Hi Guys, I have a rather peculiar issue, I have not seen this similar issue anywhere on the boards so hoping someone can help me out.
Car is a DB9 2007 sports pack.

I seem to be having my coolant overflowing out very often at random times. It always overflows out from the expansion tank.
It will overflow out sometimes when I drive it hard and other times on just normal drives, sometimes it doesn't overflow out at all even if driven hard or normally.
though it does overflow 7/10 times I drive the car any way whatsoever.

Also it always happens within 1-2 minutes of turning off the engine, however when I see the coolant leaking on the floor and I quickly turn the engine back on, the overflow immediately stops.
It is as though the engine imemdiately suck back the coolant from the expansion tank and circulates it, however if I turn off the engine again, it would flow out within a minute or so.
Only if I turn the engine on and let it idle for a few minutes then the coolant would not flow out anymore once turned off.

Once I had raised and removed the inner arch to check the expansion tank, the car was run for a while at about 2.5-3k rpm for about 5 minutes, switched off and noticed that the coolant overflowed and was boiling in the expansion tank.
This would seem that perhaps the coolant is getting too hot and the engine is overheating? However the temperature shows normal at all times.

Could this be a faulty thermostat? I will be changing this soon, and already changed a new cap, but am wondering if there could also be a blockage perhaps? Though the temperature gets up to normal quite fast and stays there.

 

I haven't experienced this, but my first step would be to replace the radiator cap...mostly because it's the easiest thing to do.
Have you seen this thread in the TVR section of PistonHeads? https://www.pistonheads.com/gassing/...&f=11&t=185578

 

Agree - Radiator cap first, then T-Stat

 

The cap has already been changed.

 

maybe not the correct cap or pressure setting.
Other thing the coolant system could be too full... the expansion tank is there to allow the coolant to expand... perhaps the coolant is at the 'hot fill' mark when it's cold

 

There is no hot/cold fill marks. Full is top cold. Excess goes into the inner fender tank when hot, then returns to expansion tank when cold. May have a cracked inner fender tank or a hose is off?

 

that's a good possibility...

 

Yup already changed the cap first, original Aston cap.

Secondly it seems to only always overflow out from the expansion tank when the car is turned off, except once, where it showed coolant low while driving.
the expansion tank is in good condition and the last time when it was raised, reving for a while (as mentioned) then engine switched off, I saw the expansion tank got full and overflowed.

I highly doubt it is the thermostat issue as if it is stuck closed, temps would shoot up, if stuck open there should be no isses in any case correct, I am in a hot country anyways.

Just in case though anyone experiences the same issue, perhaps you can try the below, I am uncertain if it works yet, but will report if it does NOT.
So far there seems to be no overflow even having driven in bumper to bumper traffic, driven hard and normal. Will take it on a windy stretch and longer journey in the next few days to ascertain.

What I did was:
I am thinking perhaps there is an air pocket stuck somewhere in the system so I decided to try one thing.
1) Got the mechanic to clamp on the hose to the expansion tank
2) Use a pressure/vacuum to suck out all the air from the system (use a pressure gauge as well). I had to drill a hole in my old cap and put in a nipple there as none of his caps in his kit could fit, it seems like the Aston caps are very different from other makes and he had over 20 different types of caps!
3) suck all the air out of the system - make sure to do step 1 above or there will be no vacuum in the system.
4) pre-mix the coolat in a pail and then let the negative pressure in the system slowly suck in the coolant in the pail.
5) there should be some coolant left, so just pour that in the radiator tank.

this should not cause any damage to the system, unless your hoses are really old, then please be careful.

hope this helps someone too.