Hello everyone. First post. This is my 4th Cherokee and the first time I've ever encountered a noise like this. I didn't get a video of it before I drained the oil (about to check the pan for any debris and take a look at the underside of the motor). Here's a link to a video that sounds exactly like what I'm hearing. I pulled out of my driveway last week and started pulling forward when my Jeep started making a tapping noise that hasn't quit since. It's a loud tapping sound that speeds up when you give it gas, is there when cold and stays when the engine heats up. I've checked the flexplate and all bolts were tight. Still removed them, added blue loctite to each bolt, then re-tightened. I couldn't see any cracks on the plate at all, but didn't pry on it either. I pulled each spark lead and every time the sound remained. So, I don't think it's rods or lifters. But, just to be sure, I'm dropping the pan in the morning to check for any debris. Maybe broken piston skirt/s? Again, would've thought pulling the leads would at least quiet and isolate the offender.

I checked the motor with a stethoscope and it's clearly loudest at the valve cover, but it seems pretty consistent front to back. No noise from the area of the flexplate and I also listened to the timing chain cover and it was pretty quiet. Also have normal oil pressure and power. I've searched the threads and have seen a lot of questions, but most seem to quiet down after warming or it's a flexplate issue. So, I thought I'd create an account and just ask if anyone has any thoughts. And thanks in advance for any ideas and help.

Almost forgot. It's a 99 XJ 4.0.

 

The video almost sounds like an exhaust leak, it is very common for the manifold to crack.

 

Good thought. I wondered about the manifold early on. Not sure if the stethoscope helps on those, but I listened and didn't hear the noise on the exhaust manifold at all. At least not from all the pipes I could reach from the top. It's been a couple Jeeps ago since I replaced one, but I thought a manifold leak would quiet down a little after it warmed up.

 

I bought a beautiful rust free '98 last month with a engine knock very similar to that, but it was down low.
Pulled each injector (Oh sure you can do that on the spark plug side. but do you want to hold the wire with 10,000 volts in it or 12?) sound might have lessened, but never went away.

Then pulled the flex plate inspection cover, bolts were not only tight, but rounded over by the PO.
Took a compression test; 155 psi consistently hole to hole, healthy engine and piston slap is not likely.

Dropped the pan, no sludge or pieces of engine in it. Actually quite clean.
Everything was in order. Timing chain wasn't tight, but not loose either.

Mechanic's stethoscope told me the problem was at #5.
Pulled #5 rod cap spun bearing.
Crap, engine's coming out.

 

Valve cover is easy enough to take off, I've heard of folks breaking rods before. Piston skirt is a possibility as well as the exhaust leak mentioned. I'd aim for easiest stuff first. Pull the valve cover and have a looksie, climb under and you might see the crack on the exhaust manifold if you're lucky!

 

Most of the manifold cracks you can just look down and see.
Right at the Y.

 

Thanks for all the thoughts. First thing in the morning I’ll crawl under and take another look. I’ll check the manifold from down there as well. If I don’t see anything the valve cover is coming off next. Wish it were summer so there was more daylight time to work or that my house had a garage.

 

Well, I inspected the manifold this morning and wasn't seeing anything. I decided to break out this cheap inspection camera I had to look around a little better and found it. It's on the firewall side of the Y and it's a huge crack. Here's the best photo that camera could take with my hand shaking.


Thanks for the replies. We're about to put our house up for sale in the next couple of weeks and the thought of busting into something big on the Jeep at this time while finishing some construction on our house was not fun. I'll live with the noise for a few weeks and replace this when we've moved. I have some new motor mounts and tranny mount to put in as well and it'll be nice and easy to do that work with the manifold side of the engine cleared out.

 

Nice work! Word around town is to not cheap out on replacement or you'll be in there again in no time. I think nickintime's ebay find lasted him 1 yr. Good luck with those bolts, they can be a pain to get a socket on sometimes

 

I got an APN header from Summit Racing $170 with free shipping, and it was here the next day! I had ordered two cheaper ones from Amazon before and I sent them both back because of fitment issues, the APN header is much better made and fit perfectly.