I figured since this coupling is such a PITA overengineered piece of crap, and I couldn't find much detailed information about the coupler and methods of bypassing it, I thought I'd share my findings with the community in some more detail. I'm talking of course about this bugger:



As these things get old, the plastic turns brittle. As long as you don't touch it, it will probably hold up fine. While swapping my engine, the connectors HAD to come out, and in doing so you will usually ruin it. In my case the plastic was brittle and probably cracked internally when disconnecting the hoses. Upon reinstalling my new engine, I could take a shower under my car when I filled it with water for the first time. I even ordered the O-ring set thinking this was the problem, but this was money wasted.

Materials I used

- 1m of 15mm inner diameter heater hose (2m if you want to connect the hose directly to the block)
- Some vacuum hose to fill the gaps
- 4 hose clamps, wormwheel type
- Hacksaw for tight spaces or angle grinder
- Torx bits (T15 i think) and small ratchet
- Your standard toolbox

Procedure

1. You will need access to your heater-core, and also the coupler will need to be removed from the interior side. Pull back the floor carpet and remove the large black plastic guard fitted against the middle console. You'll find a small torx screw holding it in place behind the carpet at the height of the gear-shift-lever. For easier access to the coupler in the firewall I recommend removing the gas pedal by sliding the pivoting pin about 3mm to the left. It should pop right out.



2. After all the coolant has been removed (I recommend blowing through the heater-core from the engine-side so your interior doesn't turn into a pool), the metal hoses will need to be cut. I cut them close to the bottom, while leaving enough length for the heater hose to slide over (I would aim for 2cm). If you have an angle cutter or anything like that, you could use that. I did handwork using a tool similar to this one:



3. With the hoses cut, you can now remove the two torx screws holding the coupler in place on the engine side. Screws removed, use a flathead screwdriver and wiggle the metal plate off. Now with some twisting and turning, you should be able to remove the coupler as a whole from the cabin side along with the pieces of metal hose still attached.





4. The coupler needs to be gutted for the new hose to pass through. Unclip the metal hoses and remove them. You should now be able to pull the plastic connectors straight out - they are not glued. Twist and turn them as you pull. Mine broke off because it was cracked to begin with. You will be left with this:



The coupling itself removed looks like this:



And finally, the reason why these things often fail. A whopping 8 possible points of failure on the cabin-side alone:



5. On to fitting your hose. First determine if you want to run the bypass all the way to the engine block, or if you're going to use the original lines that clipped into the firewall. I used the original lines, as due to tight spacing I could not twist and turn the firm hose in a comfortable position towards the engine without it kinking. If you route straight to the block, make sure they have enough slack to not kink. I routed them to the original fittings. If you do so, you should remove the clips that are stuck to the metal ends of the hoses.

Measure the correct length for your hose, and fit them through the coupler. You may want to cut the coupler's ends off a bit for extra clearance later on (see the red lines). There is no need to have the plastic stick out past the metal cover plate on the engine side.





6. The hose will probably not seal perfectly inside the coupler. You do want a proper seal to reduce noise and fumes from the engine bay enterin the cabin. I used a piece of silicone vacuum hose cut to the right length, cut open, and wrapped around the hose. Any hose of the right thickness will work for this. I wrapped it around the hose and twisted the hose through the coupler while applying pressure to the seal.







With both hoses through at the correct position and both seals applied, it looks something like this:



7. Time to install. Wiggle the coupler with hoses attached back into its original hole. You might need to bend the plastic quite a bit for it to snap back behind the white plastic airduct. Once it's in place, slide the heater hose over the metal hoses (make sure to sandpaper the sharp edges and clean any metal filings out of the pipes as well as you can). Slide the hoses over as far as you can, and tighten them firmly with the wormwheel clamps.




8. At the firewall side, first install the metal plate and tighten the torx screws. Then slide the original metal couplings into the hoses. They may be very tight, but tight is good. I used spit as a lubricant to get them in as far as I could. I wouldn't recommend soapy water, as you don't want them to pop right back off when the system gets pressurized. Again, tight is what you want. Install the wormwheel clamps and tighten firmly.



Done! Top up your coolant, and warm up your engine. Check for leaks and you're good to go, never having to worry about the coupler again.