Coolant expansion tank keeps popping off the cap

[ACas23]

Hi all, I'm looking for a little help if anyone has run into this problem before.

I have a 2008 Ford Fusion which is our second car and used as an occasional run around. It's has its quirks but for the most part runs relatively sound.

On a trip out a few weeks back I spotted a gush of coolant coming out from between the bonnet and front wing and quickly pulled the car over. The engine wasn't showing as overheating on the dash however it looked like the expansion tank cap was loose and a lot of coolant had boiled off.

I ended up dropping in a litre of water just to get me home (only a few minutes drive) and once home booked it into a local garage. They gave it a look over, had it up on the lift and ran it for a while but couldn't seem to find any issue or replicate the problem and so it came back to me.

I did a little googling to see if I could diagnose the issue and could only find that maybe it was a faulty tank cap which I promptly replaced.

It was then a week or so before running it again and as before, the same thing has happened with me getting home to find a loose cap and having the coolant boil off again.

Same as before, no warning on the dash for overheating and it seems to have come up to operational temp and run happily for the 30 mins drive.

Would anyone have any insight into the problem? Could it be a blockage in the coolant lined which is over pressurising the tank and popping the cap? Could it be the threads on the tank itself that have gone and the tank needs replacing?

Any help would be greatly appreciated and thanks in advance.

Adam.

[unofix]

It sounds like you have one of two problems. The easy one is you have a failing expansion tank, very common on older vehicles. The plastic deforms with heat, often leaks and I suppose that in your case the expansion tank cap could become loose. You should be able to find a new replacement tank on eBay for around £30.

The second more worrying problem could be the head gasket is on it's way out. This is causing excess pressure in the cooling system and is partly blowing the cap off the tank.  

[ACas23]

Thanks unofix,

That sounds like I have a couple of things to consider and I guess replacing the expansion tank should fix things in the short term and highlight the second issue you mention if it happens again.

Ill have a shop around for a new tank and take a peak at the old Haynes manual to have a go at replacing it myself.

Much appreciated!

[unofix]

The tanks are very easy to replace, just 2 or 3 small hoses and a few fixing screws. It will take less than an hour even if you have little idea of how it comes apart.

[ACas23]

Thanks for the info, I've just had a quick look and it seems easy enough so I'll give it a go as soon as I can get a new tank delivered. Managed to pick one up from amazon for £29 with delivery in just a couple of days so fingers crossed this is all that's required.

 

[unofix]

Just be sure you get the right tank. As far as I know there are 2 different ones used (might be more) on the Fusion.

[ACas23]

Expansion tank changed and all seems to be working fine again now. Have been running it for a few days without issue.

On reflection of testing the old tank after removal, the cap could be over tightened and the cap would run back round to being loose so the tank certainly was the problem, the threads on the neck of the tank were jiggered!

Thanks again for the advice unofix, your help was much appreciated.