Cracked Jeep 4.0 head

Chad Culbertson · **Posted:** Sun Oct 18, 2015 12:54 am

Thought you guys might like to see what I discovered after I yanked the head on the Jeep today.

The gasket was 100% fine, thats when I knew there was a bigger problem. After inspecting the head a bit on the cylinder that was having issues I saw a faint line between the valves, and sure enough 99% sure it's a crack.

This particular casting isn't known for cracking like the later years which crack between cylinder 3 and 4 and allow coolant into the oil. There wasn't any oil/coolant mixing here, it was just being burned and pressurizing the cooling system.

The intake valve also had some strange defect on the face of it, not sure how that happened but it didn't match the rest of them, not sure if the burning coolant caused some kind of damage to it or what. Perhaps it just had a manufacturing defect from the get go, I doubt it really hurts much.

I ordered a re-manufactured head off ebay for 300 bucks and gaskets, bolts etc from summit, so I should have her buttoned back up again in a couple weeks.

Other than the miss while the engine was running it seems to run pretty well and the oil pressure is still great, so I expect to get quite a few miles out of this jeep, it's currently got 161k and the body is in great shape, there's only a couple places where the clear has come up some and it's on the fiberglass hatch.

RodneyWright · **Posted:** Mon Oct 19, 2015 8:52 am

Makes you wonder what caused the crack in the first place. Looks like you have this one under control. Well done...

Chad Culbertson · **Posted:** Mon Oct 19, 2015 10:27 am

RodneyWright wrote:

Makes you wonder what caused the crack in the first place. Looks like you have this one under control. Well done...

Yeah, not real sure. One theory I have is that the previous owner mentioned he had the fluids changed and I believe meant the coolant also. Wondering if the shop didn't get all the bubbles out which caused it to overheat and crack.

He couldn't really tell me if it started having problems after an overheat or if he just noticed it overheating because of the problem.

I may never know, he's smart enough to do basic maintenance but he's no gear head so I doubt he truly knows exactly what happened.

Either way, it should live again, hopefully the new head and other parts come in this week sometime and I can use this next weekend to get it wrapped up.

JamesMilko · **Posted:** Mon Oct 19, 2015 11:24 am

Did it crack near the coolant outlet? The PO could have overheated it, blown the coolant out then refilled it while hot causing it to crack when the cold water hit the hot head.

Chad Culbertson · **Posted:** Mon Oct 19, 2015 1:51 pm

JamesMilko wrote:

Did it crack near the coolant outlet? The PO could have overheated it, blown the coolant out then refilled it while hot causing it to crack when the cold water hit the hot head.

The only crack I saw was between the valves, but i'm guessing there is a coolant passage up there somewhere. I believe this would be the first cylinder of the head coolant would make it to, but it would have gone through the block also first.

The PO was pretty honest and kept going around showing me little things that I really didn't care too much about lol, I think he would have told me if that happened, but who knows.

RickyBeam · **Posted:** Mon Oct 19, 2015 4:55 pm

How deep does the crack go? It's possible it was a casting defect. That's an odd place for a crack to form, but heat cycles can do weird things.

RodneyWright · **Posted:** Mon Oct 19, 2015 5:04 pm

Upon further review, I say just open up the crack with a dremel tool and then JB weld that sucker.... Will save you the cost of a reman'd head. You're welcome....

JamesMilko · **Posted:** Mon Oct 19, 2015 8:45 pm

I offered to hit it with my welder.

Chad Culbertson · **Posted:** Mon Oct 19, 2015 9:06 pm

I like your guys redneck thinking, but honestly, I do like working on cars, but I don't like installing and pulling heads for shits and giggles, lol.

Chad Culbertson · **Posted:** Mon Oct 19, 2015 9:17 pm

RickyBeam wrote:

How deep does the crack go? It's possible it was a casting defect. That's an odd place for a crack to form, but heat cycles can do weird things.

Not sure really, I didn't bother to spend the money to have it magnefluxed. I do agree it doesn't seem like a common place and I couldn't find many pics of a 4.0L head cracking here. I did find one of a 2.5L head that had been overheated due to a freeze plug rotting out and dumping the coolant, from what I understand the 2.5L and 4.0L are fairly similar.

The other possibility is there is something else wrong causing overheating and this was a result of that overheating and not the original problem. I guess i'll find out for sure soon enough. The cooling system seemed quite rusty, but no sure if that's because of corrosive gases getting in there, or somebody not keep the right amount of anti-freeze or what.

The color of the coolant was a nice brown color, except for the fill neck where it looked nice and green like normal. I had let it get up to temp before tearing it down while idling and the upper radiator hose got nice and hot, so I think coolant is flowing fine, it also didn't readily overheat while idling and seemed to hard fairly steady. Maybe if it was being driven around and hood down it would have overheated though.

Roger McDaniels · **Posted:** Thu Oct 29, 2015 12:11 pm

I've had that happen several times on old cast iron heads. They like to do that if the coolant gets low and they get too hot. Recently a plastic coolant overflow reservoir cracked and did in the head on an old Taurus, but I've also seen it from water pump seals failing, leaking heater cores, radiator end cap seals failing, etc. My cracks have always been in 2-valve iron heads; maybe there's just not enough material in the narrow piece between 2 big valves to dissipate the heat unless they have a good amount of coolant flowing through them.

Roger McDaniels · **Posted:** Thu Oct 29, 2015 12:14 pm

P.S. - One of the coolant passages in the picture appears to be mostly blocked by some type of crud. I'd de-crudify that.

Chad Culbertson · **Posted:** Thu Oct 29, 2015 1:41 pm

Yeah, I think that passage actually gets blocked by the head gasket, which probably explains the buildup there.

I'm going to probably flush the whole system onces the new head goes back on with just water for a bit. The coolant that came out looked pretty rough so i'm sure it needs a flush anyway.

RobLupella · **Posted:** Thu Oct 29, 2015 2:36 pm

Chad Culbertson wrote:

JamesMilko wrote:

Did it crack near the coolant outlet? The PO could have overheated it, blown the coolant out then refilled it while hot causing it to crack when the cold water hit the hot head.

The only crack I saw was between the valves, but i'm guessing there is a coolant passage up there somewhere. I believe this would be the first cylinder of the head coolant would make it to, but it would have gone through the block also first.

The PO was pretty honest and kept going around showing me little things that I really didn't care too much about lol, I think he would have told me if that happened, but who knows.

If I show him all the little stuff he'll think I'm honest and he won't notice the big hole in the block where a piston should be...... :twisted:

Chad Culbertson · **Posted:** Thu Oct 29, 2015 3:45 pm

RobLupella wrote:

Chad Culbertson wrote:

JamesMilko wrote:

Did it crack near the coolant outlet? The PO could have overheated it, blown the coolant out then refilled it while hot causing it to crack when the cold water hit the hot head.

The only crack I saw was between the valves, but i'm guessing there is a coolant passage up there somewhere. I believe this would be the first cylinder of the head coolant would make it to, but it would have gone through the block also first.

The PO was pretty honest and kept going around showing me little things that I really didn't care too much about lol, I think he would have told me if that happened, but who knows.

If I show him all the little stuff he'll think I'm honest and he won't notice the big hole in the block where a piston should be...... :twisted:

Lol, fortunately I spent a few years working with him and we got along pretty well so I was pretty sure I could trust him. He could have just put some head gasket repair crap in the radiator and maybe fixed it temporarily and traded it in lol.

The one weird thing about this thing was that the ECU had misfires registered for all 6 cylinders, but the other 5 appeared to be just fine and didn't sound like they were missing. Hoping that was maybe just a fluke or maybe he let it get hot and it started misfiring other cylinders.

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Cracked Jeep 4.0 head

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