BMW E9 Heater Bypass Valve DIY - Part 2

The Hack Mechanic

Rob Siegel

It's taken much longer than I thought, but I've completed (well, nearly) the heater bypass valve installation.

If you've read my Roundel columns, you know that I'm not the "do it once, do it right" type. That very phrase, in fact, drives me nuts, because it's a value judgement solemnly intoned by someone who isn't spending YOUR money. That having been said, I was surprisingly sensitive to what looked out of place in my dear E9 that I've owned for 25 years.

In the previous installment, I looked at three heater control valves. I settled on the AC Delco 15-5533, since the pipe spacing is nearly identical to that of the pipes coming through the firewall, allowing you to run very short stub hoses. The pipe size, though, is 5/8" instead of the 3/4" size of the hoses, but the hose clamps seem to squeeze down ok on the hoses.

I could've had it together in an evening by simply splicing sections of hoses with right-angle connectors (the heater inlet hose that runs from the back of the head has to take a few tight turns). I tried, several times, to splice it. It didn't look right. I tried to tuck the splices down in crevaces so I wouldn't see them. It still didn't look right. I never would've predicted that my eyeballs would've been so sensitive to this.

So I looked for a solution that would allow me to run a single hose, without any splices, from the back of the head to the inlet of the valve. I tried several, but in the end found a Gates 28476 3/4" heater hose that's 22" long, with a 18" long straight section, followed by a right angle bend, followed by a 4" straight section.

I cut about 6 1/2" off the long section and 2" off the short section. This was done by trial-and-error and test-fitting the hose each time. The short pipe at the back of the head is a bear to reach, but I got good at snaking my arm back there and pushing the hose onto it. A few drops of glycerin smeared inside the hose helped things considerably. When I had to tighten it up for good, I thanked the Automotive Powers That Be for my flexible hose clamp driver (my new favorite tool in the entire world).

I attached the long section to the back of the head, and ran it beneath the intake manifold and above the starter motor. The pic below is before I'd fully trimmed the hose. Note that the second hose -- from the pipe under the intake manifold -- was trimmed for length, and is folded back for clarity.

The pic below shows the heater control valve, with its stub hoses, installed against the firewall, with the hose from the back of the head attached. The second hose is still pulled back for clarity.

Below is everything attached.

Below is a pulled-back photo showing that the installation is actually fairly unobtrusive and doesn't cry out "some Hack Mechanic retrofitted an A/C Delco heater control valve."

Note, as per the previous installment, I've elected to use the valve without the dashpot, so I'll manually throw the lever when I want to enable or disable the heater core. I'll have to see if, over time, it stays in position or if I need to secure it in some fashion. The same valve is available with the dashpot, in which case it could be hooked to the intake manifold with a relay-controlled bypass, to enable or disable it. I wasn't interested in making it any more complicated than it needed to be.

Note also that this is all easily removable, and, if need be (if for example the valve fails on the road), the two hoses that feed the bypass valve can still be hooked together, as people do with the brute-force bypass.

I'll post again when the system is filled with coolant and tested, but this looks promising. It'll be interesting to see if the a/c works demonstrably better without the little radiator sitting right next to it.

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