'96 XJ 4.0 here, and I'm seeing something rather ominous. Although it's getting into the low 90s and slight overheating is expected this time of year, my dash temp gauge is getting up close to the red zone lately in the heat of the day (which it never has before), and I'm getting bubbles girgling into my coolant reservoir for about 10 mins after shutting the engine off. Sounds like boiling water. Now, just a few weeks back I had to remove some coolant from the top of the radiator (maybe about 100cc) at the cap as I disconnected the overflow line when I was changing out the alternator. It seemed to start overheating after that, so I overfilled the coolant reservoir so it could suck in as much as it needed. Since then, it's normalized the level of coolant in the reservoir by overflowing the excess after it got hot, but it's still bubbling after shutoff.

Is this possibly just air in the coolant system somehow, and I might be able to fix it? Or is this a leaking head gasket for sure?

Almost forgot to mention, but a symptom that started along with this overheating is my idle speed in park gets up to and stays at 900 after it gets hot. Not sure if this is a symptom of a leaking head gasket or not.

Thanks.
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Sounds like you’ve got a blown head gasket, by the way where I live the daytime temperature for the last month has been 94 better with it feels like temperature of 100 to 109 and mine does not overheat, so I wouldn’t expect overheating to be something that would happen just because the temperature got hot

 

Daytime temps shouldn't be a factor in your cooling system working correctly (if everything is in good shape).


Does the '96 4.0 have the pressurized overflow tank?

 

Bubbling into the reservoir just means the coolant is getting above the boiling point when the engine is shut off due to residual heat in the engine block heating the non-circulating water. It doesn't require a blown HG. You shouldn't be letting the engine get that any warmer than 235F. Heat cycles close to the red zone promote big issues.

A few things you can check:

• Make sure your radiator cap is working properly. I'd try a new radiator cap, specifically, I'd try a 16 psi cap that was specified for later model XJs, including yours. PNs are: MotoRad T16V, Gates 31523, and Duralast 7616.
• Replace your tstat (and housing).
• Check and replace the fan clutch if needed.
• Check and replace the efan if needed. Use a late model 11-blade efan with curved blades. These flow much more air than the 5-blade fans.
• Understand whether your heater core is leaking. If your heater core is leaking, the cooling system can't build pressure (to raise the boiling point), and it allows vapor bubbles to form in the head which is makes for very bad cooling of the head. If its leaking, bypass it (you'll have no heat, but its summer).

You should address this as soon as possible. I'd start with the items I mentioned, and then go as far as replacing the radiator (Spectra Premium) and water pump. I'd do the water pump and radiator at the same time. If none of those fix it, then its onto the head, but you'd want to replace all the cooling stuff anyhow because its important to avoid the issue after replacing the HG.