Ok now here's the deal, 77 F-100 C-4. From a stop in 'D' as soon as you get on the gas it immeadiatly slips slides into 2nd. man low works fine. I thought the trans was pooped but after playing around with the truck I found if I put it in drive, and hold the selector 'up' it works fine. What is happening is the engine is twisting and torquing the trans and causing the manual valve to be shifted half in 1st and half in second if that makes any sense.

Could worn motor mounts and front springs be making this worse? when I rev the engine the whole truck tends to twist to the right, anybody ever have such a weird problem?

 

Try revving it both in 1 st & reverse with the brake on to see which motor mount is broken.

 

Also, check the tranny mount, it could be raising in the rear under a load.

 

tranny mount is ok, I'll get back to you on the motor mounts cause the trans is out of the truck. Took it to school to rebuild it and somehow stripped out 2 bolt holes in the valve body, thats todays project putting helicoils in those.

Anybody got any more ideas????????

 

Phinx,
Motor mounts can cause some peculiar problems.
Does the shifter move at all during accells?
Can you feel the throttle pedal move at all?
You should be able to see the shifter move when you go over a speed bump or something that would twist the frame.
Good luck,
KingFisher

 

I had this happen to me in my 76 f-100

the driveside motor mount was broke and it would pop into 2nd.

changing the motor mount is a pretty easy job, or if you are lazy just chain the engine to the frame.

 

Yeah the shifter does move, when in D and from a stop when I gas it it moves toward N. Haven't really noticed the gas pedal move, and as far as the bumps I don't know about that either. So being that motor mounts is a weak area for me, what am I looking for to tell if they are broken? Can I physically look at them and tell they are broken without the engine running? What does it take to change them, as in chaining the engine to the frame??????? Knowing the habits of the previous ******** owner I'm sure when they did the engine swap they never changed the mounts.

Thanks again for the help

 

Gas pedal move?? What causes that? I think I may have noticed that yesterday while driving my truck. Also, when I start to accelerate from a stop the truck lurches a bit....could that be caused by the fact that I have broken leaf springs?

 

I was checking them out briefly today, that rubber seems kinda squishy, like a hard sponge if that makes any sense, should I change them? Is there anyway to do it instead of an engine hoist??

 

pedal movement should only occur if you have
the older linkage type of gas pedal
with the cable style you won't see pedal movement
these problems happen when the engine raises from the torque
and the linkage being attached at the body and the drivetrain
when one moves and the other doesnt something has to give

 

They will usually have a crack across them. It may or may not be easily seen. The easiest way is with the truck running and the hood open, put it in drive, while holding the brake, give it a little gas. If the engine lifts on the drivers side more than about 1/2", then it's probably broken or almost broken and stretching too much. You can also do this in reverse to check the passenger side mount.
You may not feel the gas pedal move if you have a throttle cable and not the rod type linkage.

 

Well I got a gas pedal rod type setup, I don't feel it move and now that the tranny is back in I tried it in D and R and didn't really notice the engine move but the whole frame just twists, and the whole truck just rocks when you give it gas, even in Park it does this. GRRRR.