Heater Core Firewall Coupling Bypass - Write-Up With Pics

[Boxman]

In this write-up (based on ycartf's tutorial over at Volvoforums) I will show how I bypassed the heater core firewall-coupler with regular heater hose. This is also cheaper than buying a new coupler with o-rings.

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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. 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.

[troyhyde]

Not trying to sound rude here but isn't it honestly easier to just replace the original type coupler and pipe dog-leg? The only thing that seems to damage the original firewall fitting is abuse from the inexperienced (I've broken one so I am sharing the blame/shame here).

I am not going to argue your assertion that there are 8 points of possible failure in the original design. In my ownership of quite a few of these cars I see failures of the heater cores more often than anything else, second place probably goes to heater hoses. I'm a big fan of leaving a car as originally designed and not applying too many home-remedies.

Pull a few of these from junkyard cars and you will have a lifetime supply for less than $20.

[msestina]

Excellent writeup.

Thank you Boxman!

The heater core tanks fail because the tanks are molded and not crimped to the matrix. These were both design mistakes.

[andyb5]

I did basically the same thing as OP, but ran a single length of hose from the engine to the heater core instead of having a junction at the firewall. My firewall coupling wasn't broken, but I still chose to use heater hose instead of the OEM setup because I don't like spending money on poorly designed parts - the fewer joints/couplings, the lower potential to leak.

[Boxman]

Not trying to sound rude here but isn't it honestly easier to just replace the original type coupler and pipe dog-leg?

I'm not sure it would actually be easier, as for removing the junction the proper way, you would also need to remove the heater core itself. Though if you want to keep it original for the sake of keeping the car original, you shouldn't start cutting. Honestly though, the OEM design was weak and I'm confident in saying this remedy is less likely to fail than the junction, and cheaper in maintenance (maintenance on the junction still requires you to buy a new expensive O-ring set).

Thanks for the feedback msestina and andyb5!

[Serge]

I like this solution. As long as it is sealed at the firewall, there shouldn't be any problems. The factory coupler is fairly problematic (I have broken mine before, requiring a temporary heatercore bypass), but if you have a new one, it shouldn't break on its own.

[Kevin.]

I think I'm going to do this one weekend, I had a brand new coupler fail after 1 week

[Brad850]

Nice job. My only gripe with this mod is engine movement constantly pulling and pushing the lines. The stock coupler and solid metal lines inside the car separate the heater core from that movement.

I guess if you leave enough slack it won't be an issue. I did the same thing Andy did although I used very thick silicone. It's way too tight and pulling at the metal lines enough to where I can't seal it. I already reverted back to stock and am now using d088 silicone hoses.

[Boxman]

Good point on the push/pull of the engine if you connect directly to the block. In my case the lines from firewall to the block are rather soft/slack, and I got it to seal really tight in the coupler. At least in my case the heater core sees no stresses from engine movement, but you do need to take care of making a tight seal in the junction and having as little stress ALL GLORY TO THE HYPNOTOAD possible on the engine side.

[erikv11]

When I gutted the coupler and ran heater hoses straight from the core to the block and pipe nipples, I left some slack in the hoses in the engine bay. I wasn't thinking to accommodate engine movement, just trying to make the bends without kinking the hoses, but hey whatever works ... .