Last week I was all set to move the ball down the field in the a/c retrofit of the ’79 Euro 635CSi and complete a major milestone—installing the reconditioned evaporator assembly—when a little voice inside my head reminded me of my own advice. That is, two weeks ago, I said, “The evaporator assembly sits behind the heater box, and in vintage BMWs, it’s extremely common for the foam to wear off the heater-box flaps, making it so you can’t ever fully block off the flow of hot ambient air into the passenger cabin, which obviously impedes the performance of the a/c system. The heater box has to come out in order to re-line the flaps with foam, and, depending on the model, may have to come out to replace the fan. The point is that it’s kind of silly to pull the evaporator assembly and re-install it without doing the ‘while you’re in there’ thing and addressing any needed issues with the heater box.”

The Shark’s blower fan worked fine (and, on an E24, you don’t need to remove the box to change the fan anyway), but when I reached up into the heater box and felt the footwell flaps, it was obvious that the foam was completely deteriorated.

The problem with one’s own advice is that one feels compelled to follow it.

But, quite simply, I just didn’t want to do it. This project is beginning to exhaust me. It’s dragging on week after week. I thought, “Just because the foam is deteriorated doesn’t mean hat the box is actually leaking enough warm engine air to matter.” Denial ain’t just a river in Egypt; it’s also a guy getting worn out.

So I made a deal with myself. I’d drive the car and see how much air was leaking in through the box. However, with all the work I’d done, preparing to install the evaporator assembly, the interior of the car was a wreck; it looked like the inside had been vandalized. I even had the ECU out of the car to increase the clearance I needed to drill the holes in the firewall for the hard a/c lines.

I put the ECU back in, installed the two hard lines and their big grommets to plug up the holes, re-installed the shifter surround with the power-window switches on it so that I could roll up the windows, and buttoned things up enough to be able to drive the car.

Then I drove the car for the first time in several months, taking it onto the highway with the fresh-air vents closed and the blower fan off, and feeling around the defroster and footwell vents with my hand.

When you tell yourself that you’re going to do this sort of a test and act on the results, you need to keep your word to yourself; otherwise, why are you doing it? Thus, you can probably understand my reaction of “God damn it!” when I felt warm engine air pouring in through the defroster vents. Oh, well; I suppose I can think of it like having to eat my peas before having dessert. Yeah, we’ll go with that.

Like so many things that we dread doing, however, dropping the heater box and relining the flaps with foam wasn’t that big a deal. There’s a very good how-to on removing and rebuilding the early E24 heater box here. Pull the two heater hoses, take the fan out, undo four nuts and one bolt, and the box easily drops down onto the transmission hump. It was much easier than it is on a 2002, where maneuvering the box itself in and out while managing the Bowden cables is a bit of a puzzle.

The early E24 heater box itself isn’t really any more complicated than it is on a 2002. Its construction is similar—two plastic halves held together by clips. There are, however, a lot more clips.

But even before I opened up the box, there was trouble. I saw that both of the lower corners of the rectangular foam seal where the box mates to the opening in the firewall were drooping. When I inspected them, I found that in both places, the plastic corners of the mating rectangular flange on box were broken.

I tend to think of the E24 as a generation newer than the E9, which of course it is, but we’re still dealing with brittle 38-year-old plastic. For an instant, I rued disturbing it, but for all I know it may already have been cracked.

Well, no problem, I thought. When I bought the entire set of under-dash a/c components, the seller included the heater box. I didn’t think that I needed it, but now I scarcely believed my good fortune, I gleefully dug it out from the back of the garage, and was quickly deflated by the fact that it was cracked in exactly the same two places.

I looked at both boxes carefully. On the box that came out of the car, most of the cracked-off plastic pieces appeared to be adhered to the foam seal. I thought that I could figure out where the pieces went and JB Weld everything back together. Since I knew that the fan motor worked and it didn’t smell of dead rodent, I elected to use that box.

I disassembled the box. The only surprise was that the heater valve has to come off first, and that was a bit awkward to do, as it’s difficult to reach the nuts holding it to one of the pipes. The rubber O-rings behind it were deformed and clearly needed to be replaced. I put a pair on order.

Other than the completely deteriorated foam, things looked good inside. There were a few acorns, but not wholesale evidence of mouse contamination. Still, when you lay a disassembled heater box out on the table, there’s the unmistakable feeling of wondering what you got yourself into, and hoping you’ll be able to put Humpty Dumpty back together again.

I scraped the residual foam and adhesive off the lower flaps with a single-edge razor blade, as those can be removed from the box and laid flat. The fresh-air flaps at the top of the box, however, cannot be easily removed. Scraping them in place was necessary, but proved difficult. Many single-edge razor blades gave their life in pursuit of the project. I then cleaned all surfaces with acetone to remove any traces of adhesive.

I then had to determine what thickness of foam to use to re-line the flaps. I searched on E24 and E28 sites but could not find a reference. I had some 3/16-inch closed-cell foam left over from rebuilding the heater box on one of the 2002s. I used it to re-line the lower control flaps. These rotate about their center axis and seal along the edges, so I didn’t think thick foam would be a problem there.

However, unlike the lower control flaps, the fresh-air flaps rotate about one edge. This means that if the foam is too thick, the foam nearest the rotation point acts like a foot in the door, preventing it from closing. I ordered 1/16-inch and 1/8-inch closed-cell foam from McMaster-Carr. Neither was in the Goldilocks zone, with one seeming to be too thin and the other too thick. I opted to use the 1/8-inch stuff, figuring that over time, it would compress slightly if I left the flaps closed.

As with other adhesive-backed foam tape I’ve bought from McMaster, this foam tape is of very high quality, with very sticky adhesive on the back. But I quickly discovered that the stickiness posed a problem: The fact that the flaps pivoted from the back edge made it very difficult to get my fingers, and the foam, on that edge, place it accurately, and lay it down straight. And because the foam was so sticky, I had only one shot at laying it down. I found that I could rotate the flaps down further than normal, and gain extra clearance for my fingers, by disconnecting the Bowden cables and bending a tab that’s a mechanical stop for flap motion.

But on my first attempt to align the foam with the back of the flap and lay it down, I botched it badly, and there was no pulling it back up; that industrial adhesive gripped on the acetone-prepared metal surface like stink on a week-worn sock. I had to rip it up, scrape it off, clean the surface, and try again. But on the second try I got it right, or at least close enough.

When I test-assembled the box and checked how well the flaps sealed, I found that my use of thick foam on the lower flaps was, in fact, problematic, and had to judiciously shave it in certain places to get the flaps to close correctly.

On the upper fresh air flaps, the 1/8-inch foam was preventing the flaps from closing fully in places, but judicious bending of the flaps made them seal all the way around.

After the box was reassembled, I had to deal with the cracked-off plastic pieces. I solved the jigsaw puzzle, laid the pieces down in a bed of JB Weld Kwik Set, and hoped for the best.

Once the JB Weld dried, the repair seemed solid and stable. I sanded off the high spots with a little Scotch Brite wheel on a Dremel tool.

New O-rings went on the pipes leading to the heater valve. Then the valve, with its difficult-to-access nuts requiring a quarter of a turn at a time with an open-end wrench, went on.

There was one final task before installing the box. While I was in foaming mode, I applied foam tape around the outsides of the ports on the heater box, as well as to an intermediate duct piece which mates to the evaporator assembly.

Installation, in comparison to the how-the-hell-do-I-rotate-this-thing-to-get-it-back-in heater box on a 2002, was almost trivial.

So I’ve eaten my peas. Next week, dessert: I install the evaporator assembly.—Rob Siegel

Rob’s new book, Ran When Parked: How I Resurrected a Decade-Dead 1972 BMW 2002tii and Road-Tripped it a Thousand Miles Back Home, and How You Can, Too, is now available on Amazon. Or you can order personally inscribed copies through Rob’s website: