MegaSack DRAW - This year's winner is user - rgwb
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Having to top up the coolant on my '08 Avensis 2ltr Diesel every couple of weeks, think it's time to sort it out.
Considering speculatively changing the reservoir cap. Is that a good move/likely to be the problem?
Perished hose somewhere? If you park it on clean ground can you see the leak? Other than that does the temp stay constant - might be worth a pressure test?
Being the owner of an old car with many coolant hoses and extra pumps I have a trick for this. I take the reservoir cap off and attach a compressor to the overflow pipe. I put my hand over the filler and pressurise to about 14psi. Any leaky hoses show themselves, I keep meaning to make a proper cap with a valve but I’ve never got round to it.
Take a lesson learned from me;
I had a similar leak but it was slow enough that topping up occasionally was no bother. Problem was I topped with clean water and only used proper coolant occasionally. corrosion of a steel link pipe eventually lead to a much bigger leak...
Our garage had two tricks they tried (after the usual blow on the radiator cap).
The first was to attach a pump instead of the expansion bottle screw cap, pump it up and look for leaks.
The second was some form of colour changing liquid, again in the expansion tank, that would detect any combustion gasses bubbling up through the coolant and indicate a gasket leak.
I have had 2 2.0 D4D Avensis. at 56 plate and a 07 plate (Still have one)I have had issues with the cooling on one and hope my experience may help a little.
All of the above makes sense, and is perfectly valid for detecting a leak. It is definitely worth ruling out a small leak first.
The coolant level in both of mine has always ran a little lower than the marks on the reservoir but is stable a little under the min level. When topped up. it would slowly drop down to that level and then stabilise. My first suggestion would be to monitor it without topping up and see if it stabilises. If it is truly loosing coolant the reservoir would empty completely if you let it. (don't let it)
If you cant find a leak but it does keep on dropping, you are on to further testing to discover the problem.
A replacement cap cost me about £20 from Toyota (2016) (but didn't sort my problem out) You can pull the overflow pipe up over the engine cover and watch it while the engine is running, and if coolant is blowing past the cap you will see coolant coming out of the pipe onto the covers as condensation. This then at least tells you how its getting out. It could be the cap. - or that the coolant is getting pressurised beyond usual expansion.
On mine a tell tale sign was that the cap was getting stiffer to tighten. This was because exhaust gasses were getting in, and gumming up the thread (became easier after cleaning the cap with a cloth.) - Because I had a blown head gasket. As stated previously a garage can confirm that with a kit you put on top of the expansion tank.
This was a relatively common fault on the early versions of the timing chain engine unfortunately. -And is a right ballache to change. But me and a friend did mine - but took a long time, and would be an expensive fix at the garage.- If I was doing it again (which I would avoid) Id be getting the engine out of the car, as certain bits would be a lot easier.
I hope yours is just like mine and settles a little under the mark,
Sorry for the long post!
Head Gasket would be my first thought - any mayonnaise under the oil filler cap?