Toyota Yaris Coolant Loss.

oldagetraveller

Toyota Yaris 1.4 Diesel, 2010. 7000 miles. 13 months old.
Noticed drop in level to well below minimum in coolant expansion bottle at around 11 months. It still showed a level so was never dangerously low. No overheating. Possibly it had been dropping for some time but when previously checking underbonnet didn't really pay full attention to actual level, only that there was coolant.:o Topped up with pre-mixed Toyota red coolant to "full" when cold. Took to supplying main dealer for 1st service and mentioned the loss. They could find nothing wrong. About 2 weeks after service noticed coolant had dropped again to "low" (cold). Took to dealers who topped it up and pressure tested the system (they said) nothing, apparently the pressure remained at 20 psi for 30 minutes.
Now it's below "full again (cold). I've never ever, in the numerous "sealed" cooling system vehicles owned, had to top up the level. It's a sealed system so evaporation should be virtually nil. So the coolant's going somewhere, albeit very slowly. It's only a small expansion bottle and the top up required is not a lot each time.
No signs of coolant anywhere under the bonnet, nor any visible drips from anywhere. I do have experience of pressure testing suspected leaking components, e.g. cylinder covers etc., which do not leak on test but do leak under operating temperatures and pressures. So a static pressure test can give a false indication that all is well.
Any ideas, or has anyone experienced this with their Yaris?

vax2002

The bit they did not test....The cap !
They have a thermal pressure valve in that can leak steam if they fail, steam = water
try a new cap.

RegWorts

As a preventitive , many cars have a bottle of BARS LEAKS in their system.

Available from Ebay , Halfords etc

oldagetraveller

vax2002 wrote: »

The bit they did not test....The cap !
They have a thermal pressure valve in that can leak steam if they fail, steam = water
try a new cap.

There is a covered pipe boss, just below the cap which I assumed was the test connection, so possibly the filler cap would have been in place when tested? Unless it's not for testing purposes and they use a special connector in place of the filler cap? Not dismissing your suggestion at all but I would expect signs of red coolant on or around the cap.

mikey72

A lot of my cars have run at low for years.
Whenever I fill them up, it drops again, but never actually goes below low, so I leave it there.
Just the tolerence of the pressure cap, the amount of water in the system, the thermostat, and how much it'll expand when it gets hot.

If it keeps going down, or goes below the minimum mark, I'd worry.

oldagetraveller

It was initially below the minimum mark mikey "Noticed drop in level to well below minimum in coolant expansion bottle at around 11 months".
I intend to see what it does for a while and if there is a further drop to below the minimum then it will be back to the dealer again. I've made allowances for variations in ambient temperature too as it's now a bit colder than it was a month ago. When they topped it up the engine will still have been warm because of the 12 mile drive there and them doing the checks/top up soon after I arrived. I've now topped up to full mark with the engine stone cold, first thing in the morning before any running and will check the level at a similar time and condition.

mikey72

There could be a bit of air in it somewhere, like the top of the heater matrix etc.
When you top up at cold, the coolant expands the first time you heat it, and pushes the excess out until it has a comfortable air gap in the expansion tank it can fit into. If there is air trapped elsewhere, the air will expand, and push more coolant out than normal.
Then the coolant will fall below the normal when it contracts as it cools again.

If you're brave, don't keep topping it up, and keep your eye on it and see what level it finds. (unless it starts to empty too low)

poppasmurf_bewdley

Run the engine at idle for about 15-20 mins with the radiator/expansion tank cap off and the heater on full heat with blower on. This should eliminate any air lock in the system. Then check the water level, fill to Max level if necessary and refit cap, and check every day for a week or so to see if it varies. If it never falls below Min, then it should be ok.

gilbert_and_sullivan

Have you lifted the carpets in the footwell and had a poke nose under the dash with a torch, just in case there is a small leak from the heater matrix, the matrix could be encased in a semi sealed plastic box so might not actually be enough to have leaked through yet, but some staining could be evident.

I presume Toyota have done a hydrocarbon sniff test on the coolant just in case there is a head gasket or other seepage, if you've topped it up several times there should have been enough pressure leakage back into the coolant to register by now.

Alexis27

There's probably a coolant temperature sensor somewhere in the system which slots into the pipe. If it's loose or the rubber seal has perished, coolant can slowly leak out.