Fourth Gear Fail

[mjestes17]

hey guys could anybody tell me why my truck runs great in 1st, 2nd, and 3rd gear but when it shifts into 4th, it starts shaking uncontrollably? It's an 89 with 4.0 renix (obviously) and I have brand new fuel pump, gas tank, injectors, spark plugs, tps, IAC, and CPS. I can't even get this thing to go on the highway because I'm afraid it will fall apart. the problem is ONLY IN FOURTH GEAR and I'm completely stumped.........

[mvusse]

Does it ALWAYS do it AS SOON as it hits 4th gear, or just most of the time around that speed?

 

Automatic transmission? Put the lever in 3 to keep it from shifting into overdrive (4th) and run the speed up to 65 or 70mph.

 

I'm thinking you might be experiencing death wobble, which would be a problem with the front steering/suspension and has nothing to do with the engine or transmission.

[mjestes17]

it does is AS SOON AS it shifts into fourth (auto tranny) and its definitely not death wobble beause i get doeath wobble and its scary as hell haha. this is definitely and engine or tranny issue and it scares me because i don't want my engine to fail on me and I'm still novice to the whole jeep maintenance thing. I'm learning things as i go and this problem just came up and I'm stumped so if you have any clue, it's greatly appreciate! -matt

[mvusse]

In that case I would check engine and transmission mounts. That's one issue I have never experienced myself.

 

Also, why did you replace spark plugs, but not the wires, distributor cap and rotor? They all usually get replaced together.

[mjestes17]

because the distributor and wires were all replaced by the PO and he himself didn't replace the plugs. they were dirty as all get up when i removed them. sad to see what people do

[Comanche County]

Get underneath it with a pry bar or something similar and see if there's movement in the rear u-joint. Does it still do it if you shift into neutral at the same speed?

[mjestes17]

I'll have to get back to you on that one. at work right now so I'll check it in the morning. I'm willing to bet my u-joint could use replacing because I'm guessing it's never been swapped out.

 

the truck used to drive on the freeway just fine in overdrive but at about 55-60 it would buck really hard every once in a while and I had no idea what it was so that's when i decided to start swapping parts out and upgrading because the truck seemed to be falling apart beneath me. any clue as to why it would buck like that?

[mountainman]

two options to diagnose something like this. Either borrow a go pro from a friend (seems everyone has them now) and mount it under the truck. Then watch for whats getting all crazy.

OR speak with a shop in town that has an elevated dynojet dyno rack. Put it on there and run it up the speed where the issue is, of course you or whoever's dyno your on can watch and see WTF is going on.

This is of course if you can't find it through the normal diagnosis process. Probably take it by a good gear shop and have one of the techs take it for a ride, they can usually tell you what it is straight away.

 

Hope you get it sorted.