[bob350]

OK guys, I joined cuz this forum seems to be pretty good with tech stuff. I'm a newbie with a 91 YJ 4 banger that has seen it's day. It's got 226k miles and plenty of creaks, rattles, clunks and squeaks. But amazingly, it still runs pretty good and keeps chugging around. Gotta love Jeeps!

I can deal with most of the noise, but one has me a little concerned. When I'm in 4hi, I get this random faint clunking. But it's not so much a clunk as it is more of a dull thud, like you jabbed the brakes for a split second. Or maybe like a hiccup from the engine. It only happens in 4hi (haven't driven it enough in 4lo to notice it there), and it's when I'm going straight, so not the typical turning clunks. And it's happened on snow and mixed snow/wet asphalt.

I got this Jeep pretty cheap, and I'm not looking to spend a lot of $$ trying the R&R guess work to find the problem. I've been looking at a bunch of old posts seeing if anyone else has written, but no luck. From some of the posts, it seems that it could be a bad front axle vac motor that momentarily releases (??), or maybe the t-case chain slipping?? I'm hoping it's something easy or harmless...

Any help greatly appreciated!

Kent

[Michael_94YJ]

I would check front axle u-joints and front drive shaft u-joints... I bet that's your culprit

[bob350]

OK, thanks. I'll try that. I kinda thought it wasn't them because of the way it hiccups when it happens. But I haven't actually checked anything, so that's probably a good place to start...

[rustywrangler]

It may actually be the tcase chain slipping momentarily if it is not the U joints. If you get to the point of needing to look into the tcase, first plan on doing a fluid change. 30MM socket is required for this. You can actually look inside the drain and fill plugs and see the condition of the chain. If there is alot of slack in it, your prolly looking at a new chain.

While your looking into this stuff, you may want to take some time and check all of the vac line connections as well.

[sentinal02]

my money is on the vacuum shift fork. overtime they develop play on the shaft they're mounted to and even if the vac motor is working properly the fork can have enough slop that it pops off the splines for a moment, causing a clunk. depending on how much slop it has it can be a pretty nasty clunk. happened on my 88 and on the 92. the 92 was bad enough that i thought it was an engine problem at first. it wasn't until i was running on just the front DS waiting for my new rear that i got to watch the speedo spike when it did it one day and i put two and two together.

[bob350]

Looks like I have several options. Hopefully, it's the cheap and easy one that nabs it. But Murphy being what he is....

I think I'll start checking the easy stuff and go from there. What's a good way to check the vac motor?

Thanks again.

[rhowe]

this is an "i have the same problem bump". found anything yet??

[bob350]

No, haven't had a chance yet. I won't be able to start looking until this weekend. But I'll post my findings when I do. I'm just hoping it's not the chain!

Are NP231s easy to rebuild???

[rustywrangler]

Originally Posted by bob350

No, haven't had a chance yet. I won't be able to start looking until this weekend. But I'll post my findings when I do. I'm just hoping it's not the chain!

Are NP231s easy to rebuild???

IMHO, yes. as long as you are mechanically inclined and can get the right tools togeather. Follow instructions and it is fairly simple. Time consuming but simple.

I didn't mean to jump right to the tcase, but it is possible and would be the worst case scenerio thing.

[bob350]

OK, good. I'm going to leave the t-case as a last resort. I've done several engine rebuilds, so shouldn't be a problem.... unless there is some weird special service tool. And it's not my daily driver, so time isn't an issue.

[Michael_94YJ]

Originally Posted by bob350

OK, good. I'm going to leave the t-case as a last resort. I've done several engine rebuilds, so shouldn't be a problem.... unless there is some weird special service tool. And it's not my daily driver, so time isn't an issue.

Nah, t-case is pretty straight forward. Theres like 3 or 4 different size bolts holding the casing together though. A 12 point is one of them if i remember correctly...

[bob350]

Update:
Checked out the u-joints. They seem to be very tight, so I don't think that's the issue.
Checked out the vac motor. As far as I can tell, that seems to be working good also. I took off the vac connector and used a handheld pump to pull a vacuum on each of the connections one at a time. One of the ports engaged the RF wheel, and the other disengaged it. And they both held their setting for 10-20 sec and then even when vacuum was released. It seemed strange to me that the wheel would still be engaged even without the vacuum, but I had to pull a vac on the other side to disengage it.

That's as far as I got so far. Didn't have a chance to check the t-case because I found that I can't start it anymore. Seems to be too much fuel and (maybe) not enough spark. Tries to start a little bit, but then nothing. Ugh.

[flame0086]

Mine's been doing the same thing and I am thinking it's the chain in the t-case. I just put in a posi-lok and its a different noise with the front dissconnect slips. Whe the fort collar goes, its more like a grinding noise (all the splines tapping each other). This noise is one clunck coming from under the driver seat.

My guess is it is the chain since there is a bit of play in the front drive shaft when 4wd is engaged.

[bob350]

Originally Posted by flame0086

My guess is it is the chain since there is a bit of play in the front drive shaft when 4wd is engaged.

I think you are right. That's what makes the most sense to me, but I don't really want to admit it. I've got some play in my front shaft also, makes sense...

I'm going to try to drain and look inside this weekend.

[Provojeep]

Look at the mode shift fork in the t-case also. That is where my problem was. It should be quite obvious if it is the problem.