AJV8 4.4L, 2005 LR3 - New to the LR3, trying to understand an intermittent overheating, where within first 5 minutes of driving from cold, the temperature gauge shot up to just under the red line, and then suddenly dropped back to normal halfway.

What could cause a brief spike in temperature?

Thermostat open - closed - open again?

Water pump slipping for a while?

And why would the two coolant hoses that run to the heater stay cool when all other hoses get hot? (heater doesn't work)

 

Sounds like air in the system. Air can cause hot spots that can easily cause damage since there is not coolant all around the system. And since there may be air it essentially starts to boil and surge which is why the needle may move around. I would look at bleeding the system first.

 

It's good advice, thanks. I did bleed after the spike, with bleed valve at highest point, so air could escape - but, the spike in temp actually happened without having touched the cooling system. It was working fine, no leaks, no messing with the coolant, no nothing - temp gauge nailed to the halfway mark no matter what.

And then this spike out of nowhere, and then followed by normal again...

It wasn't under heavy load either. Just cruising in traffic.

What's baffling me is why it would go back to normal afterwards.

 

When you bled it, did air come out? And did you bleed it from multiple locations?

 

I've only got one bleed location, as the one on engine top was replaced with straight connector by PO - so I've got the expansion tank valve, and I parked up a hill at an angle so it would be highest point for bleeding.

But this was after the spike, when I took the opportunity to change the expansion tank, as the old one was looking like it had seen better days.

 

No matter how you park, the tank will never be the highest point. You need to open that fitting. And if this was a new, random event after no working was being done you likely have a small leak some place, which is how the air also got in. That is if in fact this is an air in system issue like I suspect.

 

OK, I'll bleed to make sure, thanks.

Any other possibilities come to mind, if not an air lock?

 

Could be a failing water pump, blown head gasket, etc.

 

I was thinking water pump, but I can't really measure it, other than - yup, it's circulating.

And I wouldn't want to summon the HG ghosts...

 

Think I got it - under testing I heard the water pump make nasty clicking noises. So, it seems it's on its way out and I'll see if replacing it solves it.

I removed the thermostat to get the coolant circulating at maximum flow, and that's when the tick-tick-tick sound really picked up.