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Bottom End Knock

1

10334226

NAXJA Forum User
Location
Utah
1990 4.0L Renix 143,000 I have been getting a knock in my engine that has been getting worse. When The Engine is completely cold, and I crank it up it makes a really terrible noise, then it calms itself down to a dull knock right after its running. I crawled under, and It was really loud underneath. The noise is loudest towards the L/R of the oil pan. I tried to use a stethescope to get a better Idea of exactly where it was coming from, but I couldn't really pin point it. The knock increases with engine speed. I also noticed that when it gets to operating temp, the oil pressure will drop down almost to zero(The loss of oil pressure does not happen all of the time it is intermittent, and it will only lose it at idle) When the oil pressure drops the noise gets louder, and I have to tap the accelerator to get the pressure back up. Once I tap the accelerator the oil pressure comes right back up to normal, and the noise isn't as loud but it's still there. I did the oil change to check for metal, but the oil looked normal. Has anyone had a problem like this? Any suggestions would be much appreciated.
 
If the oil pressure thing is related to the knock, and it might not be, I would suspect a rod/main bearing failing.
 
is it an auto? if so, take off the inspection plate over the transmission and make sure the flexplate bolts are still tight. also try to see if the flexplate is cracked, they like to crack between the bolt holes, and when it cracks or comes loose, it sounds just like a rod knocking.



just throwing out the flywheel info because it sounds almost the same, ive had friends who have bought jeeps with "rod knock" for less than 500$, changed out the cracked flex plate, and sold them for 2k.

good luck, but i would start with checking the flexplate, if that is fine drop the oil pan and give it a look.
 
I second what GrimmJeeper said. I recently picked up a 96 with a ton of goodies on it for $400, the PO thought the motor was shot. I did too, just wanted it for the lift and aftermarket stuff on it. My buddy took a look and sure enough, flexplate. Check out this thread and watch the video in it to see if it sounds similar: http://www.jeepsunlimited.com/forums/showthread.php?t=486782

There's a few pic's in the thread as well of what the cracked flex plate looked like.
 
x4 torque converter bolts, mine loosened up last year, tightened up, no more noise. Common problem.

If that solves the problem replace oil pressure sender, my was doing funky things when it went out.
 
Re: Bottom End Knock Updated With Video

Alright, so I removed my flexplate inspection cover, and no cracks, or excessive movement found. I had two bolts that were slightly loose. I removed all 4 bolts added loc-tite, and torqued to spec. I did notice I had some really worn teeth on parts of my flywheel, but none broken. My noise is still there, I shot a video, and I will post the link at the bottom. When I used the stethescope I could hear the knock very clearly when I pointed it against the rear of the oil pan right where the half circle is that covers the rear main seal. I also placed it on top of the valve cover in several differant spots, and could hear a lot of rattling over the 4, 5, and six cylinders. I am Thinking I may have a top end problem as well. I plan on Pulling the valve cover, and checking it out. Could a valve train issue reverberate all the way to the bottom end of the engine? I am going to pull the wires one cylinder at a time too, and see what happens. I will post a update after. Check out the video, and tell me what you think.


http://www.youtube.com/watch?v=OkxFayhKDdI
 
Is that vid cold or warm? Top end sounds fine(can't really hear much over the fan). Bottom end sounds more or less like piston slap and normal noises.
 
Its at operating temp. It is much worse warmed up then cold. I think because the oil pressure is higher when cold. Is that common with piston slap?
 
I use Valvoline 10-30. When its warm at idle its about 20psi. Last week the stock guage dropped to zero on me a couple times, and I had to tap the throttle to get it to come back up. But it has not dropped on me this week. I tested it with my pressure tester, because I don't trust my stock guage. I think the oil pressure sender is going out. I was thinking about running a thicker oil, or some Lucas oil stabalizer to see if it changes the noise, or quiets it down. What do you think?
 
Eek. the whole dropping to zero thing is kinda iffy. I would at the very least change out the sending unit. I'm thinking the Lucas will either make it better or worse.
 
Yea the sender is on its way. Well I am going to try it, and hope it changes something. If it gets worse it may be easier to diagnose. I have a few more tests to run. I'll update later. Hopefully I can get a few more people to check out the video. I really do hope its just piston slap.
 
Ok this is WORSE case. I had the same thing happen to my Camaro. Low oil pressure when you raise the RPMs and there is a loud rapping sound from the bottom end. Ended up to be spun crank bearings. I'm not saying you spun them. But do remove your valve cover and shine a light at the head and look for fine gold flakes mixed in the oil. Or drain the oil and strain it with a paper towel if you dont have a strainer. If you want to go further remove the oil filter and cut it up to see if theres any contaminants. All your looking for are bearing material(gold flakes).
 
EFR98XJ said:
Ok this is WORSE case. I had the same thing happen to my Camaro. Low oil pressure when you raise the RPMs and there is a loud rapping sound from the bottom end. Ended up to be spun crank bearings. I'm not saying you spun them. But do remove your valve cover and shine a light at the head and look for fine gold flakes mixed in the oil. Or drain the oil and strain it with a paper towel if you dont have a strainer. If you want to go further remove the oil filter and cut it up to see if theres any contaminants. All your looking for are bearing material(gold flakes).
Click to expand...

x2 on the investigation, it does sounds like you may have bearing issues in the bottom end. I really hope you don't, but looking in the oil filter and checking out the oil itself would be a good idea.

:eek:
 
I drained the oil very first thing if you refer to my original post. No metal shavings, and it was still pretty clean, because I had only put 2000 mi. on it since my last oil change. I have not lost any performance either. I am starting to think I have a tired motor, or really bad piston slap. but it only has 139,000 miles on it.

Video
http://www.youtube.com/watch?v=OkxFayhKDdI
 
Last edited:
Re: Bottom End Knock New Video Pulling Plug wires

I ran a new video pulling the plug wires one at a time. When I pull the #3 it gets really loud. It was loud when I pulled #5 too, but I accidently removed #3 at the same time, so disregard #5. I am going to drain the oil again, and check for flakes a second time, because I am really starting to think I have a bearing going bad. Anyway any suggestions?

New Motor?
Salvage Motor?
Complete Overhaul?
New Jeep?
Or Just Drive it until it dies, and decide then?

New Video:
http://www.youtube.com/watch?v=4a4fjN9WQuU
 
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