Heat or Not to Heat: That Is the Heater Core Bypass Question

Forgive the eye candy photo, but who’d want to click on a pic of a heater control valve?

A few weeks ago, my wife and I drove up to Manchester, Vermont to see old friends. This is something of a Columbus Day tradition that affords a hat trick of delights—a chance to reconnect, to see the glorious New England foliage at its eye-popping best, and to take a drive in one of the fun cars.

The driving duty fell, if somewhat by default, to my 1979 Euro BMW 635CSi, a.k.a. “Sharkie.” I’d brought the car back from the Monson warehouse this summer, thought about selling it, and floated it on my Facebook page for a fair price. As I’ve written here before, Sharkie is an early Euro car with short bumpers, front and rear factory spoilers, and black stripes that give it a very particular and highly-desirable look, but the fact that the car no longer has its Euro-specific high-output engine and dogleg close-ratio 5-speed caps its value. The “float” had no takers, so the car was still at the house.

The 635CSi doesn’t have the lightness and handling snap of the smaller vintage BMWs like the 2002 (or even its predecessor the 3.0CSi), but it’s a fabulous long-haul cruiser whose air dam keeps the nose planted at speeds above 80 where its older brethren and sistren begin to feel pretty light. The fact that the drive up to Manchester is mostly county two-lanes (there’s only ten miles of interstate) didn’t exactly play to Sharkie’s strength, but even still, I figured that the drive might get me off the fence one way or another about selling it.

The 150 miles of road between Newton, Massachusetts from Manchester is barely enough to constitute a real road trip. As someone who does annual 2,000-mile hauls in 50-year-old cars for which he over-prepares and over-plans, this is just slightly further than the radius of my Hagerty towing insurance. I thought, hell, if the car dies, I could probably Uber home, pick up the Armada, rent a trailer from U-Haul, and drag it back. Still, with Maire Anne coming along, I was honor-bound to make certain that something easily preventable didn’t break and inconveniently strand us.

A fluid check immediately revealed an issue—the expansion tank was nearly empty, indicating that the car was leaking coolant. As my eyes glanced around the engine compartment, they fell on something I’d forgotten was there—a heater block-off valve I’d retrofitted in from a BMW 2002. I rotated the valve from its “off” position to “on.” It immediately began leaking, and didn’t stop when I rotated it back to “off.” Well, at least I’d found the problem.

At this point we need to take the off ramp from the Sharkie Goes to Vermont story and talk about the technical matter at hand, which is the way that heater cores are plumbed, why they’re sometimes bypassed, and what the best way is to do it.

Internal combustion engines produce heat that is carried away by coolant flowing through the block and head. The exact path varies engine to engine, but a water pump generally pushes coolant into the block where it surrounds the cylinder walls to absorb the heat generated by combustion. The coolant rises up into the head through holes in the head gasket, then flows out a hose connected to the top of the radiator. A thermostat governs how much coolant flows out of the bottom of the radiator and back into the water pump.

When the engine is dead cold, flow from the radiator is blocked to allow the engine to warm up. In a fully-warmed-up modern car, the thermostat opens and closes by fractional amounts to keep the coolant in the 195-to-220 degree range. Vintage cars whose engines aren’t primarily designed for low emissions may run cooler, more like in the 170-to-210 degree range. A minor complicating factor is that the intake manifold is often used to send non-radiator-bypassed coolant to aid in warmup.

With all that hot coolant, you’d think that it was a slam-dunk to use it for cabin heat, but a quick perusal of the history of heat in cars surprisingly shows that a variety of gas-burning, exhaust-manifold-wrapping, and even exhaust-fume-diverting techniques were employed before General Motors pioneered the modern heater core. Basically a small radiator, the heater core is mounted inside a box under the dashboard and is plumbed by a coolant inlet and outlet hose. As the engine heats up, so does the core. The amount of heat dumped into the passenger compartment is governed by either a valve on the inlet hose or a flap on the box (sometimes both), as well as by the fan that blows across the core. Then there’s the issue of fresh (outside) air. Some cars have controls that provide fresh air that does not flow through the heater box, while others always route the fresh-air flow through the heater box, heating it if the core is hot. And on a modern car with climate control, the heater core and the air conditioner’s evaporator core are inside the same box, allowing hot and cold air to be mixed to achieve a desired cabin temperature.

Wow, that was actually quite a bit of history and climate control in one paragraph.

In the BMW 2002s I began buying 40 years ago, flow through the heater core is governed by a simple cable-actuated block-off valve. It can be completely off, all the way on, or anywhere in between. The combination of heat and fresh air can then be directed to the footwells or the windshield.

In contrast, my 1973 BMW 3.0CSi—a car from exactly the same era—has no block-off valve. Instead, the heater core is always plumbed hot. You can close the flaps so in theory the heat doesn’t flow out of the box, but you can never shut the coolant flow off.

Both of these systems are now 50 years old, so on both of these cars, unless the heater box has been rebuilt, the foam that lines the flaps is usually disintegrated, letting a lot of air past. But because of the two different designs, it manifests as two distinct problems. On the 2002, since the heater valve can be shut off, the flaps leak air at the outside temperature, so in the summer, it passes too much hot outside air, and in the winter, if the heater valve is off, too much cold air. But on the 3.0CSi, because the heater core is always plumbed with hot coolant, the leakage is always heat, which, in the summer, is a big problem.

When I retrofitted air conditioning into the 3.0CSi almost 25 years ago, I rebuilt the heater box and relined the flaps with new closed-cell foam. That helped, but didn’t fundamentally solve the issues that 1) there was a little hot radiator sitting several inches away from the A/C evaporator core that was now trying to cool the car, and 2) the heater box’s newly-lined flaps still leaked hot air.

To solve the problem, I installed a heater bypass valve, the kind that, when it’s in the “off” position, bypasses the core but doesn’t shut off coolant flow. I thought that this was correct and important. I went through two iterations. In the first, I used a bypass valve that had a lever that could be pulled by a cable, but never bothered to rig anything up to it because I thought that popping open the hood and manually flipping the valve closed in the summer and open in mid-fall would be sufficient.

This worked flawlessly at shutting off the little under-dash fireplace in the summer, but with New England’s mercurial fall weather that can be in the thirties in the morning and in the seventies by afternoon, I found the need to be able to control the heater valve from inside the car, so I went all-in and installed a servo-controlled bypass valve that has a knob I can use to fully open the valve, then taper the coolant flow to a trickle, then stop it completely when the cabin and outside air temperatures are both warm.

After all this was done 10 years ago and I wrote about it on a BMW E9 forum, I had responses that I’d over-engineered it—that it could be a simpler block-off-style valve instead of a bypass valve since that was what was used in the slightly-later mid-1970s BMW 530i sedan. But the bypass valve worked (and still works) fine, so I never changed it.

Okay, back to Sharkie, but not quite yet back to Vermont.

Like I did with the 3.0CSi, seven years ago I retrofitted air conditioning into Sharkie (it was a Euro car without air). It also has an always-on heater core, and I also rebuilt its heater box and re-lined its flaps with new closed-cell foam.

However, during a long road trip in the newly A/C-cooled Shark, I found that, re-lined flaps notwithstanding, the always-on heater core basically nullified the A/C by flooding the passenger compartment with heat. I road-trip-fixed the problem by running into an AutoZone, buying a hose connector and block-off plugs, and bypassing the core.

When I got home after that road trip, I enacted a better but still an interim solution, which was to install the block-off valve I had lying around from a BMW 2002 as shown at the beginning of this piece. It was simple, small, and unobtrusive. Like the servo-actuated valve in the 3.0CSi, I could crack it open just a tad to get a trickle of heat at the footwells; I just couldn’t adjust it from within the cabin.

And then I forgot it was even there.

Fast-forward to a few weeks ago. The valve was leaking the day before the trip on Columbus Day weekend. I couldn’t find any other appropriate block-off valve in the garage, so I had to make the choice between bypassing the core (no heat) or restoring the original hose configuration (core always plumbed with coolant). As it was the second week in October and we were about to drive the car to Vermont, clearly the second choice was preferred. I mean, even if the box leaked heat into the cabin, how bad could it be?

We weren’t on the road long when I realized that, while this maybe wasn’t the wrong choice, it certainly had consequences. Even when I had the heater control knob at zero and directed the venting at the windshield, the amount of heat pouring into the footwells was oppressive.

Fortunately, I’d brought along a back-up solution. I have a set of hose block-off clamps. These are rods with two right-angle bends in them, a gate that swings across, and a knob that spins down onto the threaded end. The little ones are very handy for pinching off gas lines before you pull them off, thus minimizing spillage. I’d brought one big enough to pinch a heater hose. It took a bit of tweaking, but I found the amount of pinching that got the right amount of footwell heat. It’s nothing I’ll leave in place for the long term (leaving a rubber hose pinched like that can only shorten its life), but it was a comfort-saver for this trip.

So, with all that as background, which is the better way to bypass a heater core? Should you use a true bypass valve that leaves the coolant path intact, or should you use a block-off valve? Is the answer the same for all cars? And is it the same for heat mitigation issues like mine, for “heater-core-is-leaking-have- to-get-home” problems, and for racing applications where you want to actually remove the heater box to lighten the car?

If the heater core is leaking and you need to drive the car, you want to eliminate all sources of coolant pressure, and in my opinion, the best way to do that is to temporarily bypass the core by connecting its input and output hoses together as I did seven years ago on the road with Sharkie.

With regard to applications like mine, the argument in favor of blocking the core off is that if you bypass the core, it’s no longer there acting as a little radiator, dissipating heat, and lowering the temperature of the coolant. Thus, if you bypass the core, you’re likely taking hot coolant coming directly out of the back on the head (at least that’s how it is on my cars) that has not passed through the radiator, sending it back into the water pump, and from there, back into the block. For this reason, folks I know who race SpecE30 BMWs block off the hoses that went to the heater core instead of connecting them.

The argument against blocking off the core is that you could be interrupting coolant flow in a way you don’t understand. Maybe that’s only an emissions-related warm-up issue, but maybe it’s more. I’ll freely admit that, when I installed the bypass valve in the 3.0CSi, one of the reasons I did so was that the idea of blocking the flow of coolant out the back of the head seemed dangerous, but I didn’t fully understand that the main flow is up through the block, into the head, and out the front to the water pump.

So, as with so many things in life, it depends. You need to look at how the coolant is plumbed, see if other things upstream are relying on its flow, and read expert opinions on enthusiast forums for your make and model. And if the engine begins running hotter or cooler with the change, take note. I’m sure some of y’all will chime in with opinions on this, but I’m sticking with there not being a one-size-fits-all answer.

Oh, the trip to Vermont? Fabulous. Not the textbook interstate-inhaling road trip I’ve normally employed Sharkie for, but a fine drive through great scenery with old friends at the end. But it didn’t elicit a definitive “Gosh, I love this car! I can’t sell it!” nor a “Sell it now” response from me. As there’s a window before I run it out to the Monson warehouse, I again floated it on my Facebook page at a slightly-reduced price. But if it’s still here in a week, I’ll swap it for the FrankenThirty, and it’ll be out in storage ‘til spring, which certainly isn’t hardship.

And if it’s still here after that, I’ll likely install a servo-controlled block-off valve on the input heater hose.

Oh, one final thing. With all this background, you can see how that old-school advice that if your car is running hot, you should turn on the heat is absolutely true if there’s an actual valve in front of the heater core. Opening the valve literally brings another small radiator into the cooling system. It’s less true if the core is plumbed always hot. Opening up the flaps may help, but it won’t be as dramatic.

See? This was useful. I’ll reward you with more Shark in Vermont clickbait.

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Rob’s latest book, The Best Of The Hack Mechanic™: 35 years of hacks, kluges, and assorted automotive mayhem, is available on Amazon here. His other seven books are available here on Amazon, or you can order personally inscribed copies from Rob’s website, www.robsiegel.com.