Strange 4.0 Knock -
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post #1 of 4 Old 04-17-2016, 10:36 AM Thread Starter
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Join Date: Apr 2016
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Xj Strange 4.0 Knock

Hey guys,

I've looked everywhere trying to determine the cause of the sound my Jeep is making. Unfortunately the term "knock" always brings up the worst case scenarios and finding the correct result is pretty challenging.

Anyways, here is the sound it is making:

If I start it up cold, there is no noise.
While driving, it is not there.

The only situation it will make it is the following:
When I start the engine up after it has been sitting for about an hour, so it has cooled to about half of operating temperature. It will do it for a couple of minutes, and then it's gone. Which takes it tricky to diagnose.

It does not increase with rpms either, thankfully. It will also only do it at idle (or at least it can only be heard at idle)

My thoughts so far are piston slap, some kind of exhaust leak, or just call it a typical noisy 4.0

Another thing potentially worth mentioning is that there is quite a bit of new vibration at idle, the whole thing kind of shakes a bit.

Any input would be greatly appreciated! Thanks.

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post #2 of 4 Old 04-17-2016, 11:58 AM
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2006 TJ Wrangler 
Join Date: Dec 2015
Location: harleysville
Posts: 75
if it doesn't change with rpm then it isn't the engine.

the evaporative pump on the passenger side firewall can sometimes make a ticking noise.
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post #3 of 4 Old 04-17-2016, 08:49 PM
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1996 XJ Cherokee 
Join Date: Dec 2015
Location: central new york
Posts: 529
I watched the video about 10 trying to remember why that sound was so familiar. I believe I had a very similar sound on one of my older grand cherokees (95). It was resolved when I had the exhaust done. It was particularly noticeable at idle rpms, and whenever I would go through a drive through, it would echo off the wall right back into the driver side making it really obvious. Exhaust leaks are pretty cheap and easy, so I hope its nothing more than that.

Things I keep around that help fix Jeeps:
AeroKroil and Fluid Film
BAFX Products 34t5 Bluetooth OBDII Scan Tool and Torque Pro OBD2 App for Android
Etekcity MSR-P600 Digital Multimeter
Bernzomatic TS4000 Trigger Start Torch + MAP Gas
1/2 inch sockets, Breaker, Cheater Pipe, and BFH
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post #4 of 4 Old 04-20-2016, 06:50 PM Thread Starter
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Thanks! I really hope that's all it is. Kind of makes sense since it seems very particular with the temperature, the heat expansion probably allowing it to leak just right at certain times.
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idle , knock , noise , xj

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