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Subject: Heater box and heater core rebuild Author: DavidC (moderator) : member since December, 2003 : 12116 posts Posted on: 2004-03-07 02:51:59

The heater core in my 1986 BMW 735i had evidently been leaking for some time before I purchased it last summer, causing the unpleasant smell of antifreeze in the cabin. One of the previous owners had also neglected to reinstall the rubber gasket between the heater box and the firewall, allowing torrents of water to enter the interior of the car over the transmission tunnel and soaking the rubber padding and the carpet. The leak was so severe that my wife and I had to completely remove and discard all of the carpet and rubber padding. Maximillian Importing was able to locate new OEM padding and carpet, which we later installed (more on that in a future post).

Though some have suggested ways to access the heater core without removing the dashboard, in our case, we needed to remove the entire heater box in order to replace the vacuum-powered actuators (some of which are located under and behind the heater box) and to install a new rubber gasket at the firewall. I discovered that removing the dashboard was, surprisingly, much less difficult than I had envisioned (more on that in a separate post, too).

After removing the center console and dashboard, my wife and I drove the car (an adventure in itself) over to Ron Trochelmann's Performing Imports shop in Alpharetta, GA. Ron cheerfully performed the task of removing the heater box and installing the new vacuum actuators. By doing much of the pre-disassembly work ourselves before taking it to a pro, we avoided the typically high costs usually associated with heater core repairs. Special thanks go to Ron for his contribution to the success of this project and for being willing to work with a "do-it-yourselfer" like myself. Ron is a bimmer enthusiast, too, so I think he appreciates how much work my wife and I have put into restoring this E23 to bring it back to life.

You'll have to remove the cowling over the blower fan in the engine compartment, then remove the blower fan, too. Once that is done, disconnect the two heater hoses in the engine compartment going into the firewall. You don't have to drain the radiator, but do expect some fluid to escape when you disconnect the hoses. If you have pets, be sure to immediately clean up radiator fluid on the floor. Animals like to drink it but its toxic to them (and us).


The above photo shows the heater box and vacuum solenoids in the center console with the dashboard removed. The metal support bar above the heater box has been removed, also (above the pink vacuum-powered actuator).

Another view of the heater box behind the center console. The heater box is a clamshell containing the heater core and evaporator core.

The above photos shows the heater box being removed from the dash area. All of the wiring harnesses must be disconnected, as do the plumbing connections in the engine compartment and the AC lines in the glovebox.

A view of the center console area with the heater box removed.

A view of the front of the heater box. This is the same orientation as it would have inside the car, from the perspective of the front seat.

The left side (driver's-side) of the heater box.

The right side (passenger side) of the heater box.

The rear of the heater box. The metal fins are the external side of the AC evaporator core. The black plastic housing above it is where the blower motor in the cowling would normally be installed. Note the two metal pips exiting at the left (passenger-side) and right (driver's-side). These are the tubes for circulating hot antifreeze from the engine into the heater core. I had to replace the vacuum-powered heater valve in the engine compartment, too, along with most of the vacuum hose lines in the engine compartment. I used a hand vacuum pump ($20 at AutoZone) to pressurize and test the vacuum lines in the cabin, which were all fine, surprisingly.

The heater box is a clamshell, held together by four retaining clips (two along the bottom and two along the top). The plastic mounting point for the lower right retaining clip was broken during service by one of the previous owners, and the the rubber gasket between the two halves of the clamshell was missing. Nearly all of the foam insulation surrounding the heater core had disintegrated and fallen away.

The old heater core being removed from the front of the heater box.

A view of the heater box with the heater core removed. The AC evaporator core is behind the metal air flap.

The new heater core. The kit included new tubing, O-ring gaskets and foam insulation.

New pipes are attached to the left and right sides of the new heater core.

The old and new heater cores.

The newly assembled heater core with the new pipes and foam insulation.

The new heater core assembly slides into heater box.

The cover of the heater box is prepared to be re-attached. As previously mentioned, however, the plastic for the lower right mounting point of the retaining clip was broken. Unless repaired, the clamshell would not seal properly, potentially allowing any condensation that forms in the heater box to leak into the carpet over the transmission tunnel, instead of draining properly through the drain tubes.

 

The solution was to install a piece of metal which would allow the two halves of the clamshell to be secured together using a sheetmetal screw.

A view of the broken plastic mounting point.

Three holes were drilled into the metal brace.

The metal brace was secured to the heater box cover with a couple of nuts and bolts.

Permatex Gasket Maker was applied to the channel between the two clamshell halves where the old gasket would've been originally.

Finally, all of the vacuum-powered actuators were replaced with new ones and the heater box was reinstalled in the car, along with a new rubber gasket between it and the firewall.

• Heater box and heater core rebuild (pic) - DavidC 2004-03-07 02:51:59 (12228 views)
  • Another great write-up. - kff 2004-03-08 13:05:27 (1068 views)