LCFRJEEP

All,

Out of the blue, without any warning of any kind, I started my Jeep up in the morning and had an obvious knock. I let it run for a few min and the knock is not audible when it is idling in park but when you put it in drive, as soon as you hit the gas pedal you can hear it for a split second as the RPM's come up and then as the RPM's comtinue to increase the knock cannot be heard. The knock is easily heard from inside my jeep and harder to hear from under the hood. That alone makes me think it is something in the bottom end towards the rear. I understand a ton of things can cause a knock but does anyone have any ideas as to what this could be?

Thanks for all your help thus far.

PS, I have good oil pressure, no loss of power or MPG. Idles fine and after you let it warm up, it seems to almost go away. Who knows!

mnaemark

it could be an exhaust leak. sometimes they sound very similar and our jeeps are prone to exhaust manifold cracking ect. If it is a true engine knock just throw some Lucas oil stabilizer in there and see if goes away. Worked for me but im just trying to get as many miles out of my engine before a total rebuild

Jeepman659

yeah mine does this it annoys the **** out of me but it's just the Exhaust Manifold. I'm just waiting till i have enough money for headers! =]

1 slow 87 stang

Xjs are real common for the fly wheels cracking and they put out the same noise as you described when it happens.

Jeepman659

haha 87 stangs right about that too that was the first thing i had to do when i bought my jeep sound comes from inside the bell housing. It's a pretty loud sound.

LCFRJEEP

So after talking to a mechanic friend, he suggested changing the oil to a thicker oil. I currently have 158xxx miles on my rig and I religiously use Mobil 1 full synthetic oil for my changes. He told me that with the higher mile motors they sometimes will start to knock and or be noisy with thinner oil and with using a thicker oil (20w50) it might help and or alleviate the problem. Any thoughts?

crc

I've read that some people say they have quieter engines with thicker oil, but I don't see how you develop a knock out of nowhere just because you use synthetic. I would check for something loose under neath first and pull the inspection cover and check the flexplate.