Tapping noise in the crankcase

Hi guys,

I recently pulled the motor (only 130k miles) to replace a leaking rear freeze plug, and while I was at it, I did the rear main seal and oil pump, among other things. It worked great for 100 miles or so. Then, one day when it had run for quite awhile, I started hearing what I thought was a knock from the crankcase. It sounded like it was coming from the back of the oil pan.

So, I pulled the pan and checked everything. It all looked fine. One side of the rear main may have been a little bit under torque. Or maybe it wasn't. But the bearing looked great, so I put it back together. I didn't trust it as much this time. I let it idle for an hour and get up to temp. Dead. Silent.

And then I drove it around on some errands, and the noise is back! It sounds more like a tap than a knock. Changes with RPM, probably once a crank or cam revolution, though it doesn't get louder, so it gets harder to hear.

I'd take it apart again, but I really don't have any guesses about what is going on.

Thoughts?

Paul

Auto? Flexplate?

If the two top bolts on the transmission bell housing are not tightened down well it may be the issue. I replaced mine with regular bolts you should do the same. Other than that could be a loose starter or flexplate

Cracked header?

-torque converter bolts could be loose (almost every 4.0 auto has had that problem)
-possibly the exhaust manifold being cracked (it happens)
-Motor mounts (mine were shot and made a tapping noise at idle in gear and while accelerating)

It is an auto.

Laying under it with my ear up to the pan and bellhousing, it really sounds like it is inside the pan. Engine would also explain why it only makes the noise when it is good and warm.

I will check the torque converter bolts. I used brand new ones when I put it back together as the originals were rough. I hope to heck it isn't one of the crank to flexplate bolts! (I did torque them properly.)

It isn't the header. It doesn't sound like an exhaust leak (a sound I am VERY familiar with). The header is brand new as well. But I suppose an exhaust leak could explain why temp makes a difference.

I think the top bolts from the bellhousing are o.k. They are new too. When I took it apart I was surprised to find that one was missing! The previous owner definitely had the motor out and I bet he gave up on it. It is frustrating that when he had the motor out he hadn't replaced the back freeze plug and saved me this hassle!

Motor mounts are good enough. They are M.O.R.E. that are a little beat up, but not bad enough to make noise.

The starter went in hard because that stupid plate at the back of the engine didn't line up perfectly. Could be something there rattling. That cover over the torque converter bolt isn't lined up quite right either, but I don't think it is the source of the noise.

Thanks for all the ideas!

Paul

Check the distributor/cam synchronizer drive for wear.
Later model 4.0s, in TJs, had a FSM pertaining to a tapping noise coming from the meshing of the synchronizer gear with the cam's gear.
My 2001 XJ had a tapping noise that came from the bottom of the pan, near the oil pump.
Changing the synchronizer eliminated the tapping. You could see wear on the drive gear.

I like that theory a lot. That does sound like the neighborhood the noise is coming from.

Sunday I checked the torque converter bolts and poked all around with my stethoscope. Didn't find a thing.

But there is an interesting new development. I decided to drive it to work today. 30 miles each way. It didn't make the noise on the way in. When I got about a mile from home, it started doing it LOUD. I pulled into my driveway and ran to get my stethoscope (all excited, "I've got you now you POS"). I had to shut it off to open my garage door with the key. When I started the Jeep again the noise was gone!

I'm hoping if I drive it enough it will get worse and do it consistently enough that I can track it down.

Paul

lazyxj said:

Check the distributor/cam synchronizer drive for wear.
Later model 4.0s, in TJs, had a FSM pertaining to a tapping noise coming from the meshing of the synchronizer gear with the cam's gear.
My 2001 XJ had a tapping noise that came from the bottom of the pan, near the oil pump.
Changing the synchronizer eliminated the tapping. You could see wear on the drive gear.

Click to expand...

In my case the cam synchronizer drive gear was in good shape, but the bushings were worn. It was a tapping that I could feel if I put my finger on the sensor. The same thing can happen to a distributor in the earlier years. They are just metal sleeve bushings, no bearings.

Well, driving it around has been interesting. Today, I managed to catch it in the act. I was in a suit, so I only listened around the things you can reach from the top with my stethoscope. The noise doesn't seem to be coming from distributor. Or anywhere else on the top.

But, I am becoming more certain it isn't a bearing. I was sitting at a stop light at idle. It was making the noise loudly, but then, while the engine steadily idled, the noise quickly slowed and stopped.

Still stumped. I'm hoping if I keep driving it it'll get louder and more regular. Or the engine will blow. At least I'd know how to fix that.

Paul

Is it perhaps the purge valve solenoid on the firewall by the A/C stuff? I thought for sure I was fixing to spin a rod bearing when I first got my '99 but apparently a) the thing is naturally loud as heck and b) apparently I bumped it while changing the spark plugs and it made it resonate, which is REALLY loud.

Well, to add insult to insult, it is now undriveable.

I'm pretty sure the problem is in the tranny or torque converter. I can hear the sound now with a stethoscope on the plate that covers the flex plate.

But worse, the Jeep now climbs hills like it is pulling 10,000 lbs. The engine sounds fine, but it has no power. It is shifting through all the gears, but it just seems like the tranny is slipping.

I have no idea what's going on.

Paul

Bummer!