I have a mk3 Mondeo TDCI Titanium Sport X diesel and I noticed it doesn’t have a fuel cap (obviously has a bodywork flap to cover when the diesel goes in), there’s like a seal that only opens when I insert the pump handle. On that basis it all seems to work ok as the seal won’t open any other way. 
 

Problem is the seal won’t open for any other reason. Because it’s a relatively higher miler (180,000) I want to put some fuel tank additive but obviously I can’t - again the seal only opens for the pump and when I do that there’s no room for me to get the additive in. 
 

Is there anything else I can do to keep the seal open so I can get the additive in? Would the nozzle of a spare fuel tank work?

A 2009 Mondeo is a MK4 which has the capless Ford Easy Fuel system.

The Easy Fuel system consist of a number of valves inside the filer neck that can only be opened with an object of the correct diameter (In this case a Diesel filler handle).

For emergency filling using a jerrycan or to add an additive you need to use the Easy Fuel funnel. There is a different (diameter) Easy Fuel funnel for Petrol and Diesel.

The Easy Fuel funnel is included with each Ford vehicle that has the Easy Fuel system. However on a car this age there is considerable chance that the Easy Fuel funnel is no longer in the car.

Plenty on e bay, just make sure you order the diesel one

https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2380057.m570.l2632&_nkw=ford+mondeo+fuel+filler&_sacat=6030

 

You can get different colours 😉

https://walkthestorm.co.uk/shop/brand/shewee/shewee-extreme/

As above, you need the funnel. Check in the boot under the carpet.

I use Millers Ecomax additive and the bottle comes with a built in tube to open the flap so no need for the funnel.

Cheers all, bought one today. 
 

Looks like a She-Wee 🤣 but it did the job.