Another vibes thread

I know its discussed a lot, but I was hoping for some advice on where the source of my vibrations may be coming from.

99 xj 4.0l automatic

I recently did a ton of work on it, and it runs great but I have a pretty annoying vibration problem. I installed brown dog rubber motor mounts and after some tweaking of the through bolt, I got the vibes to a more reasonable level.

Now, I feel I have a very loud but "soft" vibration (as opposed to the brown dogs at first which felt very stiff and rattled everything). I'm betting its vaccum related?

Here's what happens:

-Start the car, it idles solid at 1000 and is SMOOTH. Can't feel anything, hard to tell if it is even on. Will stay this way until I put it into gear.
-Once I put it into D or R, RPM will fall to about 700 and vibes will get rough. Still bearable though.
-When I accelerate, it is very smooth and no vibrations.
-As I slow down, no vibrations until I come to a stop, then the vibrations ramp up and become very loud over the course of a few seconds. Accelerate again and they disappear.
-Sitting in park or neutral, the vibrations are still worse than my initial start-up but much less than sitting in D/R with the brakes on.
-The vibrations remain fairly consistent in intensity, but do kind of fluctuate in terms of loudness

The vibrations don't shake everything like did when my brown dogs needed to be tweaked; but I can feel it in the pedals/seat, sorta see it in mirrors, and it is just a very overwhelmingly loud low-pitch vibration/hum. My RPMs are very solid at about 700 (1 tick down from 1000 RPM), but will occasionally bump maybe ~50 RPM in one direction. For the most part, it is very solid.

I have cleaned IAC and throttle body, cleaned intake, new exhaust manifold, timing chain, and a bunch of other things. I have an aftermarket pre-cat O2 sensor installed, and will be switching to an OEM one soon.

I would chalk it up to jeep things, but given that is is so damn smooth when I first start up makes me think there is more to it. Other than that, any ideas where to start?

Thanks!

Did you change out your tranny mount as well?

dan1977p said:

Did you change out your tranny mount as well?

Click to expand...

yes, forgot to mention but I did trans mount at the same time.

EDIT: Also a new harmonic balancer and timing chain recently. Motor/Trans mounts were done like 9 months ago, everything else was done over the past 6 months.

So I double-checked that my Trans Mount and Motor Mounts were still good. No issues there. I also swapped in the new Mopar O2 sensor, no change.

While I was under the Jeep, I noticed the front driveshaft, right at the slip joint, has quite a bit of play. I can move it about 1/4" in in any direction - did not try rotational. This was with the wheels on the ground.

Given that the Jeep is pretty dang smooth sitting in Park/Neutral, but vibrates like crazy when in any gear and foot is on the brake - I'm starting to wonder if it is something in my transmission/drivetrain? I also get an occasional kick during up/down shifts, and it just feels a bit sloppy on the pedal.

I am still running stock drivetrain/u-joints and am currently at ~188k miles. They have never been serviced AFAIK (had the jeep since ~71k miles). I also was driving around with shot motor mounts and a trans mount hanging on by 1 bolt for many years. I would get grinding/popping sounds when turning, and that was happening for at least a year before the new mounts made it stop. I imagine the drivetrain is in bad shape...

So, it seems like the next most logical place for me to look. Plus it is something that AT LEAST needs to be inspected.

The only issue I have with this thought process is that the jeep runs smooth once I am accelerating, and at seemingly any speed. A lot of the posts about bad vibes from ujoints/drivetrain happens at high speeds. I run great until I slow all the way back down to idle, then the low frequency vibes suddenly find their way in.


Can I get a 2nd opinion? Trying to track down what could cause deep, low frequency vibrations when sitting idle in gear, but not in Park/Neutral or accelerating.

Thanks!

Just found this big thread about hte exact same issue I have..
http://www.jeepforum.com/forum/f310/vibration-while-stopped-drive-any-gear-1141298/index3.html

Lots of people saying that a vaccumm leak on PS near a rubber T fitting where it goes from pressure reservoir to interior controls. God I hope that is it. That would be sweet...