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Weird rattle in trans at startup!

n9viw

NAXJA Forum User
Location
Worth, MO
Hey all,
Before you say it, yes, I did do a search on this topic first. :D I searched on "rattle", "trans", "transmission", "AW4", "noise", "sound", "start", and even "weird" and "strange". I read through mountains of threads, but nothing that addressed what I'm experiencing, so here goes.
1989 XJ Laredo, 175k on the original 4.0 engine, AW4 auto trans, NP231, D30/35, pretty much bone stock. We recently replaced the cat and muffler (plates were broken inside, muffler was just old), and once that rattle was fixed, we found another one that it had been hiding.
It only happens upon startup, within the first minute or so, whether the engine is hot or cold. The engine starts, and after a few seconds you hear (and can feel through the floor) a rattle, almost a clunking grind. As the engine accelerates to 2k RPM (regardless of gear, even in park), it accelerates in its own frequency until it suddenly goes away.
I checked the torque converter bolts, all are quite tight (as tight as I can get them, I'm guesstimating over 100 lb-ft). The check for cracks in the flex plate comes up negative, it's very tight, doesn't flex at all. Trans fluid level is... weird. When checked with the engine cold in park (idle around 800-1000 RPM), it read low this morning. I put in about 3/4 of a quart, and after having run it a bit, the idle in park was now up to 1500 RPM, and the fluid level was OVER full.
The creeping idle is a different issue altogether (I'm guessing it's either a bad TPS or a vacuum leak), but this rattle-clunk is really concerning me, especially when the trans reads low one time, and high another. Oh- fluid is fresh, not burned or dark, as I just replaced the filter about 6-8k miles ago.

So - any suggestions?
 
No, I'm absolutely positive it's not the exhaust hitting the crossmember. I DID have a problem with it when I 'fixed' it before (head pipe rusted off at the cat flange, so I had to get creative, and an exhaust clamp was hitting it), but now there's a 2-1/8" pipe that runs from just in front of the crossmember to just after it, where the cat sits. The pipe is about 2" below the floor, and 1.5" above the crossmember, and does NOT use the factory mount off the back of the crossmember, so there is no physical contact between the exhaust and the crossmember in any way.
Should have checked the trans level before I left for work, but I was running late. I'll check it before I leave work tonight, and see what it says. I'm hoping it doesn't say it's low, but if it has been (just found a hi-pressure line leak up in front of the engine), odds are probably pretty good that I've now done damage to either the TC or the trans, don't ya think? Bugger.
 
I had a simular sound ,, it turned out to be the insides of the CAT rattling around. On start-up or hard acceration, it was the worst.

Crawl under the Jeep , when it is Not running,, and kick the CAT with your foot or hit it with your hand, and see if it makes a noise.

OH Wait , I see you all ready replaced the CAT. .... nevermind

Sticking lifters? or exhaust valves?????
 
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Yeah, I replaced the cat with a Catco 400CFM, and the muffler with a Thrush Magnum glasspack. The overall length of the two is shorter than the stock cat and muffler, so the front of the cat sits about 6" back from the trans guard. The cat is attached to the headpipe by a 15" section of 2-1/8" tubing, so there's great clearance before, through, and after the guard.
I know the engine has one sticky lifter, but I know that sound, and that pumps up within 5-10 seconds of startup, which is before this other sound starts. Also, the other sound (the one in question) is so strong you can hear it as well as feel it through the floor. The fact that it happens in any gear including park, and that you can feel it thru the floor, makes me suspect the trans (owing to the 4wd linkage between the trans and the floor). But when I had it idling and crawled underneath it with a screwdriver... er, po-boy stethoscope, I couldn't hear a bloody thing- I don't know if it had already happened, or decided not to happen that time, or what.
 
A noice you can "feel" through the floor is likely something hitting something else. You might want to check the rear transmission mount. It's on the cross-member. When mine was going out, it "thunked", especially on accelleration (when the engine torque tends to flex motor and transmission mounts).

Regarding the transmission fluid level...it is supposed to be checked hot. It will always read low when cold. If you filled it when cold, it will read over-full when warmed up.
 
Thanks for the suggestion, I'll take a look at the trans mount and see if it's torn or whatever. I do know it's badly oil-soaked from that trans line leak- that's been blowing all down the engine and drivetrain for some time now, and I've been trying to track down this pesky engine oil leak. Turns out there isn't one, it's trans fluid!
The funny thing about the clunk is that it's only on startup, and that it happens even while sitting still, for a period of several seconds. If it's something hitting something else, it's not on the outside of the vehicle, because I tried moving and shaking everything I could get my hands on- trans, t-case, driveshafts, steering, shift linkages, all that crap. My worry is that it's the torque converter (or internal to the trans... :eek:!) going bad, but there's no way to tell apart from taking the whole mess apart and hoping I'm right, in order to justify all the work!
On that last note, does anyone know of the existence of an exploded diagram of the AW4? I've never seen the guts of one apart from having the pan off to change the filter, but I'd like to know what's in there so I know what might be causing the noise.
 
I have what sounds to be the same identical problem as you do...I cannot figure it out, i know its not the cat because I don't even have one, mine does it more on bumps while coasting then just coasting, but idling and everything...i thought the same thing Torque converter or tranny...not trying to hijack the thread...i just wanna know whats going on too...thanks!
 
Well, I had a bit of a closer look around under there the other day. Seems the exhaust DOES hang closer to the guard than I thought, but it's not actually hitting it. The only way I can see it's hitting it is if the motor mounts are FUBAR, and they don't seem to be- I can't get them to move at all, except within the range of their normal rubber squooshiness.
This also doesn't address why it only happens at startup, and only temporarily- not immediately, not while driving, and not at shutdown. Dag, this is pissing me off! I think tomorrow, when the weather here in the midwest is briefly warmer, I'll pull the pan and hope (or not) to find some metal shavings or SOMETHING that might indicate whether I can implicate the trans or not! :mad:
 
Well, time for me to eat my daily allotment of crow... where's that salt shaker. :eek:

Had a crawl around under the XJ the other day, and found the exhaust quite a bit closer to the trans crossmember than it originally was. I suspect bad motor mounts, but they don't move a smidge when the wife holds the brake and revs it while in gear... who knows.
I was doing some exhaust work on the CJs, so I took a bit of torn-off rubber exhaust strap and crammed it between the exhaust pipe and the trans crossmember. So far, no more 'phantom rattle', but you do get the redolent stench of burning rubber when you stop. Whooo! :eek:
So, in spite of the fact that I was so sure it WASN'T the exhaust, turns out it was anyway. Now to fab a weird hanger out of the old one from the trans mount, to pull the pipe up and away from the crossmember. D'oh!
 
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