This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
The heating and defrosting system on the 911 is one of the most confusing and complex systems on the car. It's also one of the most common systems to fail. On top of that, these systems didn't really work too well when the cars were new. Heating and cooling systems on air cooled cars have always been difficult. Combine that with the fact that the engine is in the rear of the car, and you have some added difficulties.
Nonetheless, there is plenty that you can do to restore your heating system back to its original, somewhat lackluster performance. The heating and defrosting system for the 911 relies on hot air that is channeled from the motor to the exhaust pipes, through the sides of the chassis, and into the area behind the dashboard. Needless to say, all this traveling causes the air to lose some of its heat and velocity by the time it reaches the passenger compartment.
Throughout this trail of winding heated air, there are plenty of spots for it to leak out and lose pressure. If there is a significant break in the system then the heating power will be greatly reduced. If you have an open-air car like the cabriolet or a targa, then you will immediately recognize the importance of having a working heated defroster on a humid day.
The source of all this hot air starts in the engine compartment. Depending upon which model and year you have, the orientations of the hoses and blower motors may vary. On most cars, there is an adapter plate on one or both sides of the engine fan that funnels air down into the heat exchangers. The heat exchangers are nothing more than a exhaust header system with sheet metal wrapped around the pipes. The air from the fan is funneled through these heat exchangers, and heated by the fact that it is flowing past the hot exhaust pipes.
With some of the Porsche 356 models, there were problems with the European heat exchangers rusting through the pipes. This caused the exhaust to become mixed with the air that was fed into the engine compartment. As a result of this, a few people died from breathing in exhaust gases. Although no reported cases of poisoning have been reported with the 911, it's important to remember that your fresh air is circulating past the exhaust system. If there is a problem with a significant amount of rust in the exhaust system, then you might have some leakage from the pipes. Make sure that you inspect your heat exchangers carefully and replace or repair them if they look like they might be developing leaks from rust holes. For the early heat exchangers, there are separate replacement parts available that can be welded in for repair.
Another important consideration is that the heat exchangers are located right below the engine. This means that any oil leaks from the engine will have a likelihood of dropping oil onto the top of the heat exchangers, mixing with the air that is channeled into the cockpit. When the car warms up, this oil will start to burn and smoke, and you can actually have smoke coming out of your dashboard vents if you are not careful. With this in mind, it's a wise idea to repair all of the oil leaks that might drip on the heat exchangers before you activate your heating system. It's not a wise idea to use your heating system if you can smell burnt oil in the air.
The hoses in the engine compartment are manufactured out of aluminum and paper, and have a tendency to crack and break over the years. Inspect and replace these hoses with new ones if you find any damage to them. Insuring that these hoses are intact is the first step to improving your heating system. Underneath the engine, there should be additional hoses that connect the heat exchangers to the hoses in the engine compartment. Inspect these too, and replace them if necessary. In particular, the A/C bracket has a tendency to poke a hole in the paper hose that runs right next to it, in-between the fan and the bracket.
On the 911s made after 1978, Porsche installed a separate blower motor in the engine compartment to give an extra push to the air that's channeled to the cockpit. This blower motor is connected to the air intake of the fan through a plastic hose that often develops cracks over the years. It's a wise idea to replace this hose if it's cracked, as a lot of extra air can slip out of this hose.
Exiting out of the front end of the heat exchangers, the warm air enters what is known as the flapper boxes. These two boxes, located on opposite sides of the car, control the flow of hot air into the cockpit. They have flaps inside that are either open or closed. These flaps are connected via a cable to the two levers that are located in-between the passenger and driver's seat. These levers also have an electrical switch built-in that operates the engine compartment blower motor on the later 911s. It is important to inspect these boxes, as they are often susceptible to rust and deterioration. Also inspect the hoses that lead from the heat exchangers to the flapper boxes.
If you decide that you need to replace the flapper boxes, exercise care when removing them from the car. They are attached to the chassis via small studs that were welded into the car when it was assembled in Germany. If you accidentally break off one or more of these studs, then you might have a difficult time reattaching the flapper boxes later on. When installing a new flapper box, make sure that you use a new seal between the flapper box and the chassis.
If the heater hoses that connect the flapper boxes and the heat exchangers are broken or ripped, then you should replace them with new ones. Be sure that you only use high temp heater hose, as most other ordinary hoses from hardware stores will melt when exposed to the high heat from the heat exchangers. There are very good aftermarket hoses available that were originally designed for the aerospace industry that will work well in your Porsche.
The flapper box cable is another source of problems for the heater system. This cable has a tendency to break on the older cars. To replace it, you need to remove the small handle assembly located in the cockpit, between the two seats. Lift up the edges of the carpet, and find the small screws that hold the lever assembly to the chassis. When you lift up the assembly, you should be able to see the catch for the cable underneath. The new cable needs to be threaded through the two holes in the chassis firewall from inside the cockpit (not always an easy process), after first being threaded through the lever assembly. Make sure that you don't damage the cable during this process. You may find it easier to work if you pull out and remove the passenger's seat from the car.
After you have fixed all the components that comprise your heater system, you should test it. Most people don't know how the heater system even works on most Porsches. As previously explained, the hot air from the heat exchangers is funneled behind the dash. The red lever on the climate control module determines whether the heated air will be directed at the windshield (for defrosting) or through the dash and lower vents for heating. In addition, there is a fresh air blower that mixes air from the outside with the heated air from the heat exchangers. By adjusting the volume of fresh air mixed with the heated air, you can change the temperature of the air exiting the vents. You can also change the amount of hot air released into the system by adjusting the heater lever arms in-between the two front seats. Pulling up the handles only part of the way will limit the amount of air that passes through the flapper boxes.
When installing new flapper boxes, make sure that you use new sealing gaskets. The old flapper boxes should be taken off with care, as it is possible to accidentally snap off one or more of the studs that hold the boxes to the chassis. Once these are gone, it's very difficult to attach the flapper boxes to the car.
New flapper boxes are painted to avoid rusting. When installing these new flapper boxes, make sure that you inspect the flapper box cable and the high temp heater hoses that connect the flappers to the heat exchangers. Only use hose that has been approved for high temp use. If you use normal hose available from a hardware store, then it might melt and create noxious fumes.
In the engine compartment, there are several hoses that warrant inspection and/or replacement. The large plastic blower motor hose that acts as an intake from the fan often cracks and leaks air out the sides. This can actually have a detrimental effect on the cooling of your car, as this air now becomes 'wasted.' In order for your engine to get all the cooling that it needs, this plastic hose needs to be intact and installed. The long paper and aluminum hoses that connect the blower motor to the heat exchangers also often need replacement after many years of use. Finally, if the blower motor is not functioning properly it too may need to be replaced. Check the connections to the blower motor to see if there is any power getting to it before you replace it (see Pelican Technical Article: Troubleshooting Electrical Problems).
The hidden catch for the heater pull cable is located under the heater knobs. These pull handles have a dual-purpose. They act as a physical switch, pulling open the flapper boxes. There is also an electrical switch underneath the pull handles that actuates the blower motor. If you wish to improve the look of your interior, the little red plastic tips on the pull handles simply pop off for replacement.