Internal engine knock help!
 

Ever since I rebuilt my Jeep motor and fired it up for the first time, its always had a knock under light load around 2000rms and up. It will knock every once and a great while at idle. Ive done as much as I can think of to figure this out with no results. This is a mechanical issue.

Things Ive done so far:
Pulled the oil pan, no metal shavings
Re-torqued everything, and I do mean everything!
Pulled the valve cover off and re-torqued all the rockers (no adjustment to them)
I pulled the timing cover off, verified timing and nothing hitting the cover.

Then I found out if I pull the injector plug or plug wire off of #3 cylinder, it knocks continuously at idle. #3 is the only one that knocks when I do this. So I then pulled the brand new hydraulic lifters, rockers, and pushrods and swapped them with #2 cylinder. No change what so ever. I swapped injectors with #2 and #3, nothing. Swapped spark plug wires with #2 and #3, nothing. I even changed out the #3 rod bearing and it didn't change, didn't even need a new one either.

One thing that I did do that made a change was loosen the rocker bolts for the intake and exhaust with the injector plug off and it changed the sound. It got quieter, but didn't go completely away. Im getting plenty of oil flow, well equal to the other cylinders, out of the pushrods through the rockers.

Info about the motor is that the only internal moving parts that are old are the rods and crank. Everything else is brand new. The head is a brand new head that came fully assembled. I did reuse the old rockers, but that's it. Everything was machined/replaced for just a normal rebuild, nothing crazy on this one. I cant think of anything else to do besides pull the head off and take a look. If I had a borescope to use, that might help some. Anyone have any other ideas? Im completely lost.

Does it have headers or an exhaust manifold? If it has headers you can you a pyrometers to check the exhaust gas temps when it's knocking at idle to see which cylinder is causing the problem. I'm assuming that you already pulled and read the spark plugs. It sound like you are confident that it's not a mechanical knock and could be detonation.

It has the stock exhaust manifold. From everything I've done so far, I know it's cylinder #3. I just don't know what else to check. I know it's not the rockers, lifters, pushrods, injector, spark plug, plug wire, distributor cap or rotor, or the rod bearing. The piston easily moved on the wrist pin before install, the rings all have the correct gap, it's not piston slap because tempurature makes no difference either. I think my last thing to do is pull the head and look at the valves on that cylinder.

I should add that it's a 98 Wrangler with the 2.5L I4. It runs just fine other than the knock. It even passed smog too.

Did you mic the bearing clearance when you put it together? How about the valve to piston clearance?

I mic'ed all of the bearings. I didn't check the piston to valve clearance since I'm using stock replacement pistons that have the same dished shape as the stockers did. I would have done that if I were running some aftermarket higher compression pistons, but these are stock compression which is 8.5:1 if I remember correctly. Plus, if one cylinder was off, Id expect them to all be off.

I also did a compression check the other day and it has 140psi, minimum is 100psi.

Just because it has the same dish doesn't mean that the compression height is the same on the replacement pistons as it was on the stock ones. You could have one piston that wasn't right and is sitting higher in the bore than the others.

Also, you said you bought a head that was already assembled, did you remove the rockers from cylinder 3 and rock the valve to see if it moves? Maybe there is an issue with the guides on cylinder 3.

I just pulled the head off, I was really hoping I didn't have to do that. The pistons look fine, no nicks or anything. Same with the valves. The cylinder walls look like they are brand new.

I just went out and tried to move the valve and I cant, but the springs are still on it. Im pretty sure my neighbor has a spring compressor, so Ill take the springs off on that one and see. The valves appear to be seated flat though.

With 2 and 3 pistons at TDC, there is no difference from a straight edge on the top of the block to the top of the highest point of the pistons.

How about swapping fuel injectors to see if the knock follows? You could have a stuck open, clogged, or failing injector. It definitely a long shot, but quick and easy compared to pulling the head.

I tried that as well. I even swapped in my Typhoon stock injectors with no change.

Pulling this out of my ass. Small crack in the manifold around 3 causing it to take in more air and go lean to the point of knocking?

Any way or reason the timing could go way advanced on 3?