Another overheating Jeep!

[hear]

If you've been bored enough to read any of my recent threads, you'll know that we recently put a "new" 4.0 in a friends jeep. It's been running great, more or less, but every time we bring it back to the owner they call us a day or two later telling us it is overheating. We've had it running in the driveway for long periods of time and it runs steady at 205F (via ODBII reader). Yesterday the owner was out with it and called me because he was nearby and it was overheating. And it sure was. The coolant in the overflow tank was literally boiling. Like a rolling boil in the tank. Reader had the temp at 255 when I checked it, laser thermometer on the block had it right around that same number.

What we've already done, mainly as part of the motor swap:
* new radiator (the old one had a hole). It's not a Mopar, but those are hard to find.
* new cap
* pressure tested....first time ever having done that but it did flush out a problem with the lower hose clamp which we have since addressed.
* thermocure flushed several times
* new water pump & thermostat
* new head gasket
* burped it initially, but have not tried to burp it again
* fan shroud is in place, fan is spinning
* Coolant doesn't look great, but no evidence of oil present / oil doesn't look to have any coolant in it, although an oil change is high on my list of things to do.

The only thing I can think of that we haven't already done is to change the fan clutch. The overheat seems to be intermittent, and it was crazy hot here yesterday which was probably a contributing factor, but my TJ didn't overheat in the same conditions. What am I missing about troubleshooting this overheating issue? What should I be looking at next?

 

[KennethS]

Do a combustion gas test to rule out a head gasket leak or cracked head.

 

[TheBoogieman]

* new cap

Which one?

* Coolant doesn't look great

What's wrong with it?

 

[Link]

I’d vacuum fill it with fresh coolant. I’d also be looking at the fan clutch.

 

[hear]

Which one?

What's wrong with it?

The cap was an orielly house brand which ended up being a Motorad (which I believe bought Standt?). Was either 16lb or 18lb...can't remember. But the pressure should only serve to raise the boiling point, and it seems to be doing that job fine, lol.

What's wrong with the coolant? IDK, it's dirty. Not oily, just more of a greenish-brown than the day glow green that comes out of the can brand new.

I’d vacuum fill it with fresh coolant.

I'm not familiar with this operation?

 

[Link]

Here's a couple versions of the same tool. I have the Harbor Freight version. You use compressed air to pull a vacuum on the system. Once vacuumed down, you turn off the pressure and then open the other valve. Since there is a vacuum in the system, it pulls the coolant right into the system. Extremely simple to use.


https://www.harborfreight.com/cooling-system-test-and-refill-kit-64985.html

https://www.amazon.com/dp/B01BW39HJS/?tag=wranglerorg-20

 

[JMT]

Do you have air in the system?

We had an intermittent overheating problem in my sons.

We had replaced the thermostat with a Stant. After a year of intermittent overheating we ended up replacing it again with a Flowkooler. Been fine ever since.

 

[hear]

Overheated again yesterday afternoon. Well...temperature had been right at 210 and then about a mile before my house it shot up to almost 230. Didn't let it get any higher than that. We hung out for a while, then drove it home and had pretty much the same thing happen.

So late last night we took out the thermostat (just for the time being, just to eliminate a possible cause) and flushed the thing. No thermocure this time, but that has been done several times as part of the motor swap. Then we let it run for an hour, 45 minutes of which were with the cap off on a somewhat inclined driveway to help get the air out. Initially got the air out, but I'm hoping somebody can help me understand some other things I see:

As the engine runs, the level of the water as seen in the radiator neck will rise. We ran cap off for the better part of an hour, and it continued to do this. It makes the combustion gas test hard because it said to run for 2 minutes but in that time the water level will rise back up and potentially contaminate the fluid used in the test. But WHY does it keep rising? The only reason I can come up with is that air bubbles somewhere are heating up and expanding which pushes the water level up. When we put the cap on, the overflow tank eventually filled up to the fill line (coincidence?). I was surprised it decided to purge coolant, but maybe I don't understand how to fill a radiator.

We did also see plenty of little bubbles...almost foam? We did manage to run the combustion gas test and it came back clean (again), but still questioning if we're doing it right. It has a brand new head gasket so failing this test would make me sad.

Anyway, the point is that it sure seems to run fine at idle. Or maybe it just does fine if the temps are low....was ~100F the times it has overheated, but was only 86F last night.

 

[KennethS]

My experience doing the combustion gas test was similar, with the coolant rising and contaminating the test chamber. I then drained enough coolant from the radiator to be able to perform the test without coolant in the way.

Dunno about your overheating vs ambient temps. Yesterday it was 119 here and mine ran just a tick below the 210 mark all the way home in rush hour traffic with the AC on.

 

[Link]

Bubbles make me suspect the head gasket.

 

[hear]

Bubbles make me suspect the head gasket.

Same, but the combustion gas test seems to have passed. We did just replace the head gasket as part of the swap, so that's slightly less likely but anything is possible.

 

[Link]

Same, but the combustion gas test seems to have passed. We did just replace the head gasket as part of the swap, so that's slightly less likely but anything is possible.

There is only one thing capable of forcing air into the pressurized cooling system and that's compression. I'd be replacing the head gasket just to rule that out. I think it's very likely that's your issue.

 

[sab]

As the engine runs, the level of the water as seen in the radiator neck will rise. We ran cap off for the better part of an hour, and it continued to do this...But WHY does it keep rising? The only reason I can come up with is that air bubbles somewhere are heating up and expanding which pushes the water level up.

Keep in mind that coolant density varies with temperature. I have a spreadsheet from my days in vehicle engineering that takes this into account. I just looked at that to see how the TJ coolant volume changes with temperature. Assuming you filled it at 68°F to capacity (2.63 gallons) with a 50/50 mix of water and anti-freeze, that coolant volume would increase to 2.78 gallons at 212°F. That difference is about 20oz (liquid):

These calculations are all based on atmospheric pressure, which is identical to your conditions when running with the cap off. Now, that expansion should stop once the engine reaches a steady temperature, but maybe you didn't reach that in the time you ran it with the cap off, so it appeared to just keep rising. If you do that again, watch the engine coolant temp. Also, that 19oz should be close to the amount in the overflow reservoir between "cold" and "hot" lines typically on a reservoir (I can't remember specifically how the TJ reservoir is marked).

There is only one thing capable of forcing air into the pressurized cooling system and that's compression. I'd be replacing the head gasket just to rule that out. I think it's very likely that's your issue.

This is true. However, the presence of bubbles in the radiator does not mean air is being forced into the cooling system. Cooling systems always have some trapped air in them. It's nearly impossible to initially bleed all the air out, and even if you did, once the engine went through a cool-down cycle, it would likely have air re-introduced as the coolant cools and the pressure and volume decreases, pulling coolant and likely some air from the reservoir. Entrapped air will form bubbles as it impacts the water pump impeller and as it moves through the complex passages in the block, radiator, etc. If there are any surfactants in the system (anti-freeze contains a surfactant, and flushing solutions probably do, too), they reduce the surface tension, which results in smaller and more numerous bubbles. Think of a balloon, and the surface tension is the strength of the balloon. Lower the strength, and the balloon pops. With bubbles, that means the big bubble becomes several smaller ones.

I don't know what's causing your overheating, but I would not conclude that the bubbles or the expansion are indicative of any specific issue. I'm afraid you need more investigation to draw a conclusion.

 

[KennethS]

When I used the coolant test kit, I verified it worked by breathing onto it and placing it near the vehicle's tail pipe with the engine running. Both caused the fluid to change color. I very much doubt a leaking head gasket would not cause the test to fail.

 

[freedom_in_4low]

Keep in mind that coolant density varies with temperature. I have a spreadsheet from my days in vehicle engineering that takes this into account. I just looked at that to see how the TJ coolant volume changes with temperature. Assuming you filled it at 68°F to capacity (2.63 gallons) with a 50/50 mix of water and anti-freeze, that coolant volume would increase to 2.78 gallons at 212°F. That difference is about 20oz (liquid):

View attachment 444661

These calculations are all based on atmospheric pressure, which is identical to your conditions when running with the cap off. Now, that expansion should stop once the engine reaches a steady temperature, but maybe you didn't reach that in the time you ran it with the cap off, so it appeared to just keep rising. If you do that again, watch the engine coolant temp. Also, that 19oz should be close to the amount in the overflow reservoir between "cold" and "hot" lines typically on a reservoir (I can't remember specifically how the TJ reservoir is marked).

I just measured mine the other day. It was about a quart from the bottom to the fill line, and another quart from fill to full.

This is true. However, the presence of bubbles in the radiator does not mean air is being forced into the cooling system. Cooling systems always have some trapped air in them. It's nearly impossible to initially bleed all the air out, and even if you did, once the engine went through a cool-down cycle, it would likely have air re-introduced as the coolant cools and the pressure and volume decreases, pulling coolant and likely some air from the reservoir. Entrapped air will form bubbles as it impacts the water pump impeller and as it moves through the complex passages in the block, radiator, etc. If there are any surfactants in the system (anti-freeze contains a surfactant, and flushing solutions probably do, too), they reduce the surface tension, which results in smaller and more numerous bubbles. Think of a balloon, and the surface tension is the strength of the balloon. Lower the strength, and the balloon pops. With bubbles, that means the big bubble becomes several smaller ones.

I don't know what's causing your overheating, but I would not conclude that the bubbles or the expansion are indicative of any specific issue. I'm afraid you need more investigation to draw a conclusion.

I posted a photo in my cooling system cleanliness thread showing similar bubbles. I believe you're 100% correct on it being a result of trapped air and surfactants. My bubbles were starting to diminish after several flushes with tap and then distilled water. The heater core hoses trap lots of air since they're highest in the middle...I'm sure it takes a bit to get all that out, and probably worse without a thermostat installed. I was using a hose clamp (not the kind that retains a hose on a fitting, the kind that pinches it closed) on the upper radiator hose for 30 seconds or so at a time to encourage flow through the core and to push some out.

 

[JKP]

What year is the "new" engine? Does it have the TUPY head?