PNWjeeper

So I went mudding the other day (probably the only trip I'll get this year) and now when I push the gas to the floor I get bad vibes all around my jeep and when i let off it goes away. Under normal acceleration, I try to keep it at or under 2. I'll get rolling vibes, starts at the rear moves forward then disappears mostly, at 40 mph. At first I thought it was the tires being off balance either because of a bunch of dried mud or knocking off a weight, I plan on cleaning the tires and getting them rebalanced. Any other ideas guys?
Thanks.
1990 4.0 auto renix

PNWjeeper

I was hoping for something within 4 hours.

Asphalt Cowboy

same thing happened to me only i was in a chevy. cleaned the mud off the rims and was fine.

PNWjeeper

Thats what I was thinking at first, but then why would the whole jeep shake with WOT and stop immediately after I let off? The tires would still be spinning at the same speed.

PNWjeeper

Cleaned off my rims and I still get the vibes. I only get them while moving and get them the worst when accelerating at 4k+ RPM's, if I revv in P or N nothing.

89xjrocker

Misfire maybe how deep was the mud any get around your intake or distributor last time I went Mudding I somehow pulled a vacuum line loose caused the motor to have a rough idle until I tracked it down

PNWjeeper

Mud wasn't deep at all, didn't even reach the unibody except for the splash it barely reached the rims (best i could do on a whim with no one to go with me). I found that the hose to the FPR had fallen off plugged it back together but it didn't change anything.

PNWjeeper

So to get more detailed.
My jeep vibrates when I put the pedal to the floor and RPM's reach anywhere from 3k and up (something it never used to do). It will also begin to vibrate at 40 mph ( not from the tires that doesn't happen until 50). I have no obvious loss of power.
My initial hope was tires were off balance due to the dried mud, washed that off and no difference. I haven't got the tires checked to see if they need rebalanced but I plan to wether or not its the problem.
I've checked vacuum hoses and found the hose to the Fuel Pressure Regulator had come lose, I put it back together, again no difference.
Plenty of gas in the tank.

What else could I be missing?

JerrytheJeep

How much lift do you have? How do your motor and tranny mounts look?

PNWjeeper

Between 0 and -1" of lift at least in the back. I'll check the mounts tomorrow. I know where the engine mounts are but not the tranny mount, obviously on the tranny but that's the extent of my knowledge.

JerrytheJeep

Just above the crossmember. look for cracks, splits, or signs of deterioration. I have yet to see an xj over 12 years old with intact motor mounts, but that's my experience. Take a pic and post it on here, experienced eyes and whatnot.

PNWjeeper

Back to tires, I was near the place I purchased my tires and popped in to see if they would rebalance them since I got my tires there over a year ago. Told him what was going on and he said to bring in my jeep (i didn't have it with me for once) and they would do it, he also suggested that there might be a misfire happening at the higher RPM's. Any truth to that? Like I stated earlier I put my jeep in N and P then revved with no shakes.
I can fix things no problem it's the diagnosis that gets me all confused and frustrated.

PNWjeeper

Here is a pic of the passenger motor mount and trans mount.

JerrytheJeep

Can't see much with the motor mounts, you need to look at the rubber within and see how its doing. Tranny mount may need to be replaced as its looking a little low. Thing is, if you replace one you should replace them all, both motors and tranny at the same time. Replacing one or two but not all three is not good on the ones that are still bad. Here's the difference between my motor mounts and the new ones as well as the tranny mount.



Aside from the fact that it has broken away, you can look and see how much lower it was than the other.



Theoretically the newer rubber will transfer more happy engine vibrations to the body, but at higher rpms it shouldn't be able to shake around as much.

PNWjeeper

I'll take a look into those. Thank you.

Is there anything else I should look at?