231James

Hello,

I am a first time engine builder. I followed Larry Shepard’s guide to a T. Initial startup fired right up, went fine, had a small fuel leak due to a previous repair made on the line by the previous owner, and a small coolant leak until I noticed the remanned head was missing a pipe thread in the rear of the cylinder head near the firewall. While adding more coolant and burping the motor, i gave it a little gas, not much, but it started to stumble and a knocking sound was audible. The more gas the faster the knock and the engine stalled out. It restarted but if you give it any gas it starts to stumble at idle.

Now my brother n law yelled shut it down it’s a rod knock you threw a rod. I thought this was impossible because everything is new and just out of the machine shop. After draining the oil, pulling the starter, I can move the oil pan off to the side enough that we turned the motor around a few times with the crank-bolt and can see the rods are all intact. The cylinder walls look well oiled. I felt like the sound was coming from the rocker arms I did replace two bent rods but used the old rocker arms and old pushrods, the machine shop thought it would be fine and it didn’t have to be in order since we placed a new OEM melling camshaft. The top of the valve cover doesn’t have any noticeable damage and everything seems fine on the surface under the valve cover.

Next I pulled the spark plugs and they all had been gapped correctly and none of them are damaged.

I know I’m asking everyone to throw spitballs but does anyone have any idea what this could be?

I purchased this with 260,000 miles and the previous owner stated this engine had been rebuild 60,000 miles ago. The #4 piston disintegrated after overheating this past summer. Took it to the machine shop, they sleeved a cylinder, ground the crank, and here we are.

Any thoughts would be greatly appreciated. There is also a new high volume oil pump and pressure was great, twice what it was before it broke down. The heat had just started to get warm after adding coolant and burping before the knocking sound.

231James

I ended up pulling the whole oil pan out after raising the front end off the ground. I can’t see any issue down there. I did find a tiny piece of metal in the pan. I don’t see any fluid or anything like that on top of the pistons. Still looks pretty new up there while shining a flashlight through the spark plug openings. The spark plugs seem to get dirtier moving from piston one to six.

This could have potentially been in the pan previous I cleaned it by hand But may have been stuck back in the shroud possibly. No way to know for sure. It could also possibly have been there from the wire wheel I used on it or around it. I did spray everything out with an air hose before assembly and soaked parts in kerosene.

#1 is not in this photo but its a clean as 2-3

231James

Turning the motor over while lighting up the cylinders I do not see the pistons separating from the cylinder wall causing any piston slap or anything like that.

While peering through the #6 cylinder there does appear to be a tiny chunk of metal on top of the piston and possibly little micro divots, not sure if it’s part of the valve or not. There wasn’t a socket or anything left in there before installing the head.

I believe the timing was correct, it was triple checked with a dial indicator. If the timing were off far enough to cause that would it fire right up and run smoothly at idle?

exasemech

An engine that's been"rebuilt" twice, major over heat in it's past, first time rebuild for you, metal junk in a fresh engine... the possibilities are endless. But a couple observations:

1) no way those plugs should vary in color like that. Something weird is happening on the cylinders with the really dark plugs.

2) unclear what the noise is like but you might want to check the timing again to still if it's still ok... The high volume oil pump vs the drive for the cam position sensor issue is a bit of a red flag

dmoe69

Maybe there was something unwanted in the intake runner you missed prior to assembly. Is it small enough to suck up and capture with a vacuum and a small hose?

Seabee

What was the oil pressure reading? That would be your first "health" clue. Do you have a bore scope? They are cheap at HF and will let you take a good look in the cylinders.

231James

I agree the possibilities were endless. Was desperate for answers last night so thanks everyone.

So I pulled the head out because I wanted to inspect the top of the cylinder and bottom of the head and valves in case something did break off in there. I pulled out these two metal chips that must have gotten in there from the head itself. I bought this head directly from CHI in Texas it looked great upon arrival although it is not a TUPY head.

I dont believe it was actually ever rebuilt until now. Everyone’s idea of rebuilding is different but this one had RTV for a head gasket and was covered in oil. I did not know as much about these engines until recently.

The valves look good as well as the cylinder wall, there is some tiny tiny divots on top of the cylinder head but the valves look fully seated. It may be normal but it’s odd to me that this would cause the engine to stumble, I’m really not sure if I should just put it back on and try again, maybe give it a compression test afterwards. I did blow out the intake with high pressure air before assembly but I suppose it’s possible something got sucked in there. It had been sitting around for months while it was in the machine shop. They service the dealers first so they had my block for months.

231James

The oil pressure was over halfway, somewhere between 45-60. I can’t remember the exact pressure with all the excitement at the time. I remember when I bought it, the previous owner was telling me it needed a rotella oil change to bring it back up, I did change the oil after purchase but it never went up over the halfway mark unless it was floored, it used to hover just under half way through the gauge until it warmed up and it was barely over a 1/4. It also ran extremely hot when I purchased it, you couldnt open the hood without scolding hands and the clear was all burnt off the hood and tops of fenders believe it was from the engine heat and not the sun. Last night while burping it it stayed around 190-to just under 200. It has a 190 thermostat.

231James

I wish I did have one now that you mention it, would have been easier than pulling off the head. The guy at the machine shop just said to pull it off so I did. He also had said before I removed it even if the valves were used or remanufactured the chances of them breaking in there was very very slim. He mentioned the cam timing, but that seemed pretty strait forward. It has the cam position sensor, used the toothpick/wire method after watching a couple you tube videos on it and having it on TDC.

Seabee

I would call your oil pressure "normal." The AMC 6's always seemed to have low oil pressure after a little age. I have noticed both of my 4.0 6's carry great oil pressure, even at hot idle. They must have improved the machining processes and are able to maintain better bearing clearances in the 4.0 version.
That heat is very concerning. I have read a lot about HOT running 4.0's, but luckily I haven't seen that yet on mine. That much heat is never a good thing for sure.

231James

Luckily not too much damage. I’ll run a compression check after just to be sure. They didn’t have one in the area and I didn’t want to wait until tomorrow to inspect.

I may as well compress the springs and make sure the valve seats are alright before re-assembly

231James

The heat was pretty bad, I’m sure I’ll have to replace the wiring harnesses eventually. The air intake sensor pretty much melted when it overheated just before the rebuild. This was the engine a couple thousand miles after purchasing.





replaced the harness for the injectors all that plastic, pvc/ breather etc, was too brittle to take apart without breaking.

Sig220

"I dont believe it was actually ever rebuilt until now. Everyone’s idea of rebuilding is different but this one had RTV for a head gasket and was covered in oil. I did not know as much about these engines until recently."

Are you (op) saying that a one time it had RTV for a head gasket or you saying that you used RTV for a head gasket? If RTV was used as a head gasket, that may have been the problem...Get a fiber/metal proper head gasket to remount your head.

If I misunderstood your post #7, I apologize!

231James

Noticed the RTV when disassembling the engine, I think that’s when I first acknowledged that he did not have this rebuilt correctly. I used a fel-pro compression gasket for my build. Now I’ll have to buy another one since I took the head off. I just meant that there were many signs that this hadn’t been rebuilt recently when I purchased it but I didn’t know at the time. Seemed like it ran pretty good except for the exhaust leak and the oil all over the block, but when there’s all this info and jokes online about oil leaks it was difficult to tell until now. The guy was asking 7k in Oklahoma, I paid 6 at around 268,000 miles. So I’m keeping this and making it all new starting with the inside. Oh and the belt squeaked, it was pretty embarrassing, I put the same one back on and it doesn’t squeak at all yet since tightening it down. Will end up replacing it soon either way. It did have a new belt, and water pump, thermostat, heater core, 3.5 rough country lift, alignment, It sounded like he had recently put alot of money into it at the time. I had replaced all fluids and swapped out all the interior/exterior lightbulbs with LED’s, replaced 2 tie rods, added a double din dash, and all speakers upgraded, installed a front/rear dash cam, before it quit.

231James

Just finished the initial break in. Runs like a champ this time around. The knocking sound was the debris that ended up in the cylinder.

The stumbling while applying throttle ended up being the throttle position sensor. I looked up the part number and the old one I believe was a 10$ amazon knockoff, lucky shot in the dark with respect to that. Also purchased a stethoscope to trace down vacuum leaks if in case that did not work and fuel pressure gauge just in case as well.

While breaking in the motor by running it for 30 mins between 1800 and 3000 RPM the temp was pretty consistent at 200-210. I do not have a fan shroud yet, and didn’t have the secondary fan running. Not sure how much difference that makes.

The oil pressure stays around 30-40. Before dropping the pan it was running 20w-50 high zinc, now it has a combination of conventional 15-40 rotella and lucas break in oil. So that may explain the lower pressure this time around.