Although several list members use it, it is not recommended by Porsche. The bottles labeled "safe for aluminum engines" still contain phosphates. The only approved anti-freeze are ones that specifically state "Phosphate Free". There are several available products, including:
Check your temperature sender and gauge.
Make sure all the air is bled from the cooling system. There are a few tricks in getting most of the air out when refilling. The sixteen-valve heads are prone to trap more air than the simpler eight-valve engines.
A trick for getting most of the air out when refilling involves leaving the bleeder valve open while refilling, loading the system until the expansion tank is mostly full and then forcing the coolant through until coolant comes out the bleeder valve (how do you force the coolant through? You can perform "mouth to mouth" on the expansion tank, or you can use a coolant system pressure tester - a device which basically looks like a bicycle pump connected to a radiator cap). You may have to repeat the fill/pressurize cycle several times before coolant comes out of your bleeder valve. For all bleeding, make sure you crank the heater temperature control to the full open position or you'll be there forever trying to get the air out.
Check hoses, pipes, and paths for blockage/leakage and verify water is getting in the radiator. If your car is over heating, but the radiator remains cold it's possible either the radiator is blocked or the thermostat is stuck shut. The thermostat recirculates the water until the engine is hot, and then opens up to let the water into the radiator. If it's frozen shut you'll overheat but still see the normal reading on the gauge.
Check the fan operation - there are two. There are two relays that affect fan operation. One controls the one fan during normal operation and another that controls the other fan when the A/C is on. Only one fan operates when the A/C is off, turning on the A/C activates the other fan. Neither fan is necessary for highway cooling (in fact, they get in the way) so you may only notice over heating in stop and go traffic. The fan switch in the radiator can fail with the fan off, or on. If the fans won't run under normal conditions (sitting in traffic while your temperature climbs) the switch is probably stuck off. If your fans continue to run long after you turn off the car, draining your battery, the switch is probably stuck in the on position.
Many people have sent questions regarding problems with this thermofan switch, so I will explain here. The thermofan switch is a plug that screws into the radiator just below the inlet for the upper radiator hose. It has two wires attached to it. This switch monitors the temperature of the coolant in the radiator and decides when to turn on the fans. At one point, the normal cooling fan will kick in, and at the high temp point, it will turn on both fans. If your fans continue to run after shutting off the engine and drain the battery or do not come on at all, chances are the thermofan switch is bad. (There are more details on this below, in 7.6)
Check coolant level. Ensure that air flow to and through the radiator is not blocked. Make sure your water/antifreeze ratio is greater than 2:1 and preferably 1:1.
Removing the 944 thermostat snap ring is extremely difficult due to its location and configuration. Ron Olsen has suggested a particular tool: Snap On SRPC9045A Snap Ring Pliers (45 degree with .09" tips). It costs approximately USD $34.
Automotion also carries 90 degree snap ring pliers (around USD $45).
Or your oil has coolant in it, which may show up as a taffee-colored glob on the dipstick. Chances are your oil/water heat exchanger seals need replacing, if your car is a 924S, 944, or 944S. If you own a 944S2 or 944 Turbo, it's basically the same part but simply functions as a base for the oil filter.Al Broadfoot has written up a procedure for resealing the heat exchanger/console, which you can view here.
The coolant system is closed. If you're losing coolant it's very likely something bad. Pressurizing the cooling system may help you find the leak. Common leakages are the hose connections, water pump, oil cooler, or even the radiator splitting apart (they're part aluminum and part plastic, the seam sometimes fails). If you do find a leak, your best bet is to fix the problem. Avoid using stop-leak products if at all possible since they may cause more problems than they fix such as clogging passageways and causing blowouts.
Also, check the heater valve for leaking. As was mentioned above, avoid stop-leak as it can clog up the radiator and the heater core. If the radiator is the cause, bite the bullet and replace it (cost around USD $300 - $400). I think that it is close to impossible to repair a cracked seam in one of these (I've tried it...even epoxy didn't work), so your best bet is to go ahead and replace. This is an easy job that can be accomplished with minimal tools and in an hours time.
The coolant radiator has a temperature switch in it that controls the radiator fans. Even after the ignition is switched off (and the key removed), the fan will run if the coolant temperature is high enough. The objective is to keep the engine from "heating soaking," which can boil coolant in the head and block.
The fan switch can fail in one of two ways. If it fails "closed," it will run the fans continually, which will drain your battery. If it fails "open," the engine can heat soak, coolant will boil and will push out the coolant reservoir's overflow hose.
Coolant will also push out because of a failing head gasket. You can tell whether it's the head gasket or the fan switch by noticing when coolant is pushed. If it comes out the reservoir while driving, it's because of exhaust gases pushing through the head gasket. On the other hand, if coolant is pushed only after shutting down, it's the fan switch.
To replace the fan switch, you'll need a 30 mm. deep socket, a new switch, and a new fibre sealing washer. The switch is different for early and late model cars. Be careful not to over-torque the new switch - you are threading it into a plastic radiator tank.