One often neglected task on many cars is the maintenance of the cooling system. In general, Porsche recommends that you flush and clean out your cooling system once every 36 months, or approximately every three years. I like to perform this task on my own cars about once a year, or if I let it slip, once every two years. The reason for this is that old, exhausted coolant can actually cause irreversible damage to your engine componentsÃ¢Â"I found this out firsthand when I recently replaced the head gasket on one of my older BMWs. It looked like the previous owner hadn't changed the fluid once in the past 10 years. As a result, there were many parts of the engine that were corroded and showing severe signs of wear.
A properly maintained cooling system must have a few things in order: adequate supply of coolant, a radiator that acts as a heat exchanger with the outside air, a fan or airflow source, a water pump to keep the coolant circulating, and a thermostat to regulate the engine at its optimum operating temperature. The coolant must also have the correct mixture and chemical compounds to promote heat transfer, protect against freezing, and also inhibit corrosion. To keep your 911 operating correctly, it's important to check the level, strength, and overall condition of the coolant on a regular basis. You also need to change the coolant before it degrades to the point where it doesn't perform its job adequately.
A fact that I keep hearing kicked around revolves around the reported findings of the U.S. Department of Transportation, which states that cooling system failures are the leading cause of mechanical breakdowns on the highwayÃ¢Â"not exactly surprising, since proper cooling maintenance is one of the most neglected areas of most cars.
Electrolysis: One failure mode associated with dirty coolant is known as electrolysis. Electrolysis occurs when stray electrical current routes itself through the engine coolant. The electricity attempts to find the shortest path, and impurities in the coolant often generate a path of least resistance that the electricity travels across. The source of this stray electricity is often from electrical engine accessories that have not been properly grounded. A missing engine or transmission ground strap can also cause the coolant to become electrified. Sometimes the path of least resistance becomes a radiator, a heater hose, or even the heater core. These components are often well grounded and offer a ground path from the engine to the chassis by means of the semi-conductive path of the coolant.
Electrolysis can destroy your engine quickly. Although it's semi-normal to have very small amounts of voltage potential in your coolant system, values greater than about a tenth of a volt can start reactions between the coolant and the metal in your engine. In particular, electrolysis affects primarily aluminum engine components, resulting in pitting and scaring of the aluminum surface. This eating away of the metal can cause coolant system leaks and, in particular, radiator leaks around aluminum welds. Cast-iron components are also vulnerable, but typically the aluminum metal parts fail first. Often, electrolysis can be easily seen attacking aluminum cylinder heads (see Figure 1).
How can you test for electrolysis? Other than actually seeing visible signs of erosion, you can perform a current flow test. Connect the negative terminal of a voltmeter to the chassis ground. Test for adequate continuity by touching another point on the chassisÃ¢Â"the resistance should be near to zero. With the engine cold and running, submerge the positive probe into the coolant tank, making sure that the probe does not touch any metal parts. The voltage should be less than .10 volts. If not, methodically turn off or unplug each electrical accessory until the reading reads below .10 volts. Have an assistant switch accessories (like the A/C compressor, heater blower, etc.) while you measure the voltage.
If an accessory doesn't have an on/off switch, test it by temporarily running a ground from the housing of the accessory to the chassis. Ground each component and check the voltmeter. If the wire restores a missing ground connection to the accessory, then you've found a component with a faulty ground.
During this test, be sure to check the starter. Not only will a poorly grounded starter struggle to turn over the engine, it will also zap away tremendous amounts of metal in your cooling system components. Watch the meter carefully when starting the engine. Any voltage spike will indicate a faulty ground connection.
Coolant System Additives: Many people are rightly skeptical of coolant system additivesÃ¢Â"there are a lot of myths in the automotive industry. Luckily, the coolant system additives are in the category of good practice, for reasons I'll explain here. It all begins with chemistry. Like today's modern oils, many of today's modern coolants incorporate some of the chemicals that help cooling and increase heat flow around your cooling system components. As more and more automotive components are made out of aluminum and radiators become smaller, the use of these additives becomes more advantageous.
Aftermarket coolant system additives are known as surfactants. What is a surfactant? A surfactant, or surface active agent, is a molecule that has a water-loving end (hydrophilic) and a water-fearing end (hydrophobic). Localized boiling of coolant in the cylinder head can create large shock waves that can wreck havoc on your engine, particularly on aluminum components. Without going into too much boring detail, these surfactants also help reduce the amount of air in the cooling system and also control the amount of foam within the system.
In general, there are three main reasons why using these additives is beneficial to your cooling system. Firstly, they reduce harmful cavitations and foaming that may occur when your water pump is kicking out fluid at a rapid pace. This reduced foaming helps to prevent damage to aluminum surfaces. Secondly, the use of these additives aid in the transmission of heat from the coolant to the radiating surfaces within the radiator. Even if your car runs very cool, these additives add an extra level of protection in case a thermostat or similar component fails. Thirdly, the additives contain corrosion inhibitors. Most cars on the road have cooling systems that do not contain the ideal 50/50 water/antifreeze ratio that the antifreeze manufacturers design for. The additives help minimize potential corrosion by maintaining adequate pH levels. Even if your antifreeze already contains surfactant additives, the use of these additional additives is typically beneficial because most cars are shortchanged on the 50/50 coolant/water mix.
In general, the benefits of additives, such as Water Wetter, are that they:
Reduce corrosion due to rust and electrolysis
Increase the "wetting ability" of water and improve heat transfer, thus reducing cylinder head temperatures
Clean and lubricate coolant system seals like those found in the water pump
Reduce the formation of foam and cavitations, which can cause corrosion
Reduce the effects of "hard water" in the cooling system
In general, the addition of these additives is cheap, and it's a proven benefit tooÃ¢Â"no snake oil here. Using the additives on a perfectly maintained car can also provide a significant margin of error in case something goes wrong. Porsches are not generally known for cooling system failures, but keeping the odds on your side can prevent a costly head gasket replacement.
It's important to keep your cooling system at the correct pH as well. Water has a pH of 7 and is considered neutral. Battery acid is highly corrosive and has a pH of about 2Ã¢Â"3, whereas baking soda is very alkaline and has a pH of about 10Ã¢Â"11. In general, you want to make sure that your coolant has a pH greater than 7. Any pH less than that will result in an acidic mixture, which will start to corrode your engine. The corrosion inhibitors in additives and antifreeze are added specifically to keep the pH above 7. A properly mixed 50/50 split between water and antifreeze will yield a pH of about 8Ã¢Â"9. Over time, the glycol (one of the main components of antifreeze) will break down and degrade, creating acidic compounds. The alkaline corrosion inhibitors must be adequate enough to neutralize these acidic byproducts over the life of the coolant. Minerals in the water, heat, dissolved oxygen, and other factors gradually deplete the coolant of its corrosion inhibitors. Once gone, the mixture will become acidic and will begin to eat away at your engine.
Cooling System Maintenance: Checking the Level: It's very important to check your coolant level regularly, as this will help detect leaks that can siphon off coolant and cause overheating in your engine. You should regularly check the coolant level in your coolant reservoir, making sure that it is within the prescribed high/low marks. These marks are printed on the side of the coolant container, located in the rear trunk. The container is slightly transparent, and you can see through it slightly to see the current coolant level.
Your Carrera will lose a little bit of coolant here and there over time due to evaporation and/or sporadic leakage. However, a significant loss of coolant over a very short period of time almost certainly signifies a leak in the system. Sometimes a leak can be seen when you park the car overnight. Often the coolant leaks out and then evaporates while you're driving, leaving no telltale mark of coolant on the pavement. If you suspect a coolant leak, visually inspect all of the hoses, the water pump, the reservoir, and the radiator for seepage or the "weeping" of coolant out of seams and gaskets. Check the seal on the radiator cap. Check that the radiator cap is fastened securely. If you suspect a leak that you cannot see, a pressure test from a professional mechanic can verify the integrity of your system.
If you can't find any visible leaks and the system appears to hold pressure, then check to make sure that the cap is good and is rated for the proper pressure. Verify that the cap you have for your Carrera is the proper one for your engine. If you look inside the coolant tank and the coolant is muddy or cloudy, then you may have a serious head gasket problem. Oil may be leaking past the gasket and mixing with the coolant. This typically means that the engine needs to come apart and the head gaskets resealed, which is a complex and expensive repair as you might imagine.
If the system does not hold pressure, and you're still at a loss where coolant might be disappearing to, then you might want to start looking in the oil. A faulty head gasket will often cause coolant to leak into the oil. If you remove your oil cap and find a yellow murky substance, then you probably have a faulty head gasket. The oil level may be elevated and you will be able to see droplets of coolant inside the oil filler hole. If coolant is leaking past the gasket into a combustion chamber, you will see steam exiting out of the tailpipe, and the spark plugs will foul easily. In addition, the exhaust will be contaminated with the silicate corrosion inhibitors found in the coolant, and your oxygen sensor will be destroyedÃ¢Â"plan on replacing it if you have experienced this problem.
If you can't discover what happened to the coolant, it may be because there was a temporary overheating problem and some of the coolant boiled over. In this case, top off the coolant and keep a very close eye on it. It's not uncommon for overheating issues to suddenly destroy a head gasket.
Checking Coolant Strength & Condition: You should periodically test the strength and condition of your coolant to assure that you have achieved the optimum balance for your Carrera. This is just as important for protection against heat as it is for protection against freezing. An imbalance between water and antifreeze levels will change the boiling point and/or freezing point of the mixture. A 50/50 mixture of water and ethylene glycol (EG) antifreeze will provide protection against boiling up to approximately 255 degrees Fahrenheit (with a 15 psi radiator cap). This mixture will protect against freezing to a chilly -34 degrees Fahrenheit. On the other hand, a similar 50/50 mixture of propylene glycol (PG) antifreeze and water will give you protection from -26 degrees Fahrenheit to about 257 degrees Fahrenheit.
If you increase the concentration of antifreeze in your coolant, you will raise the effective boiling point and lower the freezing point. While this may seem beneficial on the surface, having an antifreeze content of greater than 65Ã¢Â"70 percent will significantly reduce the ability of the coolant to transmit and transfer heat. This increases the chances of overheating. As with most things in life, it's a good thing to maintain a healthy balance.
Beware: You cannot accurately determine the condition of your coolant simply by looking at it. The chemical composition and concentrations in the coolant are very importantÃ¢Â"if the chemistry is off, then your coolant may be harming your engine.
As mentioned previously, it is important to keep the coolant fresh. The main ingredient in antifreeze, ethylene glycol, typically accounts for 95 percent of antifreeze by weight. It does not typically wear out, but the corrosion inhibitors that comprise the remaining 5 percent typically do degrade and wear out over time. Keeping the coolant fresh is especially important with engines that have both aluminum heads and cast-iron blocks.
I recommend that the coolant be changed at least every two years, or every 25,000 miles. I'm not a huge fan of long-life antifreezeÃ¢Â"if these longer-life fluids are mixed with conventional antifreeze (a very easy mistake to make), the corrosion inhibitors react and reduce the effective protection of the long-life fluid. If you do have this long-life fluid installed in your car, only add the same type of antifreeze to the car. Don't mix and match regular and long-life fluid.
Unfortunately, it's tough to determine if your long-life coolant has been mixed or topped off with ordinary antifreeze. Although some coolants are dyed a separate color (like Dex-Cool in GM vehicles), when mixed with standard antifreeze, it typically isn't enough to overpower the bright green color. In general, unless you know the entire service history of your Carrera, it's a wise idea to err on the side of caution and use a shorter service interval for changing your coolant.
Okay, so how do you check the coolant in your system? I recommend using little chemical strip tests that measure how much reserve alkalinity is left within the coolant. The test strip changes color when immersed in the coolant. You can then compare the final color change to a reference chart in order to determine the condition of the coolant. Obviously, if the coolant tests poorly or is borderline, you should plan on replacing your coolant very soon.
An additional noteÃ¢Â"ethylene glycol (EG) and propylene glycol (PG) antifreeze have differing specific gravities, so make sure that you use the correct type of test strip when testing your coolant. Otherwise, you may end up with false readings. EG antifreeze is very toxic to pets and small animals yet smells and tastes pretty good to them, so make sure that you keep old coolant away from them.
Changing the Coolant in Your 911 Carrera
Okay, so I've convinced you that your coolant needs changing. The good news is that it's relatively straightforward on the 911 Carrera. Begin by getting a large drip pan to place underneath your car. My favorite choice is kitty litter boxes, as they are large, are made of plastic, and will hold a lot of coolant. The Carrera six-cylinder engines will hold about 5Ã¢Â"6 gallons, so make sure that whatever container you use is capable of holding all of that coolant.
With your Carrera cold, elevate it on jack stands (see Pelican Technical Article: Jacking Up Your Porsche 911 Carrera) and remove the plastic protective panels that cover the radiator hoses on the underside of the car. Place the heater temp controls all the way to high, turn the ignition to the on position, and turn on the passenger compartment fan to its lowest setting. Do not start the car. By turning the heater on, you are opening the valves to the heater core, which will allow you to drain the coolant located in the core. Move to the rear trunk and slowly remove the radiator/reservoir cap inside the trunk to allow any coolant system pressure to vent out.
Now it's time to empty the coolantÃ¢Â"refer to Figure 2 for the location of the hoses that need to be disconnected. At the bottom of the engine, open the drain plug located in the oil pump housing and let the coolant empty into your large bucket (photo3). When the flow has stopped, replace the plug using a new O-ring and torque to 7Ã¢Â"11 ft-lbs (10Ã¢Â"15 Nm). Next disconnect the two large radiator hoses that feed the supply and return coolant lines to the front of the car. I also like to disconnect the front lower hoses from the radiators as well, but this requires removing the front bumper cover (see Project 68 and Pelican Technical Article: Porsche 911 Carrera Radiator and Fan Replacement).
Reconnect the hoses when all of the coolant has drained using new hose clamps. I used to only recommend the use of good quality German screw-type clamps, but I'm slowly coming around to the annoying spring-clip type that were used in the initial assembly of the car. These clips have an advantage over the clamp type in that they apply constant pressure when the hose expands or contracts as the car heats and cools.
Next, disconnect and empty the coolant from the two heater hoses. Reattach with new hose clamps. Now it's time to refill the coolant and bleed the system. In the rear trunk, on the left side of the engine bay is the coolant tank (red arrow), remove the filler cap (blue arrow) and fill the tank up to the top of the fill level, then flip up the metal clip that opens the bleeder valve (green arrow) (Figure 4). Start the car and run it at idle, topping off the coolant to the maximum level, until no more coolant can be added (Figure 5). Rev the engine, let it settle down, and top it off again if the level decreases. Be sure during the whole process that the car does not exceed 176 degrees Fahrenheit (80 degrees Celsius) while bleeding the systemÃ¢Â"if the car gets too hot it will interfere with your ability to fill and bleed the system to the proper level.
Now, reinstall the reservoir cap and let the car continue to warm up at about 2,500 rpm for 10 minutes or until the thermostat for the front radiators opens up. When the thermostat opens and coolant starts flowing forward, the electrical radiator fans should turn on. Now allow the car to continue to warm up a bit more, revving the car to about 5,000 rpm every 30 seconds or so. Remove the reservoir cap slowly, letting any built up pressure dissipate. There should not be any tremendous pressure built up because the bleeder valve is still open at this time. Top off the coolant in the tank to the maximum level, reinstall the cap, and repeat the process of revving the engine to about 5,000 rpm for another five minutes.
Now, allow the engine to idle for a few minutes until you hear the radiator fans cycle on and off at least once. Turn off the engine, and slowly remove the reservoir cap again, relieving any pressure that might have built up there. Top off the coolant until it reaches the MAX level indicated on the coolant tank gauge, located on the side of the tank. Flip down the metal clip to close the bleeder valve. If you have an automatic transmission car, then replace fuse B1 in the driver's side door kick panel.
I'm sure one question you're about to ask is "What type of coolant should I use?" The Porsche factory manuals indicate that the coolant used inside the Carrera engines should be considered a "lifetime" fluid, and mixing regular fluid with this life-time fluid is not recommended. I prefer to use the Porsche factory coolant in my cars. At $35 a gallon, it can be somewhat more expensive than generic coolant, but the Porsche coolant is not premixed. Plan on using about 19 liters (5 gallons) of coolant for cars without a center radiator. The center radiator cars take about (6 gallons) of coolant. Add one more liter as well if you have an automatic transmission. If ordering coolant for your flush job, I would be sure to order an extra gallonÃ¢Â"you might need them to top off down the road, and the Porsche OEM coolant can be difficult to find in a pinch. The part number for a 1-gallon (3.79-liter) container is 000-043-301-05-M100, and it costs about $35 per gallon from PelicanParts.com.
PLEASE NOTE: The redesigned coolant cap is part: Part #: 996-106-447-04-M100
This photo shows aPicture of the thermostat area of a BMW cylinder head that has been partially damaged by electrolysis. Notice how the aluminum has been eaten away and eroded by the chemical/electrical reactions. The process works somewhat like electrical discharge machines (EDM). These machines work by passing a large electrical current through metal, literally zapping away bits of material until nothing remains. Unfortunately, the electrolysis process works in a similar way, zapping bits of metal in proportion to the amount of electrical current passing through the coolant. A poorly grounded starter can literally destroy a radiator or head within a matter of weeks, depending upon how often the car is started. A smaller current drain, like an electric cooling fan, may slowing erode components over many months.
This photo shows the various lines under the rear of the 996 Carrera. BLUE ARROW: Cooling line to upper right side of engine GREEN ARROW: Coolant line from the water pump on the lower left engine RED ARROWS: Heater supply/return lines. PURPLE ARROW: Radiator Vent line, which should not need to be disconnected.
Under Body Hoses for the 997 Carrera (connected to metal pipes). From water pump (red arrows).To upper engine block (blue arrows). Heater core hoses (yellow arrows). Vent lines (hidden above, green arrow)
This photo shows the removal of the coolant drain plug located in the oil pump housing.
This photo shows the coolant expansion tank (red arrow) the coolant tank cap (blue arrow) and the expansion valve (green arrow). You flip up the metal clip in the bleeder valve to bleed the system. Remember to close it when done.
I personally like to use the Porsche factory coolant, which is a bit more expensive, but specially formulated for the cars. However, if you use a standard off-the-shelf coolant that meets or exceeds the factory's specifications, then that should suffice as well.