weird vibration

[CountryBoy704]

Hi all, working on a '94 extended cab 4.0 5speed 2wd

A VERY pronounced vibration is going on only when I am accelerating and generally at lower rpm's, all gears. It goes away if I let off the gas or push in the clutch and does not happen when engine braking

I just replaced the driveshaft center support bearing and it didnt help, also no detectable free play in any u-joints

If you have ideas of what/where to check next post up cause I'm at a loss

 

[black_demon69]

Hi all, working on a '94 extended cab 4.0 5speed 2wd

A VERY pronounced vibration is going on only when I am accelerating and generally at lower rpm's, all gears. It goes away if I let off the gas or push in the clutch and does not happen when engine braking

I just replaced the driveshaft center support bearing and it didnt help, also no detectable free play in any u-joints

If you have ideas of what/where to check next post up cause I'm at a loss

sounds to me to be bent/out of balance drive shaft or and most likely u joints.

 

[CountryBoy704]

thats what I thought at first but this vibration happens at the same exact rev range in all gears. That makes me think its something in/towards the engine itself but I could be way off track there. Transmission and engine mounts look good

 

[black_demon69]

thats what I thought at first but this vibration happens at the same exact rev range in all gears. That makes me think its something in/towards the engine itself but I could be way off track there. Transmission and engine mounts look good

with or without load on drive shaft? have u joints ever been changed to your knowledge? still betting on u joints they are fairly cheap and easy to replace if it is a split drive shaft it has 3 u joints should be pretty straight forward. when i did mine i replaced all 9 (3 in front axle 3 in front drive shaft and 3 in rear drive shaft also did carrier bearing)

 

[ab_slack]

I assume since you aren't new here that this isn't your first manual trans vehicle and that the vibration isn't the typical vibration you get when lugging the engine.

With the u-joints, when you checked for play did you rotate the drive shaft to other positions? A bad u-joint may be loose in one position and binding in another position....in fact if the u-joint is just binding you might not see looseness.

I would expect bad u-joints to exhibit vibration even at higher speeds so it does seem odd that you experience vibration at low RPM in every gear because obviously in higher gears you are going much faster.

You have an engine miss by any chance? What you describe is a loaded verses unloaded condition. It is possible to have a miss that only occurs under a loaded condition or is minor till there is load. This sort of issue would typically occur at lower engine RPMs. At higher RPM the engine dynamics change and either it isn't noticeable or it stops missing.

Typically such a issue is related to spark. You can have spark but it is harder to get a spark under more compression. When there is no load the engine isn't getting as much air/fuel so spark is easier. When you put it under load and the air intake opens up it can get more air thus the pressure in the cylinder increases and it has a hard time sparking.

So it might be worth checking your plugs. If you have one or more fouled and/or gap is off fix that. If plugs are good, make sure wires are good and fully seated so spark doesn't need to bridge an extra gap. Check out the cap and rotor.

We recently dealt with an miss only under load issue like this in a jeep we have. At higher RPM there was no problem but at typical driving low RPM it was horrible and would actually buck a bit at some RPM ranges. When we checked the plugs they were so fouled that we were amazed it ran, gave it a full tune up, plugs, wires, cap and rotor....and it still had the same sort of miss after....turned out we damaged the rotor while replacing it and had bent the conductive element so it had to bridge an abnormally large gap in the rotor. Replaced the rotor again and problem went away. I suspect originally the plugs were the problem. It was only that we damaged the rotor that it became the problem afterwards. It goes to show that more than one thing could result in the same symptom.

Anyway this would be my best guess for something in the engine and not much to lose checking since it is pretty simple to check out.

 

[Dune Runner]

I agree /\
Sounds like it needs a tune up.

 

[CountryBoy704]

set of plugs and wires did the trick! passenger rear plug looked like it had been dipped in tar haha
Thank you all for the help

 

[ab_slack]

set of plugs and wires did the trick! passenger rear plug looked like it had been dipped in tar haha
Thank you all for the help

Great, good to hear you got it resolved.

Isn't it great to find something turns out to be so simple?

Watch that rear plug. Sounds like that may have been the miss. So the question is did it look that way because it was not sparking (like a bad wire) and getting coated with fuel? Or did something foul it (like oil or fuel injector pouring in too much fuel) causing it to then miss?

No need to worry but you may want to make a point to check it out again in 6 months and see how it is doing compared to other cylinders if it continues to run fine.