Ok first of all I'm new to the forum so whats up everyone. I'm gunna try and sum this up as short as possible. I have a 2000 classic 4.0 and I over heated the motor a couple weeks ago, I towed it to my buddys shop replaced the water pump thermostat , spark plugs, coil rail. I was gettin a misfire in cylinder 5 so I replaced the injector. Still misfired. So I pulled the head and sent it out. They machined it cause it was warpd and replaced the valves. So I slaped the head back on to spec according to my shop manual. And I flushed the motor over night with kerosene to make sure all the coolant was out of the motor. I did a oil change and let it run for like 10 mins, then changed the oil again. It idled perfect for 45 mins I thought I was finally in the clear, no check engine. I bumped the throttle a little bit then it started makin a knock ( not a tick). But it was coming and goin. It also dropped rpm a couple times and almost stalled out. I pulled the valve cover and all the rods were pumping oil. But like I said it will run fine one second then knock a couple times then bog out. My one buddy thinks its a oil pump, my other buddy says I spun a bearing, and my other buddy thinks its a lifter. I don't know guys please help I'm lost.

Welcome to the forum!

I'm probably way off but maybe bad gas fouling things up? Sounds like you've already checked everything I would have. I'm interested to see what comes of this.

I would check for a crack in the block - it is a 2000 model with the 0331 head

Update... i pulled the head back off replaced push rods and lifters. Head and valves looked alright , new head gasket , head bolts and yet another fresh oil change. Idled great for about 45 min. With the valve cover off everything was pumping oil, oil pressure good, good temp, no check engine. Drove half way around the block and knock came back. Still no check engine and no uh oh flakes in the oil. I'm lost...

Have you been able to figure out when it does it? Can you repicate the sound when you want to, or is it just random? You need to figure out what type of action causes it to make the noise, like does it do it when the engine speeds up or is slowing down, light or moderate load etc. Can you hear be better from the top with the hood open or from the bottom if you stick you head down under the bumper?

something very similar happened to me.turns out my timing chain was VERY loose, and my flywheel was cracked. i dont know if the flywheel had anything to do with it but the timing chain definitely did.(both were making knocking noises) hope this helps in some way.

I think I'm in the same boat as you, a couple months after doing a transmission swap (the bellhousing on the old one cracked and I drove home for an hour with no OD) mine developed a really bad knock.

If the knock is lower than the head, but still in the block area, it is a main or rod bearing.

After 45 minutes the oil is thoroughly warmed up and thinned out. So the thinner oil will not cushion an excessive clearance.

The near stall is caused by excessive friction/drag.

Without being there, this is my best guess.

After I posted that I decided to go outside and drop my transmission. Turns out that I have a cracked flexplate.

A good way to check this is to put a mechanical oil pressure gauge on it. If the bearings are worn out you will have next to no pressure when the oil warms up.

Alright thanks guys.. I'm actualy gunna drag it out to the farm and put it on my lift cause I'm sick of being on my back. I'm gunna check the fly wheel, timing chain , and POP that pan off to check the bearings. I kinda thought it was a spun bearing to begin with but I guess I didn't wanna believe it. I'll get back to you guys Thursday let kno what's up. Thanks again for the advice.

Good luck. I had an old 6-cyl Mustang I bought cheap because it had a hard knock in it down low when it warmed up some, or was put into gear.

Turned out it was a cracked flexplate.

Which I found after I pulled the engine for a rebuild.

OH WELL.

I've never delt with a flexplate. Can that be viewed through the access plate on the front side of the bell housing for inspection? That might be something to look at too. I always try to do the easiest stuff first before ripping into large components.

You can try, but they usually crack around the bolt holes that the bolts go through to attach the flexplate to the engine crankshaft.

That's where the major stress is.

Well fly wheel isn't cracked, converter bolts tight. I ran for a little while and it slowly went away been drivin it all day no problems yet. Starting to think them lifters jus need to work them selves in. Idk thanks for helping guys