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89 Front U-Joint hits downpipe

M

mattbred

NAXJA Forum User
Location
Canada
On my mostly stock (upcountry) 89 cherokee, when I'm in 4-Hi and turn right, I get a scraping/clunking noise that sounds like my exhaust is hitting something. I looked under it the other day and sitting on level ground, the front U-Joint has about a hair's clearence between the downpipe and the U-Joint itself. I imaginie when I turn right, the jeep flexes enough and the front driveshaft is moving so it hits the downpipe.

What would cause this? A couple mm of clearence just doesn't seem right (you couldn't stick a penny through it).
 
Its pretty close, thats why the down pipe has that crimp in it.

Do you have any lift at all? What shape is your trackbar in, is it worn?
 
Its pretty close, thats why the down pipe has that crimp in it.

Do you have any lift at all? What shape is your trackbar in, is it worn?
Click to expand...

I didn't see any crimp next to the U-Joint. It's not lifted at all either, only the upcountry factory package. Havn't checked the tracbar..

IGOCOMMANDO said:
How are the motor mounts?
Click to expand...

Motor mounts seem fine, however I havn't looked that hard at the driver side. I did look at it and rev the engine and the bolt moved a bit, and you can see the whole engine shift in the engine bay, but I imagine that's normal.
 
norcalchris said:
no crimp?...it must not be the stock exaust then.
Click to expand...


Ding, ding, ding. we have a winner.
 
norcalchris said:
no crimp?...it must not be the stock exaust then.
Click to expand...
Yeah it mustn't be. I'll take another look, but see it never did it a month ago, and only started doing it 2 weeks ago.
 
mattbred said:
Motor mounts seem fine, however I havn't looked that hard at the driver side. I did look at it and rev the engine and the bolt moved a bit, and you can see the whole engine shift in the engine bay, but I imagine that's normal.
Click to expand...
Uhmm No,also replace the transmission mount!
 
RCP Phx said:
Uhmm No,also replace the transmission mount!
Click to expand...

I dunno, I'll try and describe it.

When you rev the engine, the whole engine vibrates in the engine bay for a split second before shifting about an 1/8 of an inch. You can keep doing this and watch with the naked eye as the engine shifts. Looks like the engine mount is in tact. It was just rebuilt 50,000km ago, so who knows. The fan isn't striking the rad yet though.
 
Ok I found the crimp in the exhaust, it's a lot higher than the level sitting position for the front U-Joint. It only comes into play under flexing. My U-Joint is hitting my exhaust on level ground and only when turning right.

Also I took a video of my engine flexing in the bay. I don't think it's excessive. The first few revs are good, sorry I didn't have a tripod but you get the idea.

http://www.liveleak.com/view?i=cbe_1198623591

Just added another video. You can somewhat see the flexing.
http://www.liveleak.com/view?i=072_1198624020
 
Last edited:
that doesn't look very bad, but the best way to replicate how the engine is gonna move during use would be to hold the brake and load the drivetrain,
i.e. put an auto in gear and "blip" the throttle or on a stick slowly let of the clutch pedal untli you can get it to load against the parking brake.

I would look closely at all three mounts and the attachment points for the exhaust as well. There's not alot of room in there, but if everything is where it belongs, you should have a little more than a penny between the shaft and downpipe.
 
the torque of the engine shouldn't just flex the engine itself...it should tilt the whole jeep, IF your engine mounts are good. With yours, it looks like its just the engine moving....

its probably a combo of motor mounts, tranny mount and possibly track bar allowing the DS to hit the exhaust.

have you checked for play in your exhaust? engine off (and cool...lol), can you move that head pipe at all?
 
Had the same problem with my '88. Turned out to be motor mounts. It's hard to see how bad they are, just looking at them, especially setting in the drive way.
When your "wheeling", your engine is under torque not developed at idle, or when "reving" her up setting still. Also, when driving over rocks, or gullies, the axle is also moving arround. The combination of axle flex and bad motor mounts result in the front u-joint hitting the down pipe.
After changing the mounts, problem fixed; however, I destroyed the driver's side again. The stock mounts are not very robust.
 
Interesting. When I'm in the jeep, and I rev the engine, the whole jeep itself flexes as well, but when you look at it in the engine bay it looks like just the engine moves.

Tell me, is the bolt in the engine mount supposed to move at ALL? Because when I rev it up, the bolt has some play as the rubber "stretches" in the mount.
 
you know you have two motor mounts, right?

take a look at the M.O.R.E. engine mounts, by looking at them, you wouldnt have much play/stretch in them. thats how it should be.
 
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