My 2004 V70 has a very intermittent coolant leak. Every once in a while the coolant in the expansion tank vanishes. Every once in a while I can smell coolant but this doesn't seem to have any connection the the times when the expansion tank empties. When this happens what fits in the expansion tank is all the antifreeze it will take. This doesn't seem to have a connection to how long I've been driving, its happened and not happened with short drives and road trips. Sometimes the car will go months without losing any coolant. I was rear ended 2 summers ago and this started after that but not immediately, I don't know if there's a connection there or not. I replaced the expansion tank cap, that didn't help.

I plan to give it a good cleaning under the hood and then pressure test it. Looking for ideas to narrow this down. Usually coolant leaks have been pretty obvious

 

Usual suspects for coolant leaks

1) overflow reservoir, pressure cap
2) hoses to/from reservoir
3) upper and lower radiator hoses
4) radiator (look for effluence along side seams and any dents etc to the fins)
5) heater hoses, o-rings to the heater core, heater core (will often give a "sweet" smell in the cabin on a cold start, may leave some dampness to the carpeting in the front footwells)

Worst case scenarios
a) head gasket issues or a cracked head/block (moves onto the list if you've ever had an overheating event)
b) radiator/transmission cooler internal leak
Both of these may mix and discolor fluids so a quick pull of the tranny and engine dipstick, and inspect/test the coolant

 

Since it is just the coolant in the expansion tank leaking. I would pop it out and inspect for cracks where it can be leaking. Check the bottom of the tank too. No evidence of any coolant anywhere in the engine bay? Did you also check the coolant for hydrocarbons to see if you are burning it? Although if you were burning it, you would see more regular coolant loss past the tank.

 

The irregularity of this is what makes it challenging to figure out. There just doesn't seem to be a pattern. Returned a week ago from a 4000 mile, 3 week trip, added maybe 1/2-3/4 of a cup of coolant part way through the journey - not enough to really have bothered about. Went up over mountain passes and well into the Arizona desert region, engine temp stayed good once I turned off the AC up past 10,000 ft elevation. Oil is ready to be changed but there is nothing concerning about its look or feel. The car ran like a champ, not the slightest cause for worry.
Going to drain and flush the coolant system and check the hoses and connectors and see how it does after. Wondering if the sensor could be getting wonky? Haven't noted any dark exhaust and it hasn't thrown any codes, but could it be getting ready to go on strike?

 

are you saying it had started to overheat until you turned the AC off?

 

Taking a vehicle up a very steep, high road can cause the engine temp to rise. The temp did not get into the red zone but the car had reduced speed and performance on the approach to a mountain pass that topped out at over 11,000 ft with 5.5 - 7% grades. Turning off the AC made a significant difference. This is recommended by the Colorado Dept of Transportation and many newer cars computers will automatically cut the AC when the car in on a steep incline.

I included this information just because this situation did not affect the coolant level.
I have ordered a new expansion tank, coolant sensor and radiator cleaner/flush and will be checking the hoses and connectors. There is an occasional, brief scent of antifreeze but it isn't consistent with anything I've noticed such as a cold start and doesn't seem to relate to times when the coolant level dropped. Haven't had any carpet dampness noted - that would cause a smell inside the car. It's just weird!

 

you're going to want to check the plastic end tanks on either side of the rad, they're prone to leakage at the seams once they get up to temp and pressure.

the expansion tank is the fill point and also the location of the sensor, just because that's where you're noticing it missing ( thats the only place TO notice it missing, for you or the sensor) doesn't mean that is the location of the leak. i would think you'd see evidence of a leak from the expansion tank pretty easily, in the form of residual coolant after the water has evaporated.

there's also the pump and the pump seal, they can start to go as early as 80,000mi, if it's never been replaced that'd be another place to check, if you replace that might as well do the timing belt and tensioner while you're in there, they both need replaced around the same mileage.

i've never noticed any reduction at altitude... Got to love that turbo.

 

Parts should arrive today. Did tensioner and timing belt about 40k miles ago. Didn't do the pump then, it had been replaced the previous time and my mechanic partner thought it was fine. That may be worth checking.
This model doesn't have turbo. The XC70 I got cheap a while back had a laundry list of issues and the turbo is on it. Looking forward to getting that one all in order! The 200s were a lot easier to make run happy.