So before the "YOU SHOULD HAVE SEARCHED IT" train runs me over, I DID! I used the search, I did find a bunch of posts with engine knocks (on this forum and others). However, non were EXACTLY like mine, and the solutions were all over the map, but I made a mental note of all of them.

So... I am going to describe what is happening, and maybe someone can pipe in? Let's hope? Please?!! :P

First, my XJ specs:

1999 Jeep Cherokee Classic. Auto. 4.0, 4X4
197,700 KM
Brand new Spark plugs, no misfires etc
Oil has always been changed (so I was told and it seems true)
I am in Canada, but that doesn't change anything, just a forewarning about weird Metric stuff ahead

So here is what happens.

1. The XJ starts up just fine. Strong start, no noises, smooth idle etc.
2. I drive it around until it warms up to temp. (In my XJ, that is with the arrow a little under middle, like 90-93 Celcius - so about 195-200F)
3. When it finally gets to temp for a bit, and I stop anywhere allowing the engine to idle, I hear "tok tok tok tok tok" (doesn't matter if it is in Park or Drive or Reverse, as long as it's not moving - read below for more on that)

So that's the big issue. The knock.

Here are two INTERESTING tidbits about it, though:

1. Can't notice or hear it over 1500RPM - so once it's off idle a bit, it's gone
2. This is the fun one: If I shut off the engine, and say pop into a store to buy a candy bar, or literally wait 30 seconds, and restart the engine, IT'S GONE. It is QUIET. But then after about 15 seconds or so, it'll start to knock AGAIN.

I tested it the other day. I pull into the store parking lot. Engine was knocking at idle, when the car is in the parking spot. I shut it off, and go inside to buy some anti-seize/return a socket (A whole 5 - 7 minutes in the store, max). I come back outside, and this time I listen for the knock. Fire up the engine, NOTHING. So I sit there waiting and sure enough, 15-20 seconds in, the knock returns.

Oil pressure is OK. I can include photos if anyone asks for it, but when I start the engine up COLD (like, from sitting overnight) oil is at 3.1 - 3.2 bar (46 PSI)
When the engine warms up, at idle at a Red light, it is 2.7 ish bar (39-40 PSI)
Driving 80KM (55mph) - 3.1 BAr (46PSI)
Under harder acceleration, it may climb up to 3.8 or so Bar (closer to 55PSI)

So oil pressure is within spec, based on what others on this forum have said, in other threads.

One thing I noticed, but maybe because this is my first XJ ever and I am reading it wrong, but the oil level seems higher than it should be on the dip stick - which leads me to think that isn't good (P.O. did the last oil change)....(But I will be doing an oil change soon anyway, so I'll get it back within spec)

So my brain is going:

- Is it a rod bearing? Seems odd that it would be so inconsistent?
- Is it a lifter issue? But those only seem to happen the other way round - hear it when cold, goes away warm
- Flex plate bolts? But again: Why so inconsistent?

WHAT COULD IT BE?

If anyone has any experience with this, OR if you guys and gals need more info please fire off any questions that I can answer, that may help me!

MANY MANY Thanks in advance!

 

This is just off the cuff, so to speak, but it could be the evap canister purge solenoid doing it's thing. The reason I suspect that is my 2000 Grand Cherokee does the same thing. You can disconnect it to see if that's it, but it might set a trouble code that will have to be cleared later, or it might clear itself. Mine is very faintly heard and you have to be listening for it. Yeah, it freaked me out too!

 

I could give it a whirl if nothing else, but I highly doubt it will be that (not to say I don't appreciate your input and attempt at helping me ) . The reason I doubt it, is because it's rather noticeable. With the radio off and windows up, my dad was riding with me and said "Whoa...is that a rod/crank bearing going? That's loud". It sounds sort of like my old Subaru Outback with the piston slap . (the old EJ25's that would tick tick tick)

 

First off - are you SURE it's coming from the engine? Could it be as simple as exhaust hitting the crossmember? The exhaust heats up and cools down rapidly (compared to the engine), so that would account for the "going into the store" thing.

What about engine accessories? Possibly some bearing noise from a worn pulley?

Sorry, I didn't closely read the post, but skimmed through and didn't see a confirmation that it's the block internals. Sounds are tricky and these engines are arguably the most durable passenger vehicle engines ever made*, so I would definitely not jump to an internal engine problem unless you're absolutely certain that's the case.

* I won't annoy the regulars with talking about my own experiences, but I read an interesting comment online somewhere in a general discussion of durable engines (I read it on the internet, so it must be true). So several years back here in the U.S. there was the "Cash for clunkers" fiasco - for those that may not be aware, this was a grotesque gov't program during Obama's first term that was supposed to encourage people to get rid of their old cars and get a newer, more fuel efficient replacement. The gov't would cut you a check, but only after your old "clunker" had the engine permanently destroyed, typically by pouring some chemical into the engine and run it until it died.
Well this guy was one of the people who were doing the actual destruction - they had to certify the engine had been destroyed beyond repair. He wasn't a Jeep guy, wasn't advocating for the Jeep 4.0, simply noting that of the many different vehicles/engines that he performed the destruction of there was one that stuck out to him. It was a mid-90's Jeep Cherokee (4.0). I don't remember the exact words, but he basically said that of all of the engine, it was the hardest to kill. It apparently ran a lot longer than anything else, knocking and banging (of course) before it eventually died.

 

Okay, judging from that, I'd say it's not the purge solenoid! That was the only "off the wall" thing I could think of. Just to let you know, loose converter bolts will usually only rattle in neutral and be quiet in gear.

I'd put a mechanical oil pressure gauge on it to see what your actual oil pressure is at HOT idle, as in heat soaked at operating temp for a while. My XJ will drop down to 25 psi at hot idle, but jump right up at anything over 1000 rpm. If it's just gotten up to temp, it will show 35 or 40 depending on how long it's been running. I'm using Rotella T6 5W-40 synthetic diesel oil or it would be about 5 psi lower at hot idle. I say this with caution, but the worst case scenario is you may have one rod bearing that has more clearance that it should have and at low oil flow levels at idle is telling you so. If that's the case, you can check that by standing next to the engine and giving the throttle a little quick tweak. If you get one or two louder "tocs" as it speeds up, my guess is that's it.

 

You are going to have crawl around unde,r preferably with a stethoscope to eliminate an external knock

the stethoscope should be applied to the engine to narrow down the whether the noise is eminating from inside the engine, or other moving part

pulling each individual spark plug will sometimes alter the tone of the noise just enough to isolate to to one cylinder

 

Thanks awg. I forgot about the spark plug trick! I'm old school and a piece of hose works pretty well as a stethoscope.

 

I must be even more old school, despite my younger age, because I was just going to press a rod against various parts of the engine on one end, and to my ear on the other! hahaha

Well, I guess I may be crawling under the XJ this weekend, after driving it for it to warm up, and doing some "noise listening". Yeesh...

These are supposed to be bulletproof engines! WHY ME!? haha

 

I have also just used rods, or screwdrivers before, but the stethoscopes are so cheap, and just a little more discerning, not to mention keeping your head away from moving parts

guess an engine age factor comes into it, with number of cold starts more so than mileage coming into play

dont know much about the XJ motor, but the things internally that seem to give an issue from what I have been told appear to be

failed lifter, and associated problems
worn conrod big ends
cracked piston

a compression test might tell you something

hopefully its something minor

 

First off, don't think "it can't be this or that".

Check the flexplate bolts, unplug the evap solenoid. Get a stethoscope and do some "noggin pokin" right where you're at.

Everyone can pontificate til the letters are worn off their collective keyboards, but there's no substitute for hands on.

 

I also found what Dave said is true and seems to be only the xj evap that does it. Or maybe you hear it better because it's mounted right by the front passenger side firewall. Disconnect it and see. Mine definitely makes a pronounced thunk at hot idle, in time with the cylinders firing. Disconnect it and it's gone.

 

Any chance it could be from the water pump? The first time I had one that was noisey I thought my bearings were shot. It took me a while before I realized it was the water pump.

 

Well, if folks would buy a $4 mechanic's stethoscope instead of farting around on the interwebs, asking questions about sounds only they can hear from THEIR Jeep at their location and posting unintelligible videos etc, these problems would be solved in a "heartbeat"......

How hard is this? Here's a link for crying out loud.

https://www.harborfreight.com/Mechan...ope-63691.html

 

Jeez, I wasn't even online for a day and haven't even responded to your first recommendation of the stethoscope, and I am still getting yelled it - it's like dealing with my dad!! hahah

I'll probably go this route, do some "noggin poking" in and around the engine with a stethoscope or whatnot. I feel that'll be the only way, just pinpoint the source and PRAY to the Jeep Gods, that it isn't a Conrod bearing.

EDIT: To update, I ordered the stethoscope online (much cheaper to do so here in Canada. It's $3.80 in the US or on ebay for me, but locally the cheapest is $20 bucks with tax)

I do agree with your "can't be this or that" statement, it very well COULD be one, couldn't it? Never know til I check.

I do also appreciate the input from the others, about the evap or water pump, those will be looked into!

 

I was just under mine, and do you have to pull the damn starter just to get the dust cover plate off ???

The tx cooler lines are bolted to one side and the exhaust pipe is bolted to the other. Some different size bolts, too.

Everything was so dirty that I moved it off the ramps and hosed out underneath with a wand. Now it has to drip a while.