Radiator boils over, but engine doesn't overheat

DAA

Well-Known Member
Short version... Is there any such thing as an intermittent head gasket leak?

Trying to diagnose an issue with my '06 LJ, Jeep 4.0. Could use some thoughts...

Twice, over about a month this summer, the radiator let loose with a sudden blast of coolant out of the cap. Once about a minute after turning it off and the other time while driving. And the engine immediately overheated both times. But was not overheating, before the cap vent. My guess, the sudden depressurization caused the sudden overheat. But, I actually don't have a clue.

Both times, stopped, took the cap off, let it cool down. Started it up and the problem was gone.

So I replaced the cap. The old one was a 20 lb parts store cap. I've had cruddy luck with parts store parts the last couple of years. So I ordered a Mopar cap, which is the correct 18 lb. Drove close to 2,000 miles without a hiccup and thought life was good.

This weekend, after almost three days of driving, over 600 miles total, about half offroad. The cap started to steam and "spritz" coolant. Not a violent eruption like with the previous 20 lb cap. Mostly just steam, and just enough coolant to make my windshield a mess (my hood has a vent just above the fan). But, the engine wasn't over heating. Temp was running like maybe 4 degrees warmer than usual, but holding steady there.

Stopped, took the cap off, let it cool down. Started it up and the problem was gone.

Next day, it started up again, worse on the steam and coolant spritz out of the cap. But the engine still wasn't overheating. But, while stopped and letting it cool, it was gurgling bubbles out of the radiator into the overflow bottle.

Stopped, let it cool down. Topped off the rad with water, drove 350 miles home on Sunday without a hiccup. Been driving it since I got home, hasn't happened again, yet.

So, I'm thinking, blown head gasket, cracked head, or cracked block. Exhaust gas getting into the coolant and over pressurizing the cooling system. The parts store 20 lb cap, held it until it let go and just blew a single big burp of coolant, depressurizing the system and causing an overheat. The new 18 lb cap, is doing it's job and venting steam to keep the system not over 18 lbs and so no overheat. All pretty much wild ass guess.

Today, did a pressure leak down test of the radiator. Held steady. No leak down. Used a "block tester" to check for exhaust gas coming out of the coolant. Zero, nothing, nadda. Both test results would indicate, NOT a head gastket.

But! This issue is definitely intermittent. I'd like to see how those tests would go while it's gurgling bubbles from the radiator into the over flow bottle. I suspect, they'd be different.

So - is there such a thing as an intermittent head gasket leak? Have any of you ever encountered that? I'd think a cracked head or block, would not be intermittent. But could a leaking head gasket be intermittent?

My next step, I guess, is pull plugs. They are new, went in just before this trip, have only about a thousand miles on them. I inspected the old ones and saw nothing unusual. While the plugs are out, I'll bore scope to look for indications of washing on the pistons. I'll probably pull the valve cover and check the head bolt torque.

I can do compression and leak down tests, but with the intermittent nature of the issue, I'm not sure I want to bother when the issue isn't manifesting itself. I'm going to keep the block tester in the Jeep and if it happens again, try to test for exhaust gas in the coolant system while it's gurgling into the over flow.

Any other ideas any of you have? For what it might be, or how to continue to diagnose?

Doing the head gasket isn't a huge, huge deal, but it's more than I really want to eff with just on a hope and a prayer. Especially since, if I don't see an obvious problem with the gasket, I'll be having the head magnafluxed and checked for warp/resurfaced just on GP, before bolting it back on.

- DAA
 

I Lean

Mbryson's hairdresser
Vendor
Location
Utah
It sounds to me like you have all your bases covered.

One question, do you have an accurate temp measurement to go off of? Beyond the stock gauge I mean. Something like a Scangauge or whatever?
 

RockChucker

Well-Known Member
Location
Highland
I don’t know what years were afflicted. But I know some 4.0 heads cracked between 2 and 3 or 3 and 4 cylinders. My brother had that issue. He had coolant in his oil as evidenced by milky color. He didn’t have over heating issues as I recall though. Have you done an oil change?
 

Kiel

Formerly WJ ZUK
Had an xj 4.0 that did that mainly under load, parleys, other big passes etc. It was the head gasket. Cylinder 5 to 6 on a 98
 
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UNSTUCK

But stuck more often.
A while back I posted a picture in my build thread of coolant sprayed all over my windshield. I figured the radiator had blown. The dumb thing didn't blow the seams out like I thought. I topped it off and hasn't done it since then. Must have been the cap for some reason. My temps always stay at 210 according to the stock gauge. I haven't messed with any of it, other than to check the level a couple times.

On my old Ecodiesel, I (and most owners) would lose coolant out of the degas overflow tube. Lots of theories, but no one really knew why. I went as far as putting a pressure gauge in the engine to degas bottle hose, then ran the gauge into the cab so I could watch it. The gauge would slowly rise to around 19 psi, which was cap pressure and then hold there. Putting around town it never moved.
I put my boat on the truck and hit the hills. On every downshift with the pedal on the floor I could watch that gauge go up to 23-24 psi instantly. When I let off it would go back down. It really acted more like a boost gauge then a cooling system pressure gauge. I tested the cap and it would start to open at 18.5 psi. If I pumped the tester really hard and fast I could get the tester gauge to show 19, but couldn't get it to go past that.
My thinking was I either had a very small head gasket leak that only showed itself under load and boost, or the head(s) was lifting. The dealer, of course, could not duplicate the issue, so there was no issue. Even though I showed them video of the gauge hitting 24 pounds and then dropping off when I got off the throttle.

That was the long way of saying I have no idea what your jeep is doing.
 

DAA

Well-Known Member
It sounds to me like you have all your bases covered.

One question, do you have an accurate temp measurement to go off of? Beyond the stock gauge I mean. Something like a Scangauge or whatever?

Just going by the factory gauge at this point.


I don’t know what years were afflicted. But I know some 4.0 heads cracked between 2 and 3 or 3 and 4 cylinders. My brother had that issue. He had coolant in his oil as evidenced by milky color. He didn’t have over heating issues as I recall though. Have you done an oil change?

I can't remember the years or the casting number off the top of my head either. But mine is the later "tupy" supposed to be good one. Doesn't mean it can't crack too, but it's not the one known for it. I haven't drained the oil yet, but dipstick and filler cap look good. No sign of milkshake.

- DAA
 

Kiel

Formerly WJ ZUK
Yeah, It was fine 98 percent of the time, freeway, driving to work etc, fine. Trying to accelerate uphill for a couple minutes like the section on parley's it would boil and pop
 
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jeeper

Currently without Jeep
Location
So Jo, Ut
I had a clogged and dirty radiator that caused similar issues.. but It was consistent (towing trailer up hill, long mountain climb, etc.
 
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SoopaHick

Certified Weld Judger
Moderator
Just going by the factory gauge at this point.




I can't remember the years or the casting number off the top of my head either. But mine is the later "tupy" supposed to be good one. Doesn't mean it can't crack too, but it's not the one known for it. I haven't drained the oil yet, but dipstick and filler cap look good. No sign of milkshake.

- DAA
IIRC the dumb head was like a 343 or at least started with a 3. The good one starts with a 6.
 
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SoopaHick

Certified Weld Judger
Moderator
I've had this happen a couple times as well and the only diagnosis I could ever figure out is that I slightly overfilled the reservoir. After the slight overfill the next time I worked the engine hard it would overpressure and blow the cap.
 
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