Indeed, this is a very common question, and one that is often not easily answered. Obviously, if there is the end of a rod sticking out of your engine case, then chances are, its time for a rebuild. However, with more subtle noises, broken pieces, and poor performance, the rebuild decision may not be crystal clear. In this section, I will provide you with some questions to ask yourself and some answers to common myths, in the attempt to correctly determine whether your engine needs to be rebuilt.
As with any serious medical condition, its always a wise idea to get a second opinion. The same is true with 911 engine rebuilds. Very often, I have heard of unscrupulous (or even over-meticulous) Porsche mechanics who have recommended, or even insisted on a rebuild, when not all of the signs pointed in that direction. Keep in mind that no matter how well-intentioned your mechanic may be, he may have a vested financial interest in seeing you rebuild your engine. Of course, not knowing that youre armed with this book and prepared to do it yourself, he might recommend a full rebuild. Rebuilding engines is a good business, and will guarantee about 40 hours of labor for complete job.
My recommendation is that you take your car to a second, independent mechanic, and pay to have the car evaluated. Have him perform a leak-down test on the engine (see later in this chapter), and let him know up-front that you have a master mechanic friend waiting in the wings to rebuild the engine for you. The goal is to try to get an independent, unbiased expert view of the condition of your engine. Many of the problems with 911 engines can be somewhat subtle, and difficult for a novice to detect and decipher. Ill give you some hints, tips, procedures, and clues to help you in the following sections, but getting at least two expert opinions is always a wise idea.
High Mileage Engines
Each derivative of the 911 has its own quirks and problems. Some of the engines are known for their longevity, and some are decidedly not. Just because your 911 engine has a lot of miles on it, doesn't mean that it's automatically time for a rebuild. With proper care and maintenance, certain 911 engines can easily last 250,000 miles or more. Of course, some years have had better track records than others, but the basic rules apply: if the engine was well cared for, and not abused, then it should last a long time, and gradually wear out. In general, the rule of thumb is that high-mileage is not a good yardstick measurement of engine condition. The methods by which the car was operated and maintained during its life affect the condition of the engine much more than the mileage total.
High mileage engines often show signs of their age in compression and leak-down tests, described later in this section. As the engines age and mileage increase, the small tolerances within the engine slowly become larger. While this usually doesnt result in a catastrophic breakdown, high-mileage engines will gradually see their performance degrade as the mileage increases. Such an engine may be referred to as 'tired'.
Stock engines almost always last longer than modified engines. Higher compression ratios, aftermarket turbos, or superchargers will almost always place added stress on engines and make them wear out or fail quicker. Engines driven constantly on the track may especially show signs of wear. Race engines have such a typically short lifespan that their usage is usually tallied in hours run, rather than miles traveled.
Stock Engine Reliability
The 1978-89 engines are probably best known for having the longest life. Its not uncommon to find a good, well-maintained 3.0L engine running strong with more than 200,000 miles on the clock. Then again, I have seen poorly maintained 3.0L engines with many broken head studs and barely over 100,000 miles on the odometer. Its really a combination of maintenance and the luck-of-the-draw as to whether your 1978-89 engine will be one of the good ones that last or not. Just give you a reality-check, in the late-seventies and early 80s, there were very few cars that lasted significantly past 100,000 miles, so the Porsche 911 engine was indeed way ahead of its time. The 911 Turbo engines also fit into this reliable category, despite the added stress placed on them.
The 1974-77 engines are decidedly the most unreliable of the bunch. Thats not to say that they are a bad engine. They have major design flaws that need to be fixed and repaired in order to become reliable and long-lasting. A properly repaired and maintained 2.7L should be able to last as long as its 1978-89 brethren. However, its very rare to find a stock 2.7L engine over 100,000 miles that has not been rebuilt. The 2.7L engines suffered from a host of problems: pulled head studs, worn valve guides, weak cases, and overheating problems caused by thermal reactors used on the exhaust system.
The 1969-73 engines were generally more reliable than the 2.7L engines. Despite using the weaker magnesium case, these engines didnt stress the limits of the case, and were generally long-lived. 911T engines in particular, with their lower compression heads, seem to just run forever. The higher-compression S models tend to wear out a little faster, but they are still probably good for 110,000 miles or so. One of the bigger weaknesses of these engines were the Biral cylinders, which dont last nearly as long as the later-style Nikasil ones. Again, the longevity of these engines during this era was unprecedented, and helped Porsche earn its reputation for excellent engineering. In addition, with todays modern motor oils, the life expectancy of these early engines is actually increasing.
Failed Smog Tests
Out here in sunny California, we
have one of the most strictest emissions tests in the world. Cars are held up to high
standards that seem to get higher each year. Recently the California Air Resource Board
(CARB) instituted a dynometer test where the wheels of the car are placed on a roller, and
tested for emissions at a specific speed. In addition, the tests monitor hydrocarbons,
carbon monoxide, and nitrogen oxides (NOx). These tests are designed to monitor the
emissions for engine conditions that might produce smog. Unfortunately, as the tests get
tougher and tougher to pass, more 911 engines tend to fail. In some cases, the tests hold
the cars to emissions standards that they were never designed to meet.
Just because your car fails the smog test, doesnt necessarily mean that its engine needs to be rebuilt. In fact, a recently rebuilt engine will most certainly fail the test if it hasnt been fully run-in yet. The best thing that you can do to get your car to pass a smog test is to make sure that it is running perfectly. Most of the time a non-passing car simply has its timing set incorrectly, or has a fuel injection problem. You must make sure that all of your fuel injection and ignition components are working 100% properly before you can assume that the engine mechanicals may be suspect. A compression or leak-down test should be able to let you know if your failure to pass smog is caused by internal engine wear.
Poor Performance and Poor Gas Mileage
When rings and valve guides begin to wear, the result is an increase in burnt oil inside the engine. Also seen is a decrease in compression. Both will have a negative impact on the power generated by your engine. The burnt oil is a contaminant in the combustion chamber and will interfere with the combustion process. The loss of compression will reduce your compression ratio, and limit the power output of the engine. Both will result in poor performance and poor fuel economy.
However, there are plenty of other factors that can affect fuel economy and power. Most notably, the fuel injection system needs to be maintained in top shape in order to achieve the most power out of your system. Make sure that you have eliminated both the fuel system and ignition system as a potential source of problems before you decide you need a rebuild. Also try to isolate and fix other obscure problems that you might not think of. Improper suspension alignment can seriously reduce power, as can improper tire inflations. Brake problems (especially with the emergency hand brake) can drag on the wheels and create some pretty significant drag. The other book in this series, 101 Projects for Your Porsche 911 has more information on fixing these problems.
Strange Engine Noises
Air-cooled engines are designed to expand and contract as they heat and cool. As such, it is very difficult to diagnose strange engine noises that occur when the engine is cold. It is not uncommon for the engine to make some unusual tapping or knocking noises when started stone cold. Its the strange noises that are made when the engine is warm and running that are the ones to watch out for. All engine also tend to get noisier as they age, and clearances between parts inside the engine become larger.
Engine noises are indeed difficult to hear at times. What may be a loud noise from one area of the engine, may in fact be inaudible from another angle. Sometimes sitting inside the car, you will hear more of the lower-pitched noises, as the higher-pitched ones are filtered out by the carss insulation. Closing your eyes when listening to the engine helps to eliminate potential distractions, and allows you to concentrate on isolating the engine noises from one another. An automotive stethoscope is a useful tool for listening closely to the engine. This tool works best when placed against a solid piece of the engine. Local sounds from troubled components can be heard better through the stethoscope because it helps to isolate outside noise. A long wooden dowel is a good alternative to the stethoscope, but be careful not to stick it in your ear, as intermittent engine vibrations can sometimes knock it into the inside of your ear. A piece of rubber vacuum hose will work as well.
There are four basic types of noises that can come from the 911 engine. Intermittent noises occur at irregular intervals and seem to have no reasonable pattern to them. An example would be something rattling around inside one of the valve covers. There are noises that emanate with the crankshaft speed, and occur once every revolution. Then there are valve-train noises which come and go once every two revolutions (the valve train operates at half the speed of the crankshaft). Such a noise would include the rockers and valve noise. This is probably the most common noise heard on the 911 engine, and the fix may be to simply adjust the valves.
A common noise to hear is a loud squeaky noise from inside the engine while running. Such a noise can often be attributed to worn alternator bearings. Take the fan belt off, and run the engine for no more than 10-15 seconds and see if the noise disappears. If it does, you know the problem is with your fan, fan housing, or alternator.
Another common noise is piston slap. This is the sound that the piston makes on its power stroke when clearances between the piston and the cylinder are somewhat excessive. Its a dull-thud clunk that can be heard every two rotations of the crankshaft. Piston slap is most commonly heard when the engine is warming up, before the piston to bore clearances have decreased due to the pistons expanding.
There are a whole host of noises associated with problems such as rod knock, noisy valves, broken rings, chain tensioner failure, detonation, and broken or pulled head studs. Unfortunately, I have discovered that its nearly impossible to accurately describe these noises in writing so that someone can diagnose them. The best suggestion would be to take your car to your mechanic and have him listen to the engine. The 911 engine can be a loud, noisy engine, and if you havent listened to a whole lot of them, your imagination can get the best of you. Of course, listening to other finely tuned 911 engines in cars owned by your friends will help you with an idea of what a normal 911 engine should be sounding like.
Oil Consumption & Smoking
As your engine ages, it will consume more oil. When the engine is brand new, all of the clearances inside the engine are easily filled with a thin film of oil. As the surfaces wear, the clearances enlarge, and oil begins to slip by them. This oil is then burned in the combustion chamber, as it seeps past the valve guides and piston rings. The wider the clearances, the more oil will be burned away. Also, some oils have different viscosities, and tend to burn at a higher rate than others. In general, thinner, lighter-weight oils have a tendency to flow more easily past worn parts in the engine. Use of a heavier weight oil in a tired engine may help to slightly reduce oil consumption.
In addition, excess clearances mean that the oil films that float the crankshaft bearings require more oil to work properly. Looser gap clearances between bearings means that oil flows more easily around the bearing journals. The result is that more oil is required to do the same task, and there is a corresponding drop in oil pressure and an increase in wear. This small drop in oil pressure can sometimes be seen if careful observance to oil pressure readings are taken over the life of the engine. In general, an increase in oil consumption, coupled with a decrease in oil pressure, is a sure-fire sign that the clearances in the engine have increased, and the engine needs to be rebuilt. In addition, the presence of oil in the combustion chamber may have an adverse affect on the combustion process. Oil tends to lower the effective octane rating of the fuel mixture, thus making the engine a bit more prone to harmful detonation.
So how much oil should your 911 engine be consuming? One quart per 1000 miles is about the standard amount for the 911 engine. Newly rebuilt engines with about 5,000 miles on them will usually burn this amount. If your engine is consuming significantly more oil than this, you have a problem. Consumption of two quarts per 1000 miles is certainly cause for concern.
There are two places that the oil can be lost, either past the piston rings, or the valve guides. If the car is excessively smoking, then there is significant oil being burned in the combustion chamber. Air-cooled engines expand significantly when they are run. Its not uncommon for the entire engine to expand more than 1/8 of an inch side-to-side when heated from stone cold to operating temperature. This means that certain clearances that are designed to be optimum at operating temperature are sometimes not ideal when the engine is cold. Oil seepage when the engine is cold is considered normal. Just about every 911 engine smokes when its started, primarily because some oil has seeped into the combustion chamber when the engine was cold. This smoking is not necessarily a sign that the engine needs to be rebuilt. A more accurate test would be to check for significant smoke when the car is completely warmed up.
What smoke should you look for? White smoke is typically caused by condensation in the engine, and is generally harmless when seen on an air cooled engine. Black smoke means that there is a lot of unburned fuel in the combustion chamber that may be a sign that the car is running too rich. In general, blue, sooty smoke is burning oil. If your engine puts out a big puff of bluish smoke when pulling away from a stoplight, its probably a sign that the rings are significantly worn.
Worn rings also produce what is known as blow-by. Just as oil can enter into the combustion chamber, exhaust gases can also be blown into the crankcase when the piston fires. Such blow-by, as it is called, often comes out of the crankcase through the crankcase breather hose on the top front of the engine. This hose connects to the oil tank, and the exhaust gases are recirculated back into the engine through the filler neck on the oil tank. On other cars, blow-by is typically funneled back into the air filter through the positive crankcase ventilation valve (PCV).
Worn valve guides can also contribute to oil loss, although typically less than worn rings. In the mid-1970s, Porsche experimented with new types of valve guides that did not last long at all. As a result, many of the 1974-77 engines had to have their guides replaced at about 60,000 miles. Most of these engines have had this repair done, however, if you find that your engine has not, then you can expect that your guides will be well worn. Worn guides not only leak compression, but also can cause the heads of valves to overheat and break off. This is because close valve guide clearances are necessary for proper cooling of the valve. It should be noted that puffs of smoke on deceleration are usually a sign of worn guides and valve seals.
In addition to the oil burned naturally by the engine, your 911 engine can also lose a lot of oil due to leaks. Many 911 oil leaks drip onto the heat exchangers and are burned off by the high heat. As such, sometimes it's very difficult to gauge exactly how much oil is being burned by the engine, and how much is actually being lost to oil leaks.
Air-cooled engines are infamous for oil leaks. Whether its a Porsche 911 engine or a Volkswagen engine, air-cooled owners will fondly describe that burning oil smell that is characteristic of these cars. To be fair, the air-cooled cars must get their passenger compartment heat from heat exchangers that wrap around the exhaust pipes. If there is an oil leak onto these pipes, then the smell of burning oil will waft up into the passenger compartment. This is the reason why many air-cooled owners diligently try to chase down and repair oil leaks in their engines.
The 911 engine can leak oil from one of many different places. Fortunately, many of these oil leaks can be repaired without tearing down and rebuilding the engine. Project 21 from the book, 101 Projects for Your Porsche 911 details all of the common leaks that can be easily fixed without engine disassembly. If your main goal of rebuilding the engine is to fix some of these major oil leaks, I suggest that you read that section first.
There are a few major leaks that cannot be fixed without major engine work. Crankcase parting line leaks require disassembly, as do leaks between the heads and the cam towers. Leaks from between the chain housing and the cam towers also require major disassembly. Many times a leak will appear to be coming from one of these places when in fact it is leaking from a different point that is significantly easier to fix. Wash the underside of the car and track down all of the easy oil leaks before you decide that its time for a rebuild.
Head Stud Problems
One of the most common failures associated with the 911 engine is the breaking or pulling out of the cylinder head studs. These are long, shanked studs that are thinner in the middle than on the end sections were the threads are located. Shanked studs are typically designed in this manner to achieve a strong, coarse thread while keeping the center diameter of the stud at the optimum thickness often designed with thermal considerations. When you remove your valve covers, you should do a quick inspection of your head studs. The head studs can be seen by looking deep down into the heads in the areas where the spark plugs are located. There are usually two problems associated with head studs they either break off, or they pull out of the case and become loose. Both will result in the head stud barrel nuts falling off, and rattling around in the valve covers. Both are a bad sign for the engine.
The 1974-77 magnesium cases are most known for their pulled head studs. How does this happen? Magnesium is softer and lighter than aluminum. On the very early magnesium cases (1973 and earlier), the engine didnt stress the limits of the magnesium case too much. With the advent of newer, and tougher emissions restrictions in the late 1970s, hotter running engines (hotter engines give off less emissions) began to stress the limits of the magnesium case. In addition, the increase of the displacement of the engine placed additional stresses on the engine cases. Particularly if the engine becomes overheated, it will expand and place additional stress on the threads of the studs that are inserted into the case. These stresses were exacerbated by the fact that the cylinder spigot was enlarged to accommodate the larger 2.7L cylinder. The enlarged bore removed crucial material in the area near where the cylinder head studs are mounted. The result is that the metal threads in the magnesium case yield, and begin to pull out. Once the engine cools down to its normal operating state, the stress subsides, but the studs remain pulled out of the magnesium case. The result is that the barrel nuts on the end of the studs become loose and fall off. Subsequent attempts to re-torque the nuts almost always results in further pulling of the stud. The appropriate torque for the heads can longer be maintained, and as a result, the heads become loose. This often creates a distinctive exhaust leak at the interface between the head and the cylinder. This phat-phat-phat sound is typical of a 2.7L engine with pulled head studs.
How can you tell if you have a pulled head stud? Simply pull off the valve covers (upper and lower), and check to see if you have any head stud barrel nuts floating around the inside the spark plug area. If you do, then chances are good that your magnesium case has pulled at least one stud. If the barrel nuts are still attached to the head studs, take a 10mm allen-head tool, and attempt to tighten them to factory specs (see Appendix X for the specification for your year engine). If the nuts simply spin and spin, then you probably have a pulled head stud. Dont keep tightening it, as you can pull the stud so far out of the hole that it will be difficult to remove the nut later on when you disassemble the engine.
The repair involves a complete teardown of the engine, as you cannot easily and effectively repair the case without the use of a precision milling machine or drill press. The fix for pulled head studs is the installation of what are known as case-savers. These are threaded steel inserts that are installed into the case and reinforce the threads to be stronger than the bare metal threads within the case. The case-saver has a larger diameter than the original head stud, which results in more material contact area to grip the time-sert. In addition, the case-saver is a larger, coarser thread, which means that there is more material between each thread. More material translates into greater strength along the axis of the head stud. In simple terms, the case-saver creates a new hole for the head stud that is stronger and tougher than the original one in the case. Case-savers can still pull out of the case, however, this is usually only seen in high-stress, high-compression race motors, or if the engine has been overheated. Placing the case-savers in the magnesium case should make it more than strong enough for street and most race track use.
The 1978-89 engine cases were manufactured out of aluminum. Porsche realized that the magnesium cases, although lighter in weight, were more susceptible to the heat and stress of larger displacement engines. However, with the stronger case, arose a new point of failure for the infamous head studs. Attempting to better balance the stress of the engine from hot to cold, Porsche used a new steel alloy called Dilavar for the construction of its head studs on the exhaust side of the engine (intake and exhaust for 911 Turbo engines). These studs were designed to have tensile (strength) properties that were similar to the earlier steel studs, however, expanded and contracted more like an aluminum alloy. Matching the expansion and contraction properties of the Dilavar stud to the overall engine meant that the stress on the case, and the stress on the cam towers would be decreased.
It was a good plan, however, the strength of the Dilavar studs did not hold up. Particularly in harsh climates, the studs are prone to snapping and breaking. This occurred only on the lower row of head studs, because Porsche still used the standard steel ones for the upper row, with the exception of the Turbo engines which used Dilavar on both rows. The reasoning was that the temperature gradients on the exhaust side were much greater than on the top of the engine. The studs were somewhat susceptible to the environment. Condensation in the air combined with scratches on the studs can create small pockets of rust that form what are known as stress concentrations. The concept of stress concentrations can be easily explained by comparing it to the opening of a bag of potato chips. The bag itself cannot be easily torn or punctured, however at the top there is a small notch in the material that allows you to tear it open from that point. The v-shaped notch creates what is known as a stress concentration in the material, and makes it easier to break and shear the material apart. In a similar manner, rust or abrasions on the studs can create stress concentrations in the studs. It is not uncommon to see a broken head stud that has snapped at a rusty scratch on the surface.
Stress concentrations aside, the Dilavar studs were not robust enough to hold up to the stress exerted by the larger displacement 3.0L and 3.2L engines. So the big question that many people ask is what stud do I use? Based on the consensus of Porsche engine rebuilding experts that I've asked, it would seem that the original steel studs used on the early cars are the best bet for use in rebuilding any 1965-89 engine. It is very rare to see these original steel studs snap and break. They have also been used in many 3.0L and 3.2L engines without any reported problems.
Another solution to the head stud problem is to use aftermarket RaceWare or ARP head studs. These are aftermarket head studs that are very effective, and very strong, and also very expensive (about $550 per set). If you have the money, or are building a high compression race motor, then the RaceWare head stud set would be a good bet. However, in most cases, the early steel head studs should be more than adequate.
The repair procedure for the broken studs doesnt involve a full rebuild, but instead can be performed only by removing the top end. Unfortunately, the top end accounts for about 80% of the engine, so if youve gone this far, you might as well tear down the bottom end as well. Removal of the old studs can be a big problem, as the aluminum case combined with the red Loctite used to install the studs creates a pretty tight grip. Special tools and a torch are required to remove the studs.
Okay, so youve read about the head studs. Youve checked and youve found that youre one of the many 911SC owners out there driving around with a broken lower head stud. Many 911 owners have a panic attack when they see this, and many Porsche mechanics will instantly recommend a full top-end rebuild to repair the stud. I have heard of engines that will run fine for a very long time with one or two broken head studs. However, the chances of further damaging increases if you drive the car with a broken head studs. While finding a broken head stud doesnt mean that the car will be damaged if you drive it for another 100 miles or so, prolonged use of the car in this condition can cause irreversible damage to your cylinder heads and cylinders. If you find a broken head stud in your motor, then you should start planning for a rebuild in the very near future.
If two studs break on the same head, then its definitely time for an immediate rebuild. In addition, if you can hear an exhaust popping noise when accelerating, its time to rebuild. Driving the car with an exhaust leak can cause the head and cylinder to vibrate and knock against each other. If this problem is left unresolved, then the exhaust leak can end up damaging the head and the piston. In general, if you have an exhaust leak at the junction between the head and the cylinder, its best not to drive the car.
Reading Spark Plugs
The spark plug is really the best way to visually see what is going on inside your combustion chamber. You need to pull out all of the spark plugs to perform a compression test, so you might as well take a close look at them while their out.
While today's modern fuels make plug-reading much more difficult, you can still glean a lot of information from looking at them. A good, well balanced engine will produce a plug that is light brown in color, and dry. If the engine is running too rich, the plug will often be coated with a lot of extra carbon. Keep in mind that the rest of your combustion chamber probably looks the same. An engine running too lean will have a powdery white coating on it, and the outer porcelain ring may have a burned appearance.
When reading spark plugs, pay close attention to the white porcelain ring around the plug. This white area will give you an excellent background to inspect the color of the plug, and to help determine how your combustion chamber looks inside.
If the plug is wet with oil, then that indicates that there is significant leakage into the combustion chamber past either the valve guides or the piston rings. This is generally a bad sign, and an indicator that your compression test may not yield good results.