2002 KJ Limited V6 - coolant expansion tank

PaulP38a

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Apologies in advance for the long story, just trying to explain what I've done.

TL-DR - I think my new-ish Coolant Expansion Tank is stuffed. Looking for validation and/or alternate ideas.

Have been tracing an odd thing with the 2002 KJ V6 for a while and haven't been game to drive a long distance. Coolant regularly partially fills the vented side of the tank.

My understanding from reading several posts here and youtube videos is that the 3.7 V6 (and the 4.7 V8) is notorious for air locks in the cooling system.

A few things I've done:
1. Coolant Expansion Tank and pressure cap were replaced a couple of year back. In the past few months I have replaced the cap again as I suspected it may be the problem. Haven't traced a genuine one so they are always aftermarket... yeah, maybe my problem here?
2. Water pump replaced 2 years ago. Has developed some gasket leaks so replaced it again a few weeks ago. No more leaks.
3. Thermostat and top/bottom hoses replaced recently. Thermostat bleed hole at top as instructed.
4. New electric cooling fan installed. Seems to cut in ok around 90C-ish (194F) and the observed temp quickly drops across top and bottom hoses as measured with a handheld 'laser temp meter'.
5. Flushed and reverse flushed the engine (thermostat removed) and radiator with tap water. Good flow in/out. No observable crap in the system.

The hex head on the orignal coolant air bleed screw is butchered and I haven't been able to budge it with a 'reversing tool' let alone find a replacement screw in Australia, so I inserted a new bleed cap into the top hose to the radiator (see pic). Seems to work ok but very quickly fills with coolant. I expected more air bubbles.

Coolant flows from the small top return hose to the expansion tank almost immediately when the engine is switched on, hot or cold. Is this normal?

Took it for a run today, maybe 30kms (20 miles). Car drove fine, temp didn't go past half on the gauge. Popped the bonnet (hood) at home and saw that the pressurised side of the expansion tank was about half full and so was the vented side(!!). My understanding is that as the system cools, vacuum on the pressurised side will suck coolant back from the unpressurised side. When I undid the cap there was very little pressure with the coolant temp around 80C (176F). Let the engine cool for around 30mins and drained the coolant from the unpressurised side and topped up the pressurised side to the cold fill mark.

Ran the engine while stationary to see what happens with the tank with the cap off and the unpressurised side empty . As the temp rises towards normal operating temp the level moves a little. I guess that is a combination of the return line constantly flowing coolant in to the tank and the thermostat opening around 90C (194F) and the thermo fan cutting in. When the level in the tank got to around 3/4 I put the cap back on and immediately noticed the unpressurised side of the tank starting to fill with coolant!

How is that possible? The level in the pressurised side was nowhere near the cap and line to the unpressurised side, and that was a lot of coolant in a short time to be vapour condensation.

What are the chances there is an internal crack in the tank between the pressurised and unpressurised sides at around the 3/4 mark?


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JeepJeepster

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Very nice!

To answer your question, yes, its normal for coolant to immediately flow out of that hose. While the bleed screw is needed on other configurations, its not as important on your Jeep b/c of that hose. It will immediately allow any trapped air to escape into the overflow tank. Its a nice design and I like it, Im not sure why they dont use it more. For some reason they did away with it on the 07's, probably to reduce costs of having to make the tank strong enough to withstand the pressure of the cooling system.
 

PaulP38a

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Very nice!

To answer your question, yes, its normal for coolant to immediately flow out of that hose. While the bleed screw is needed on other configurations, its not as important on your Jeep b/c of that hose. It will immediately allow any trapped air to escape into the overflow tank. Its a nice design and I like it, Im not sure why they dont use it more. For some reason they did away with it on the 07's, probably to reduce costs of having to make the tank strong enough to withstand the pressure of the cooling system.
That makes complete sense, thank you.
I kept seeing all these videos and comments saying how important the bleed screw is but it seemed to me that any air should get forced out that hose.
Having the in-line bleed cap is probably redundant now. Looks nice though
 

PaulP38a

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Out of curiosity I cut the old tank to see if I could find the source of the leak.
Pretty obvious in the pics the the middle join is uneven and not sealed properly.
This was an aftermarket tank so you get what you pay for.

Car is running great now with new water pump, thermostat, upper control arms, lower ball joints and outer tie rod ends.
New front tyres too.
 

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DadOSix

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Out of curiosity I cut the old tank to see if I could find the source of the leak.
Pretty obvious in the pics the the middle join is uneven and not sealed properly.
This was an aftermarket tank so you get what you pay for.

Car is running great now with new water pump, thermostat, upper control arms, lower ball joints and outer tie rod ends.
New front tyres too.
Great follow up!
 

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