My first experience of an air-cooled Porsche 911 heating system wasn’t good. Being a poor student at the time, I used to hitch-hike from place to place, and one day was picked up by a complete nutter in a Volkswagen Beetle. As he drove at insane (for a Beetle) speeds along a winding country road, he apologised for the hot cabin – explaining that there was something wrong with the heater. I’ll say there was! Not only did I get out at the end of my ride shaking and sweating, my rubber-soled baseball boot had quite literally melted to the inner sill, where the hot air was pumped in. The whole experience put me off Beetles for life.
Thankfully, though, it didn’t put me off Porsche 911s, which have essentially the same heating system. Both have air-cooled engines (we’re not including the newer 996- and 997-model 911s here), which caused the cars’ engineers much grief when it came to designing a cabin-heating system.
Most car engines (and here we can include the 996 and 997) are liquid-cooled, and it’s a relatively simple matter to tap off some of the coolant to heat up a radiator (usually somewhere inside the dash, through which air is blown into the cabin. This works very well once the engine has reached operating temperature, and the coolant acts as a heat reservoir which means that the cabin temperature can simply be regulated, regardless of engine speed.
Over the years, car manufacturers have experimented with alternative systems (the infamous Austin Maxi, for example, had an electric heater, which had the advantage of giving almost instantaneous heat as soon as you started the engine), but always come back to the obvious method of diverting hot coolant from the engine.
But with an air-cooled engine that, of course, isn’t an option. Instead, an alternative way of heating the cabin had to be found. In Porsche’s early days an optional Webasto petrol heater could be fitted in the luggage compartment of left-hand-drive cars, but that idea never caught on. Instead, Porsche settled on using the hot exhaust system to warm air for the heating system, just as Volkswagen did with the Beetle.
The principle is basically the same on all air-cooled 911 heating systems. The two exhaust manifolds are enveloped in steel shrouds through which fresh air from outside the car is forced and heated on its way to the cockpit.
On early 911s, this air was propelled solely by the large engine-cooling fan. Simple mechanical valves at the front of each heat exchanger were controlled by levers between the seats – push the lever one way and the warm air was vented out under the car; in other words, the heater was turned off. Push the lever the other way, though, and the valves directed the air inside the car’s sills and out through flaps into the cockpit. By closing these flaps you could direct the warm air up to the windscreen for demisting purposes.
It was a crude system but one that worked – sort of. The problem was it was reliant on the engine-cooling fan, so the faster the engine was turning, the more air passed through the heat-exchangers, which meant the system was less efficient if you were, say, sitting in traffic.
From the 911’s launch in 1963, Porsche’s engineers continually refined the system in a desperate attempt to give controllable and constant heat outputs.
The first improvement, in 1968, was to replace the flaps in the sills (which were basically the same as those on the Beetle) with dash-mounted controls that allowed the driver to choose between the warm air coming into the footwells or being diverted up to the screen. At the same time, a fan was added in front of the scuttle to draw fresh air into the cockpit. Mixer valves allowed this fresh air to be mixed with warm air from the heat exchangers to give a greater degree of control over the interior temperature. This meant that the 911 no longer required opening front quarter windows.
However, there was still the problem that the system was reliant on engine speed – when sitting in traffic, the fan would be running slowly and so could only push a small amount of warm air into the cabin. Hence in 1974 Porsche’s ever-resourceful engineers added an auxiliary electric fan in the engine compartment of non-Turbo cars to help things along.
At the same time more conventional-looking (but not exactly intuitive) slider controls were added on the dash for adjusting the airflow (temperature was still controlled by a lever between the seats)
This improved matters but there was still no getting away from the fact that the cabin temperature was very much dependant on engine speed and temperature, car speed, and ambient temperature. Which meant that owners were constantly fiddling with the heater controls to try and maintain a constant interior temperature.
Porsche’s solution was to offer a thermostatic temperature control on the Turbo from 1975 and on all models from 1977. In the UK this was standard, but in most other markets it was optional. Instead of a lever between the seats there was now a rotary controller that was used to set the required temperature.
The system consisted of two temperature sensors – one inside the ducting in front of the left-hand heater valve forward of the heat exchanger and one in the cabin between the sun-visors (moved to the dashboard in 1986). In addition, below the rotary controller was a servo motor that opened and closed the heater valves to control the flow of warm air.
This was a huge step forward and, once you got to grips with the intricacies of the controls, it worked reasonably well, especially from 1986 when the dash vents were enlarged.
However, by the end of the 1980s the whole system was beginning to look crude and out-dated (which it was) and it still didn’t work as well as that in even the simplest car with a water-cooled engine. Something better was required, which is what Porsche gave us with the 964-model 911, launched in 1989.
Gone was the controller between the seats and the oddball dash-mounted levers, and in their place was a conventional-looking control console on the dash. This was identical in operation to systems on other cars – there were slider controls to direct the air up to the windscreen or down to your feet, rotary controls for temperature and fan, plus buttons for recirculating the air and defrosting. In other words, anyone could jump into a 964 and know at once how to use the heater.
However, behind this high-tech new interface lay basically the same system, albeit refined to give more consistent cabin temperatures, and with heat exchangers made from long-lasting stainless steel. Now there were various servo-operated valves and two multi-speed fans at the rear of the luggage compartment. In addition, the warm air from the engine (which was now less dependant on the engine-driven fan) and outside air (or air-conditioned air if fitted) were fed into a mixing chamber, to create the correct temperature of air, before it was fed into the cabin. Furthermore, an extra fan in the engine bay helped blow excess heat out of the system and into the atmosphere when it was not required for cabin heating.
This, at last, gave the 911 a decent heating system and was carried over in basically the same form to the 993-model cars which came along in 1993.
In 1998 the all-new 996-model 911 was launched. This had a water-cooled engine so Porsche’s engineers could, after 35 years, relax and not have to worry about the complexities of heating the cabin with an air-cooled engine.
What goes wrong?
Over the years, an air of mystery has developed over the 911’s heating system. However, in its early form, at least, it’s a fairly simple system – certainly less complex than some water-based ones.
If your 911’s cabin isn’t getting warm, the first things to check are the heat exchangers and the rear-mounted heater valves (on pre-1989 cars only) in front of them.
Let’s start with the valves. These mild-steel items are located on the underside of the car so are exposed to moisture from the road. In addition, they are in close-proximity to the hot exhaust system, so any lubrication/rust protection that is applied soon burns off (adding an unpleasant aroma to the cabin air in the process). Once these valves start to rust (and they will) then there’s a chance that they’ll seize up, thus jamming the heater on or off (or partway between). The only satisfactory solution when this happens is to fit new ones, which cost around £50 each, depending on type. To help avoid your valves from seizing, then, make sure that you regularly alter the heater’s settings from cold to hot, to encourage the valves to operate. A small amount of spray lubricant at service time will help, too, but don’t overdo it or you’ll get a smell in the cabin as the oil burns off. In addition, the valves have springs to help them return to the ‘cold’ position and these springs lose their strength over time, which can lead to the valves not operating over their full range.
Heat exchangers on pre-1983 cars are made from mild-steel so, like any exhaust parts made from this material, they will inevitably rust away over time. The Carrera 3.2’s exchangers, however, were made of much better-quality steel, while the later 964 and 993 cars had long-lasting stainless-steel items. If holes develop in the outer shrouds of the heat exchangers, hot air will simply pass out through the holes rather than continuing through the heating system and into the cabin.
If your heat exchangers have corroded, then the only solution is to renew them – they cannot satisfactorily be repaired. If you’re planning on keeping the car for some time, it’s worth investing in aftermarket stainless-steel exchangers, which should last a lifetime. Prices of mild-steel units start at around £250 each, while stainless-steel versions begin at about £400.
Another possible air (and therefore heat) leak is from the flexible pipes between the heat exchangers and the valves. Because the former are mounted on the engine and the latter on the car body, the link between them has to be flexible. This large-diameter piping rots and splits over time. Also, it’s held in place by steel clips which rust away, causing the pipe to fall off. You should always renew these pipes (which cost about £20 each) and clips as a matter of course when fitting new heat exchangers or valves – stainless-steel clips are a sensible upgrade.
Cars with a thermostatic controller between the seats have an extra potential trouble point. If the heater appears to be on full all the time, first check that the roof-mounted sensor (on pre-1987 cars) is intact and connected – if it isn’t the system will always assume the interior is cold and so set itself to hot all the time.
Next, check that the servo between the seats is moving – you should be able to hear it whirr as you move the rotary knob. If it is doing, but the temperature is not changing, remove the little cover from the side of the unit and check that the rod connecting the servo to the valve has not come off its ball-joint – a simple enough matter to replace.
If the servo is not operating, you’ll need to disassemble the unit and check that all the wires are in place. If they are, it’s possible that the electronic controller has developed a fault and needs replacing. This is an expensive item, so some parsimonious owners choose instead to retrofit the simple lever system from an earlier 911. As a short-term measure, you can alter the valves’ position by pushing the control rod manually through the access hole in the side of the unit.
Note that early versions of this automatic system also had a red-handled lever to act as an over-ride in the event of the thermostat failing. This should never be operated at any other time – doing so can break the internal linkage from the servo.
When it comes to 964 and 993 cars, things get rather more complicated. As with earlier 911s, the heat exchangers and ducting are still the first things to check if a fault occurs, although the former are stainless-steel and protected by an engine undertray, so should be pretty much maintenance-free.
As regards the rest of the system, the various sensors, servos and fans make it difficult to diagnose faults without an in-depth knowledge of the various components (some of which are very inaccessible), and a compatible Bosch diagnostic tester. There are, however, some simple checks you can make yourself.
First, if the system isn’t maintaining a constant (well, fairly constant – it’s never brilliant) temperature, it could be that the sensor built into the control unit is dirty. Use a pair of Blaupunkt-type radio removal tools to take out the control unit from the dash. At the rear you’ll see a tiny fan unit projecting out. Unscrew and remove this to expose the temperature sensor inside the main unit. Gently blow any fluff or dirt away from this, and at the same time check that the little grille on the face of the control unit is clear.
Inconsistent temperatures, and heat not emitting from certain outlets (screen, footwells, dash) suggest that one or more fans and/or valves are not operating. Although it’s very hard to access these, it’s worth just checking certain fuses haven’t blown; namely, those for the rear blower fan (in engine compartment fusebox) and forward blower fan (in luggage compartment fusebox).
A detailed diagnosis of the system may indicate that the control unit has failed (and such a failure can be caused by a non-functioning servo elsewhere in the system). If this is the case, you need to ensure that you get the correct replacement for your car; different units were fitted to early and late 964, 993s, left-hand and right-hand-drive cars, and cars with air-conditioning. Check the part number of the rear of the box.
Finally, on all air-cooled 911s, unpleasant smells emitting from the heater are not uncommon. These can be caused by a rusty exhaust causing noxious fumes to enter the heat exchangers, although if just the outer shroud of an exchanger is rusty this will not lead to exhaust gases entering the system.
More common is a smell of burned oil, caused by oil dripping from the engine onto the heat exchangers. The only solution is to find the source of the leak and repair it. Remember, too, that lubricating the heater valves in front of the heat exchangers can lead to an oily smell as the lubricant heats up.