knocking help

I've searched, lots of help found - this is a great forum!

89 limited 4.0 AW4 242; 120k miles

Knocking at idle and higher rpm; changes with rpm.
Knocking at cold and normal temp.
Knocking under load and coasting.

Noticed some noise around thanksgiving when the jeep came home for break from school (it was 3 quarts low on oil!).

Oil pressure seems ok. around 37 at normal temp.

Tried the flexplate test - load the transmission/power brake. Still knocking.

Pulled the pan (just did rear main last weekend) and pulled front two mains - looked ok (so did the rear last week).

Pulled all rod bearing inserts. Top of #1 was worn to copper at top of cap, bottom insert on #1 had slight wear. All the rest looked pretty good. Took these to local NAPA machine shop mechanic - he said he wouldn't think the wear would be enough to cause a knock - "keep looking" he said!

Checked play (front to back) in crank - none apparent.

The sound is a definite knock, not a tick (tappet) sound. Tried to locate with stethescope - sounds more pronounced at lower front.

Timing chain looks tight from what I can see under the cover from behind.

Harmonic balancer (?) appears OK - no separation.

The jeep would normally go back with my son to school - 550 miles to get there. Any ideas or suggestions would be great.

(I have a .wav file but can't post attachments)

Pulling one plug wire at a time is a good way to narrow down the possibilities. If the brass is showing through the rod bearing it will usually have enough clearance to make noise (knock). I always use plasti gauge and check at the top center of the rod bearing. Thats usually where the most wear is. They wear kind of oblong or egg shape.
With pan off you can see most all of the rear side of the flex plate. A good time to look closely for cracks and do a little prying (gently) with a bar to check for a ring crack around the bolts.
With the pan off, you can also run the pistons to the top and look for scuff marks on the cylinder walls.
I fixed most of my knocking with a set of rod bearings, but still have a little wrist pin knock (for the last coupe of years), that doesn't seem to be getting any worse.
Something else to look at is plug color. Really white plugs or a lean running motor seems to knock more.

Thanks for the reply -

I'll show my ignorance:

1. - Pulling a plug wire (while running, I assume) what will happen? And what is indicated?
2. - Scuff marks on piston walls indicate what? Does this contribute to knock? Wrist pin problem?
3. - When you replace your rod bearings, do you replace all or only those that are out of spec (as indicated with platigage)?

Running-Snail said:

Thanks for the reply -

I'll show my ignorance:

1. - Pulling a plug wire (while running, I assume) what will happen? And what is indicated?
2. - Scuff marks on piston walls indicate what? Does this contribute to knock? Wrist pin problem?
3. - When you replace your rod bearings, do you replace all or only those that are out of spec (as indicated with platigage)?

Click to expand...

If you pull a plug wire on a cylinder that is knocking, it will often stop knocking (or quiet down significantly). It's nice to be able to narrow down the exact problem cylinder. Pulling a plug wire can be an electrifying experience, especially if your crotch is pressed tightly against the fender.
Scuff marks on the cylinder don't really mean much, except things are getting really loose. Piston slap is noisy, but can go on for years without causing significant trouble. The only way I've been able to check the wrist pin is with the piston out, the clearance is really small normally and even when they have excessive clearance it's only a few thousandths of an inch.
If one connecting rod bearing is worn, probably all are, or close to being worn out. I check a few at the top of the bearing as an indicator. Then swap them all out if needed. The only hard part is checking all the new bearings, for proper clearance. I don't trust the specifications on the package, to allow for the clearance for oil thats needed in the bearing. I always double check (at the bottom which is easier). Some motors have oversized bearings from the factory, though I haven't come across one yet. I've seen connecting rod bearings test within spec at the bottom and be 3 and half thousandths out (or more) at the top. When the clearances are correct the connecting rod end rotates on a film of oil between the bearing and the rod end. Metal to metal contact is supposed to be minimal.
Plasti gauge melts in oil, so a couple of cans of spray brake cleaner are necessary to clean things up before testing. You can leave the plasti gauge on, the hot oil will dissolve it pretty quick.

8Mud said:

Pulling a plug wire can be an electrifying experience, especially if your crotch is pressed tightly against the fender.

Click to expand...

That's what I like about this forum - some real insight (and handy advice=)

Thanks for the interpretation with the other items. How do I know what size bearing inserts I have presently? The caps on 2-6 are red on the inside; the cap on #1 is a "normal" steel color.
Do I plastigage the existing inserts?
Buy a few and return if the wrong size?
Or do I just take the beast to a mechanic that knows what they are doing?

Running-Snail said:

That's what I like about this forum - some real insight (and handy advice=)

Thanks for the interpretation with the other items. How do I know what size bearing inserts I have presently? The caps on 2-6 are red on the inside; the cap on #1 is a "normal" steel color.
Do I plastigage the existing inserts?
Buy a few and return if the wrong size?
Or do I just take the beast to a mechanic that knows what they are doing?

Click to expand...

I'd plasti gauge a few of what I got. I generally check those near the thicker main bearing caps.
I'd get a book and do it myself. Not that big of a deal, just time consuming.
There are some code numbers stamped on the block near the distributor that tell which bearings (if any) are over sized. I usually buy a set of standard and test the clearance on the new bearings (I always look in the book!!!) but if I remember correctly .0015 is about right. The connecting rod caps are numbered, don't get them mixed up or on backwords. Oil up everything well before reassembly. Clean the threads and nuts and Loc Tite the nuts on.
I have to go to work quick, maybe somebody else can look up the oversize code letters for you.
Old_Man says there are .001 oversized bearings available if the standards don't tighten to spec. The standard oversize is 10 thousandths over.

I looked up the letter codes for oversized journals. "B" is cylinder bore .010 over, "C" is camshaft bearing journal .010 over, "M" is main bearing journal .010 over.
And *P* indicates connecting rod journals .010 oversized.
The book says connecting rod bearing to crank clearance should be .0010-.0030. I normally try for .0015 on replacement bearings for a clearance. In my experience too tight fails before too loose. The crank rarely wears truly round and consistent across the contact face for the connecting rod bearings.
Main bearings should be between .0010-.0025.
The allowable clearances changed a little in the early 90's.
The motor should be driven like the original break in for a new Jeep. Keep the RPM's down for the first few hundred miles.
I'm fairly certain these numbers are accurate, but I always check the book myself. It rarley hurts to double check any measurement.

Replaced all the connecting rod bearings, put pan back on & fired up - still knocking. (Grrr)

My son took the Jeep to a local garage for a compression test (on to the next step) - mechanic came out and listened to it, said "sounds like your water pump, I'd bet a six pack on it."

I pulled the belt - no knock. (When searching postings before starting this project, I read this advice several times but wouldn't believe the sound I was hearing was a water pump.) I delivered a six pack of Bell's Stout to the mechanic the next day after replacing the water pump.

All is back to normal - albeit with new bearing inserts and a rear main.

Thanks for your assistance and advice 8Mud.

I'll file that away in my never seen it before but most anything is possible file.
I generally chase down knocks and such with a piece of plastic tubing. It really helps localized the problem.
If you could see the brass on the connecting rod bearing, it was probably time anyway.
Thanx for posting the fix, many people don't bother.

"Thanx for posting the fix, many people don't bother."

Yes that is a good thing,thanks again