Failed block test

Hello everyone, I took my jeep to the mechanic because it is overheating after a couple minutes when at idle. As soon as you start driving, the temp goes back to normal. They did a block test, and it failed.
I asked about the oil, and they found no signs of coolant in the oil or vice versa. They said the head gasket is bad and needs to be replaced though because it failed the test.
Is there anything else that can cause a failed block test? It just seems strange that there’s no other signs. The liberty runs and drives great (other than heating up at idle). Thank you for any advice.

The jeep has 145,000 miles

It was a chemical block test too if that matters

The head gasket has lots of different passages in it, depending where it blows it may not contaminate the oil. The other option is a cracked cylinder or liner allowing leakage to the water jacket.
Sounds like a small leak at the moment, it won't get better I'm afraid.

KJowner said:

The head gasket has lots of different passages in it, depending where it blows it may not contaminate the oil. The other option is a cracked cylinder or liner allowing leakage to the water jacket.
Sounds like a small leak at the moment, it won't get better I'm afraid.

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Darn. I’ll get another block test done just to rule out any error, and make an appointment with the shop. Most shops can’t get to it for a month so hopefully it’ll be good to drive till then

JPK34 said:

Darn. I’ll get another block test done just to rule out any error, and make an appointment with the shop. Most shops can’t get to it for a month so hopefully it’ll be good to drive till then

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Keep plenty of coolant in the car to top it up and try to avoid it boiling up, the 3.6 is known to drop valve seats if it gets too hot.

KJowner said:

Keep plenty of coolant in the car to top it up and try to avoid it boiling up, the 3.6 is known to drop valve seats if it gets too hot.

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Copy that. It hasn’t really dropped coolant levels at all. It also only overheats when at idle for a long time. Like you said, it might just be a really small leak. I was able to get an appointment for next weekend with a shop

If you notice the coolant level dropping, you can loosen the radiator cap to prevent the system from pressurizing. It'll still cool the engine but won't build up the pressure to push the coolant where it doesn't belong (ie. combustion chamber, oil passage, or all over your engine). Of course you would not want to make that a permanent fix, but it will get you around until you're in the position to properly fix it.

Could also be the water pump impeller. Slow at idle, litte / no flow Faster RPM, the pump goes faster and may move coolant - also, convection thru the radiator helps cool.

Not sure what a 'block test' is - There is a kit you can get with a colored fluid that is held to the coolant filler hole. If combustion gas is coming back thru the cooling system, the dye fluid changes color. that is a positive sign of a head gasket or warped head / block.

Yeah the fluid test is what the shop did, and it changed color. So combustion gas is getting in somewhere

JPK34 said:

Yeah the fluid test is what the shop did, and it changed color. So combustion gas is getting in somewhere

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Ok. So head gasket til proven otherwise.