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Need advice. Engine knock, perhaps?

123Grains

NAXJA Forum User
NAXJA Member
Location
USA
Hey everyone,
Let's start with the usual.


99' XJ 4.0, AX15, NP231


And now the problem. I was on my way home from work tonight when the old 4.0 started making a terrible racket. It started when I took off from a stop sign at around 2000 RPM just before I shifted into second gear and continued any time I was over approximately 1200 RPM. It is a loud clacking or rapping sound that seems to come more from the exhaust side of the engine and I'm crossing my fingers that the manifold suddenly cracked but I doubt I'm that lucky. Oil pressure is 45-50lbs at idle which is normal for this particular rig, Idle is nice and smooth around 750-800, power is still good but the noise is awful. Hopefully I can make a link to my video work, bare with me if it doesn't.
I know there is a long list of things it can be and I automatically think worst case scenario any time this happens to my daily driver. So what are your thoughts?


-Crushed Lifter?
-Bad bearings?
-Broken exhaust?
-Piston slap?
-Broken piston skirt?
-Wrist pin noise?


I have not done any testing besides checking gauges and taking a video. It's late, dark and cold here right now so that won't happen immediately and I'll be driving this to work for the next two days. Wish me luck.


Here's the link, don't forget to unmute it. https://imgur.com/a/SlC9K1l


Oh and I forgot to mention the blue smoke right at the end of the video...that's new.
 
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I would definitely start under the valve cover(dead lifter/backed off pivot bolt) and go from there. A compression check would also be a good idea at this time.
 
I will heed your advice this weekend, RCP Phx. Until then I have no choice but to drive it to work. It's so odd how suddenly the noise showed up at and at such a low RPM. I've had motors go on me before but it was always at high RPM, not just accelerating.
 
If it sounds like it's underneath, check the torque converter bolts. If it's upfront, take off the serp belt and make sure it's not an accessories or the harmonic balancer rubbing. I see some smoke coming up from the engine bay when you were reving it, so perhaps an exhaust leak?
 
I will try the serp belt removal this weekend too. No torque converter because it's an AX15 M/T. I noticed that smoke too, kinda interesting.
 
Any chance it's the harmonic balancer making all that racket? The noise comes and goes throughout the RPM range and I made my 30 mile drive to work without any problems besides the noise. Oil pressure is still good.
 
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