when taking off if your jaws are a little loose. Also, some 5th wheels are adjustable and some are not.

 

Spring u bolts, rears?

 

If you set the brakes and act as though you are trying to lift a load, ( trying to get moving) does your gear shift lever flop from to the side? Thats a sign of a broken or loose motor mount. I see you said they were replaced. Mayby someone forgot to tighten a bolt? If its while moving others have already covered that scenario.

 

A clutch with hot spots or bad springs will do that. Check everything else first.
Found some work better with just a touch of throttle and easy off the clutch. Yes that is slipping it just a bit. Against what I was taught but I had to drive it and that worked.

 

^^^^^^^^clutch yes^^^^^^^^

 

fifth wheel will make it jerk an bang when you take off.

 

Ya, are both sides of the "squares" fully clicked in.

 

If after you grease the 5th wheel it gets bad you may need to have the 5th wheel jaws adj. most shops have a pin on a weld on arm that they use to adj. it to where it is tight but will still let you pull the unlatch with/out it hanging up and being hard to get unlocked?

 

Check your suspension. I had a broken spring underneath an axle that caused a similar problem for me. It was in a place where the break wasn't visible during inspections. It finally pulled free, and I found the problem when I was backing into a dock. I noticed that the drive tires weren't parallel because the axle was offset by about 2 inches.