S80 Volvo Heater Core O-Rings
The above picture shows the clip insterted the opposite way. I personally do that on all of them as it seems a bit easier. The clip will work either way as long as it is seated against the core. Now just re-assemble and clean up residual coolant spillage.
If you have coolant leaking on the drivers side floor of your S-80 and you think the heater core is at fault, most of the time it's just the o-rings on the end of the pipes going into the heater core that are at fault. This will shine a little light on the process of replacing those o-rings.
Start by removing your lower dash trim 2-25torx screws and the lower console trim. This picture shows the fastner on the console trim, rotate it so it's straight up and down, pull it away from the console (at the fastner is) then slide the trim towards the rear of the car releasing it from the console.
With that the trim is out of the way you can see the heater core and the aluminum piping feeding it.
The above picture shows the metal levers that activate the blend door flap. You will need to turn the key on and turn your heat selector to full hot in order to get those levers out of the way. The below picture shows the full hot heater setting.
Now that you have clearance your next step is to remove the aluminum clips holding the pipes in the heater core. I personally use a pick tool to remove these(as shown below)Yes they are tight and can be difficult to remove, however a bit of patience and finesse will get them out.
They may bend a bit while removing but thats ok,you can straighten them out for reassembly.
Remove the bottom clip first. Then place an old metal coffee can with shop rags placed around it, pull your lower tube out of the heater core (but don't bend the tube) and let the excess coolant run out of the core into the can. Take that tube and slide it towards the console (once again without bending it ).
Now remove your upper tube the same way but I slide that tube towards the firewall. You will see the o-rings on the end of both tube's. Replace those o-rings with O.E. or equivilant.
I spray down the o-rings with silicone spray to make them slippery and for ease of re-assembly. Now slide your tubes back into place. The above picture shows me using a very DULL flatbladed screwdriver to push the tube back into the core. You will feel a bit of a snap once the tube is seated into the core. If it's not seated the clip will not go in. However you don't want to force it and tear the o-ring's or damage the tube's or core then you'll be back to square one or worse. You can tell if the tubes in all the way by trying to slide your retaining clip back in, if it comes across the tube flange then your in. Do the upper tube first then the lower. This process does require a good bit of finesse but like stated before, "Just be patient and don't force it"