Cracked Flexplate Noise

[AZJeff]

My 4.0 is making a strange ticking noise near the back of the engine, and it has a strange pattern to it.

 

Let me describe the sound:  it is not as metallic as a lifter, and it’s too. Low down in the engine to be valve train.  It’s not as deep or dull as a main bearing knock.  Engine load (or lack thereof) does not change the noise, so it’s not a rod issue.  None the less it’s a sort of tapping sound, but with a temperature component I will describe below.

 

When I first start the engine, the noise is not present.  As the engine warms up, the ticking is audible, but only at idle.  Once the engine comes even slightly off idle, it’s inaudible, and of course as engine speed increases, the general engine and road noise may be masking it.  
 

When the engine is FULLY warmed up (running for 20 minutes or more on the road), the noise is no longer audible at ANY engine speed.

 

If I shut the (fully warmed) engine off and restart it within about 5 minutes or less, the noise remains gone.

 

On the other hand, if I wait longer than 10-15 minutes and restart, the noise reappears again until the engine re-warms, at which time it disappears.

 

Oil pressure is good (within spec.) and compression is uniform at 140 +/- 3psi on all six cylinders.

 

My stethoscope investigation points to the noise being near the back of the block, but I cannot pinpoint it.

 

Any suggestions?

[scaleless]

Hard to tell without hearing it, but temp-dependent noise like you're describing is usually an exhaust leak.  Renix exhaust manifolds very commonly crack and cause a leak.

[AZJeff]

It’s a 2000XJ engine, with OBD-2.  It has new exhaust manifolds and the trick gasket some guys on here recommended (I cannot remember the brand name).

 

The engine has the Calif. emissions package for a 2000XJ, so it sports two cats, one hanging off each bank of three cylinders.  I have checked for cracks where the two cats join up to the exhaust pipe.

[MiNi Beast]

flex plate issue or loose bolts.

or bad lifters be my guess. 

[AZJeff]

3 hours ago, MiNi Beast said:

flex plate issue or loose bolts.

or bad lifters be my guess. 

I wondered about the lifters.  I put a new head on it about 2 yrs ago (famous “0331” head crack) and put new pushrods/rockers on it, but just cleaned all the existing lifters in lacquer thinner and then soaked them in oil before reinstalling.

 

I have a camera with a fiber optic.  I wonder if I could see the crack in the flexplate somehow with that device....

[MiNi Beast]

have you pulled the inspection cover yet? i had some with loose bolts and was able to handle it through there. you should be able to get in there with tool to see. 

but I'm betting your tearing something apart either trans or head again. 

[MiNi Beast]

you can pull the valve cover and with engine running and see if have a bad lifter as well. 

[cruiser54]

Next time you start it and hear the noise, shut it off. Restart after 10 seconds. Noise gone?

[AZJeff]

7 hours ago, cruiser54 said:

Next time you start it and hear the noise, shut it off. Restart after 10 seconds. Noise gone?

OK, I did your suggested test.  I waited until the engine was fully warm (where the noise is normally virtually gone), and I shut it off.  Then I restarted it after about 10 seconds.   The noise was unchanged, and was almost inaudible.

 

I repeated the test, but instead of waiting 10 seconds, I waited about 1 minute.  Upon restart, the noise appeared for about 3 seconds, and then disappeared.

 

Lifter leak down?   

 

I found a video on YouTube of a guy with a TJ that has almost exactly the same noise/location, but he has not yet figured out what it is.   Here is the link to his video (mostly useful as audio): TJ 4.0 Noise

[cruiser54]

I was hoping you would do the test cold also.

What oil are you using and which oil filter? 

[scaleless]

The video you posted sounds exactly like piston slap, and the comments seem to agree.  As oil heats up and flows better, it makes sense that the noise would quieten up as well.  Doesn't sound like flexplate or flexplate bolt noise, which would be rythmic with respect to the flexplate position, but if you slow it down and time it, it seems to hit about once or twice per stroke for 700rpms, which would mean any of the reciprocating components.  Given it's louder on the bottom end, piston/rod-related sounds more likely than valvetrain like you said.  Then again, that's diagnosing his  vehicle, not yours.

 

If I were in your shoes, once I was done with Cruiser's tests I would start pulling plugs and stick a borescope down at #6 BDC and moving forward looking for abnormal wear.  You mentioned you had one, so you're already set up and it doesn't hurt to look.

[AZJeff]

I considered piston slap, as I had an XJ with some of that, but it slapped pretty much all the time at idle speeds.  

 

But let's rewind the tape a bit, and give the history of this engine:

 

After purchasing the vehicle a few years ago, and spending about 11 months making it roadworthy, I took it for a shakedown cruise to a car show where my son lives, about 300 miles away.  All went well.  I had put fresh oil (10w30 Pennzoil) just before the trip.

 

When I returned, I decided to change the oil.  What came out was.....odd, sort of like a very thin pancake batter.  I sent a sample off to Blackstone oil labs, and they confirmed my suspicion--I had coolant contamination in the oil.   Knowing this was an engine from a 2001XJ that had been transplanted into my MJ, I did the recommended "0331 head" inspection, and found the classic crack in one of the rocker bosses on #4 cylinder by using UV light.  So...off to Clearwater Cylinder head for a new head and valve parts.   I also purchased new rockers, rocker pivots, and pushrods.

 

WHen I disassembled the engine to install the new head, I noticed some signs of what caused the crack---some evidence of a previous cylinder head gasket failure and streaking of coolant down the manifold side of the block.  I also noted the head gasket that I removed was NOT a Chrysler gasket, but was Fel-Pro.  Someone had obviously been into the engine to deal with some sort of overheating failure previously, but was not aware that such an injury to a 1999-2001 "0331" head would almost certainly crack the head.  I inherited that issue.

 

When I did the new head, I also took all the lifters out and cleaned them in lacquer thinner on general principle, dropped the pan and installed a new RMS, and a new Melling oil pump, and cleaned the pan and put in a new one piece pan gasket.  While I was down there, I dropped a rod cap to check the #6 rod bearing while I was doing the rear main.  Except for a bit of dirt embedded in the shell, the bearing look ok, and not wiped in any way.   I did NOT check clearances on the rod or the main.

 

I checked for the classic signs of piston slap by turning the engine and watching for cocking of the piston skirt, and did not see any.   The cylinder walls were perfect, and there was little to no carbon on the pistons, and no ridge on the cylinder walls.   Once the engine was reassembled, the compression on all six cylinders was between 140 and 143 after an initial run-in.  I figured all was well.

 

The noise started up shortly after the new head was installed.  I run Mobil One 10w30 with Mopar or Wix oil filters.  The engine is smooth, with no shaking, no oil consumption, and no coolant losses.

 

I am kind of mystified, as I would think that a sloppy piston would cause the compression readings to be a bit more varying.   Input is still welcome.

[cruiser54]

Good recap. Let's try the cold start thing. 

[MiNi Beast]

still thinking lifter or flexplate. winner has to fix it? 

[cruiser54]

I'm thinking lifter also.

[Eagle]

On 4/18/2021 at 8:23 PM, MiNi Beast said:

flex plate issue or loose bolts.

or bad lifters be my guess. 

 

I disagree on the flex plate.

 

Been there, done that. A cracked flex plate sounds like a bad piston or a bad rod bearing when idling in neutral. Put the tranny in gear so the torque converter is loaded, and the noise goes away.

[MiNi Beast]

34 minutes ago, Eagle said:

 

I disagree on the flex plate.

 

Been there, done that. A cracked flex plate sounds like a bad piston or a bad rod bearing when idling in neutral. Put the tranny in gear so the torque converter is loaded, and the noise goes away.

true, but whos to say not if havent even pulled inspection cover to take a peak. makes sense per loading principals. I would be more inclined to assume lifter with the history, however can't hurt to check either way. 

[MiNi Beast]

and to be clear this is assuming its an auto using the word "flexplate" but has yet to confirm weather its auto or manual.  

[AZJeff]

1 hour ago, MiNi Beast said:

and to be clear this is assuming its an auto using the word "flexplate" but has yet to confirm weather its auto or manual.  

It’s an AW-4.  I know the diff between a flywheel and a flexplate, and used the correct term.  Besides which, I don’t think 4.0 flywheels crack around their mounting holes in normal use, unlike the flex plates☹️

[MiNi Beast]

 not typically no. little thicker and seen junk flywheels from starters and only heard 2 times of flywheel issues at crank due to bolts not tightened down. 

 

but i understood your terminologies and further more still suggest inspection cover pulling for a peek and checking lifters fully. but hey your world boss do the work.  

[AZJeff]

So I have added two short videos of the engine running (really all that is needed is the audio).  

 

The first one is after the engine had been at operating temperature on the road for about 20 minutes.

 

The second one is of the engine on a hot restart after it sat for about 5 minutes after I shut it off from the 20 minute road trip.

 

[AZJeff]

3 hours ago, MiNi Beast said:

 not typically no. little thicker and seen junk flywheels from starters and only heard 2 times of flywheel issues at crank due to bolts not tightened down. 

 

but i understood your terminologies and further more still suggest inspection cover pulling for a peek and checking lifters fully. but hey your world boss do the work.  

I get the idea of looking at the flex plate with my camera.  Once the engine cools, I will dive into that.  (Even though it never changes the noise between loading the engine versus no load.)

 

But on the lifter inspection, what are you suggesting I will see?  A pushrod flopping around under the rocker, or something else?  Wouldn't that be most likely detected by looking for lash to return very shortly after the engine is shut off?

[cruiser54]

Have you tried the cold start with the noise, shut it off and restart in 10 seconds? 

[AZJeff]

Just now, cruiser54 said:

Have you tried the cold start with the noise, shut it off and restart in 10 seconds? 

I will let it cool and try that, and advise.  Thanks.

[cruiser54]

It's NOT flexplate. Did you tell us which oil and filter you're using?