Ok thanks!
Im not sure either, the plan is to plastigage to have some kind of info at least.

the plate on the engine says
0002MX21

what I can find out is that it means
1990
feb 21st
but still need to understand the MX part

 

Since you have apparently already opened that poor thing up why don't you just measure the journals and look at the specs in the shop manual?
​​​​​​
Also. At best any codes on the block will tell you what was going on when it was built: it won't tell you what might have been turned undersize later on

 

^this

MX is chrysler code for a 4.0 block

 

That also makes sense

Still dont understand the codes though if MX is just a generic 4.0 block id? Not important but just interesting

 

Yes those are the two main suspects now.
The knock starts when warm which would point away from piston slap that normally behaves the opposite way from what I’ve learned. Rod bearings are worn down to the copper so makes sense. No metal chunks in pan but plenty of metal sludge.

Removing the head coming week to check pistons and rings.

I finally found the stamped codes on the engine- they’re actually behind the motor mount below rhe dizzy. Mine says B wich would indicate oversized bores. I assume the pistons would have corresponding embossing.

 

PROGRESS!

So FINALLY I got the head off and after inspection of my pistons it´s pretty evident where my knock is coming from!
Piston 6 had half of the top ring missing and is also chewed on it seems. Luckily all cylinders seems ok and other pistons look great.
Will replace no 6 piston and rod+main bearings.

More clues:
-After more diving into the engine bay I found a leaking freeze plug as well, where my block heater was installed.
-When I came home after buying this Jeep and driving it for about 20 mins I notices zero coolant in the overflow bottle.
-Looking at the valves it seems that no 6 has incomplete combustion?
-There was a tiny but of coolant in the oil pan but very little

Questions:
Would this add up?
Coolant leak- overheating on cylinder 6- banging detonation in no 6 destroying the piston.
I found it hard to determine wether the head gasket was blown or not using my zero experience.
Looking at the pictures- what are your thoughts of this mess?

 

Yeah thats not good.

#6 would have had low compression, so that might explain the sooty look. Cant really tell from the pic but /maybe/ a head gasket leak between 5&6? That wouldnt cause your piston to kill itself though. It doesnt look like its been burning coolant, but Im not sure what the discoloration on the piston is from.
Could be detonation, but what caused it? Could also be that the ring broke first and ate the piston. Whatever happened, it looks like its been going on for a long time.

 

You were obviously running extremely rich, but no telling if that was the cause or result of your damage. having a hole in your piston can"t do any good for your engine or mixtures passing over your o2 sensor. the aluminum might have not done too much damage to your cyl wall but im sure the hardened rings might have done some damage. Are your injectors sized properly? I had issues with my stroker almost since the day I built it...#2 misfire all the time, but the whole motor was running lean due to my miscalculations on injector sizing. Also always low on coolant...obviously leaking into #2 cyl. new cyl head with proper injectors and it runs like a top now

 

I can't believe the cylinder for that piston is still in acceptable shape, but even if its in mildly poor shape, you could go with it. Just gonna burn some oil and have lower compression.

 

ah ok so that’s more what a head can look like, interesting!

I have been running very rich- spark plugs where completely black and sooty with No 6 smelling of fuel.

Im not sure what you mean with sized injectors- you are referring to the ones on the fuel rail yes? They look like factory to me, it seems that everything so far have been original on my xj.
I’ve cleaned and tested the injectors and they seem fine but I’m planning to switch them out for the Bosch Volvo ones just because why not. Next up is cleaning the fuel rail and installing a new fuel pump, dont think that’s done before either.

Flushed the cooling system today. A very brown story but looks good now! Also checked the thermostat in a pot of hot water. Fingers crossed I can just get all this together again now that I ripped most of it apart…

 

Is the engine out or are you doing this "on-wing"..secondly not sure if the 90 had a regulator built into the front of the rail, they go bad and bypass way more frequently than a bad fuel pump,That will cause running rich every time. .. And from what Ive heard...the replacements are junk just to replace a functioning pump "because". You need to mic or didital caliper the journals to find out how worn for replacement bearings. Also find a clean piston...stamped on top will be an oversize # or nothing if stock

 

the block is still in the jeep so winging this a bit from the prerequisites I have. Is the regulator the round thing?


thanks for the tip on after market junk
parta. I’ll just hold off on the fuel pump for now.
I guess I don’t have any problems with the fuel delivery if all the cylinders are getting fuel and the fuel injectors works fine on the bench? Have not tested the PSI properly tho, have no such tool.

the plan is to plastigauge the journals to be sure I have the correct clearance. a few of the top main bearings where 001 and the bottom std. From the FSM I learned that this is the correct way to mix them if needed. The main bearings actually looks really good- do you think I should swap out those anyways? On the topic of OEM quality- maybe I should just put them back and just swap the rod bearings?

 

At low rpm/higher vacuum... as sensed by the black elbow and vacuum line back to the intake, the regulator bypasses fuel back to the tank..therefore lower pressure at fuel at the rail. At high rpm/lower vacuum, the fuel is blocked from bypassing and you have higher pressure at the rail and your injectors. If the internal diaphram in the regulator is compromised ...fuel is sucked through the black vacuum line and dumps directly into the intake manifold causing very rich running. You might be able to test it by sucking on the vacuum port with a piece of hose and if you can suck air its blown, but usually we just disconnect the black elbow with the engine running and if fuel spills out of the regulator its shot.. The connecting rod bearings are easy to replace with pan off...the mains are slightly more of an issue. You need to verify your dimensions and get new ones before your reinstall.

 

Hi!
So my tare down grew slightly bigger than I first planned, I figured why not do a proper engine bay refresh when half of ther stuff is already out.
If the engine wont start then it will at least be nicer to look at!

I have one question before I put everything back together again:
If the compression was within spec on the healthy cylinders, the rings are all free and ring lands etc look good- do I have to install new rings or can I go with the old ones?
All pistons and cylinders look really good with visible cross hatch, no damage on the pistons or walls and rings and rods free.

I guess- "if already there you might as well", but that same logic would also go for oil pump and a lot of other parts.
I have a hard time drawing the line on where to stop and the spend on this thing is well beyond reasonable at this point. Already Just Emptied Every Pocket so to say.

 

New rings would be a good idea, but if everything looks ok its not super necessary.