Hey all new to the forum thanks for the add! Just have one question I recently purchased an 04 grand Cherokee 4.0 with a knock. Turns out it had a cracked number 6 piston! Jeep had 204,000 on but has been very well kept! I haven't found a spot of rust on the thing. Even crawling under the jeep I can not believe how mint this thing is. Anyways because of how well kept it was I decided to drop another motor in it. I put a motor in out of a wrecked jeep Cherokee with only 80,000 on it. Jeep runs great now and seems very strong. I finished this motor back in December, long story short it has been very cold around here. About a month ago we got a very warm day around 80 and after about an 4 hours of driving this thing started making a very loud grinding noise to the point I couldn't get over 10 mph to limp it home. When I pulled in my garage and put it in park it went away. But when I reveled it pretty good it came back loudly. Called my buddy that help put it in to ask him what he thought but he couldn't make it over for a few days to look at it. When he got there a few days later, the thing fired up and ran like a champ couldn't get it to do it again. We ran the crap out of it. I been running it every day for another month now been running like a champ. Yesterday it got up to 78 here, after about 4 hours of driving it started it again. Got it home let it sit till this am and drove fine again! What the heck is going on? It has us stumped please help.

 

This may not have anything to do with it, but check the bellhousing and torque converter bolts for having loosened up. This problem sometimes pops up after an engine change. In the worst cases, this will crack out the center of the flexplate.

 

Thanks for the tip I was thinking about this too. It almost sounds like one of the Flexplate bolts is coming around and catching the bell housing. Could heat bring this on and when I put the motor in it to torque they hit?