Using the expertise gained from years of designing and building engines, Porsche developed what is known as the M96 engine for the introduction of the Boxster in 1996. The horizontally opposed engine was developed with a throwback to the traditionally air-cooled motors, having kept the opposing cylinder or boxer layout. The engine was designed from the outset with the goal of providing a common platform for both the Boxster and the upcoming Porsche Carrera. The motor was indeed scalable, encompassing a displacement that ranged from 2.5 liters all the way up to 3.8 liters at the end of its production run.
The M96 (and subsequent similar M97) water-cooled engine is definitely a strong-performing engine, however along the stages of its development it has suffered from some design deficiencies that have been identified and corrected by Porsche over the 11-year lifespan of the engine. It's not uncommon to find a car listed for sale with "new factory engine recently installed" in the advertisement. No one but Porsche knows exactly how many engines were replaced under its recently discontinued engine exchange program. However, simply swapping out a broken engine for another one does not address known weaknesses in the engine due to design deficiencies. The purpose of this project is to identify some of the problem areas of the engine and offer up solutions on how to fix and/or prevent any damage from happening to your engine.
Rear Main Seal Leaks: When the M96 engine first came out, it was perhaps most known for its rear main seal (RMS) leaks. While some of the RMS problems were probably actually intermediate shaft cover leaks (see next section), there were definitely some problems with the seals on the early cars. For the most part this was a "cosmetic" issue, as the leaks did not tend to affect performance, unless they became so severe that they began to affect the proper operation of the clutch. But many engines were torn apart and/or replaced by Porsche under warranty due to this problem, because when you pay $75,000 for a high-performance sports car, you expect it not to leak.
It's not really 100 percent clear what the causes the leaky rear main seals. One cause may be the fact that the crankshaft has insufficient support on the rear end. It also may be caused by the fact that the crankshaft carrier support is only pinned minimally in one plane to the outer case. This can lead to shuffling of the carrierÃ¢Â"shuffle pinning the crankshaft carrier as is commonly done when prepping an early 911 engine for the track can help the problem.
The seal has been updated to a 997 "Cayenne-style" part number since the engine was originally introduced, and for the most part the engines no longer leak from this area when this new and improved seal is installed. If you find that your engine is leaking from the rear main seal (also known as the flywheel seal), then simply install a new one while adding a little bit of Curil-T sealant to help keep it dry (see Photo 10 of Pelican Technical Article: Clutch Replacement on the Porsche 911 Carrera).
Intermediate Shaft Bearing Failures: The intermediate shaft bearing is probably the most troublesome of all of the M96/M97 engine problems. The intermediate shaft bearing (IMS bearing) supports the intermediate shaft on the flywheel end of the motor. Porsche designed these motors using a sealed ball bearing that is pressed into the intermediate shaft. These types of bearings are typically used in devices like copy machines and other machinery used in dry conditions. In theory, the area where Porsche designed the bearing to sit is supposed to be dry. However, after years of use within the engine, it would appear that oil and contaminants from the engine seep past the bearing seal, wash out the original lubricant, and become trapped inside. The result is that the bearing now operates in a less-than-ideal environment and begins to wear prematurely. When the bearing wears out, the timing chains on the engine may disengage, and the engine will quickly self-destruct. When the bearing does fail, foreign object debris from the bearing circulates throughout the engine, causing further damage to other areas in the engine.
On the early cars, Porsche also used a center bolt to secure the IMS bearing that was too weak and sometimes snapped. If this bolt breaks, then the intermediate shaft begins to float around in the bottom of the engine, and you can soon experience catastrophic engine failure.
This area is also highly prone to leaks. The seal around the intermediate shaft cover can leak, and it has since been updated and redesigned to prevent leakage. In addition, the three bolts that hold the intermediate shaft cover are through holes that exit into the cavity of the engine case. You must coat these bolts when reinstalling them in order to prevent oil from leaking out through the bolt holes. In general, if this area is leaking, it may indeed be a sign that your intermediate shaft is failing and you should inspect it immediately.
The good news is that the IMS bearing problems are all fixable, thanks in part to a retrofit kit that can be installed with the engine still in the car. See Project 14 for full instructions on how to update your engine.
Cylinder Liner Cracks: In an effort to reduce costs during production, Porsche utilized a type of insert-mold casting process to directly incorporate Lokasil cylinder liners into the case. While this is a neat way to reduce the total number of parts used in the engine, this design basically casts a wearable part into the engine case. There is no factory replacement for the linersÃ¢Â"when they wear, the factory expects you to buy a new engine case. In addition, the design of the cylinder liners allows them to "float" within an area filled with coolant.
Excess vibration and twisting from the normal operation of the engine appears to be causing some cracking in these liners, resulting in a small chunk of the liner breaking off. This "D-chunk" problem seems ironically to occur mostly in gently driven cars. 911 Carreras that are driven hard at the track or on the street do not tend to see this type of damage. At least with respect to the track cars, one theory is that these cars tend to have their oil changed much more often. The problem affects mostly the 2.5 Boxster and Carrera 3.4 engines. When this failure happens, you will see oil and coolant begin to mix together or a slight unexplained coolant loss.
If your engine experiences this failure, it can be rebuilt using LN Engineering's Nickasil liners installed. They take your old case, machine out the cracked or damaged Lokasil liners, and install an aluminum Nickies insert, which is stronger and more reliable than the factory cast-in liner. In addition, with the installation of the liners it's fairly easy to increase the bore of the cylinders, which translates into increased displacement and more horsepower. If you go this route, you will also need to use some aftermarket pistons and perhaps update the software in your DME to accommodate the larger displacement.
Engine Casting Porosity: As mentioned in the previous section, Porsche used a new cost-effective method to cast most of the oil and water cooling passages directly into the engine case. This reduced the total part count for the engine and also helped reduce assembly time and production cost. Unfortunately, the advanced casting technique seems to have led to a number of engine cases experiencing what has been called "engine porosity." There is not a lot of information available on this problem, but it seems to be related to problems with the initial casting process.
In some cases, there appeared to have been a leak through the internal crankcase walls. The process of pouring the molten aluminum must be tightly controlled, otherwise pockets of air forming in the aluminum may result. Most of the time, post-casting inspections will reveal these flaws, but apparently some were still manufactured into running engines. The result is that oil and water became mixed within these engines. This resulted in coolant being found within the oil (turning it a milky brown color) or oil being found inside the coolant tank.
The expansion and contraction of the engine due to the heat of normal operation can expose this problem as well. I have also heard of engines that simply wept a slow bead of oil right through the walls of the engine case when running. Unfortunately, there's nothing that can be done to fix this problem, short of scraping the engine. The good news is that most of these problems were discovered on the cars when they were new, and the engines were since replaced under warranty.
Chain Tensioner Failures: There's been some chatter lately about chain tensioners failing on some of the M96 motors. If your car is noisy on startup and then suddenly quiets down, it may indicate a problem with your chain tensioners. Porsche updated the design of the tensioners in 2000 (TSB Group 1 NR 8/00) and replaced them with an improved design. I recommend that you update and replace your chain tensioners if they are the older style. See Project 16 for more information on how to identify and replace them.
Cylinder Head Cracks: In general, the cylinder heads are pretty well designed on the M96 engine. However, on some 3.2, 3.4, and 3.6 engines, small cracks can sometimes develop around the seats of exhaust valves and extend to the spark plug hole. The mounting point for the cam follower housing is also a weak point. Often these cracks can lead to coolant and oil mixing together. This is not an uncommon problem with automotive cylinder heads in general and can often be repaired by a skilled machine shop that can weld aluminum heads.
Oil System Inadequacy: The air-cooled predecessor to the M96 engine incorporated a dry-sump system that was designed to keep a significant amount of oil in reserve for extended performance driving. With the introduction of the M96 engine, Porsche moved away from that design, primarily due to the high cost of implementing a separate dry-sump system. The M96/M97 motors instead were designed with a compromise system, which has an oil sump built into the bottom of the engineÃ¢Â"a kind of hybrid between a dedicated dry-sump system and a typical wet sump. As a result of the lower oil holding capacity and other factors, the M96/M97 engines tend to suffer more from oil starvation problems, particularly during high performance driving.
There are a few things you can do to protect against oil starvation problems. Firstly, be sure that your oil level is always at the high-level mark of your dipstick. The M96/M97 engine doesn't have a vast extra supply of oil, so if you're a quart low, it's a significant amount. You can also add in a deep-sump kit (see Pelican Technical Article: Installing a Deep Sump Kit - Porsche 911), which will expand the oil capacity of the sump by about a half a quart. Finally, you can install an Accusump oil accumulator system that will protect against unexpected oil pressure drops (see Figure 5 of Pelican Technical Article: Track Prep for the Porsche 911 Carrera).
In addition to the standard issues associated with the non-traditional sump system, the flapper windows on the bottom of the engine are manufactured out of plastic and can break off inside the sump and clog the oil pickup tube. This leads to oil starvation and complete engine failure. The solution is to remove the bottom sump and inspect the oil control windows (see Pelican Technical Article: Installing a Deep Sump Kit - Porsche 911) and replace them if they are missing or damaged. You can also add aftermarket stainless steel windows for added protection.
As mentioned previously in Pelican Technical Article: Oil Change - Porsche 911 Carrera, I do not care for Porsche's standard recommended oil change interval of 15,000 miles. The oil in engines tends to become contaminated with fuel and coolant, particularly as the cars age and seals and piston rings begin to wear. I generally like to run a thicker oil and change it every 3,000 to 5,000 miles. Keeping the oil fresh may help prevent some IMS bearing issues and generally prolong the life of your engine.
Connecting Rod Bolt Failures: The connecting rods that are used on the 911 Carrera are forged out of steel and utilize what is known as a cracked-rod design. This means that the rods are forged and machined, and then broken along pre-set stress points. Then the bearings are installed and the rods are put back together again. This cracked-rod design is cheaper to manufacture, and the rod bolts don't need to have integrated "guide" pins as part of their design (like the rod bolts used on the older air-cooled engines).
Unfortunately, it would appear that the rod bolt diameter may be too small for the large loads that these engines place on the rods. A number of recent failures in some early high-mileage engines have hinted that the rod bolts are too small and may be a failure point for the connecting rod.
The failure occurs when the engine is consistently revved at the high end of its rpm range. The stock rod bolts are designed to stretch and permanently deform when tightened down to their final torque values. At rpm's of 6700 or higher, the rotating mass on the end of each rod (namely the piston and the mass of the rod itself) has a tendency to stretch the rod bolt further. Repeated stretching of the rod bolts causes them to deform and loosen up which can result in rod separation and complete engine failure.
Unfortunately, due to the cracked-rod design, the connecting rods cannot easily be rebuilt. The solution is to install aftermarket connecting rods that can accommodate larger, race proven fasteners like the ones available from ARP.
Variocam Solenoids: It's not uncommon for the variocam solenoids to fail on one side, which will result in a uneven or lopey idle. Moisture can get into the mechanism causing it to corrode and eventually fail. The DME computer should easily be able to detect this failure and trigger a check engine light (CEL).
Paper Oil Filters: The oil filter system on the 911 Carrera is a bit lame in my opinion. Using the stock paper filter can lead to a disintegration of the filter, which can then clog the oil passages of the engine. Although this is a relatively rare problem, I have heard of it happening with cheaper-brand oil filters. Stick to the good-quality brands, and also consider upgrading to the LN Engineering screw-on oil filter upgrade (see Pelican Technical Article: Oil Change - Porsche 911 Carrera).
Air-Oil Separator Failures: The air-oil separator is an emissions device that draws vapors from the engine crankcase and then sends them back into the intake manifold. When this unit fails, the result is oil sucked out of the engine and into the intake. While the air-oil separator will not cause immediate mechanical damage to your engine, it may make it smoke tremendously and/or run roughly (see Pelican Technical Article: Porsche 911 Air Oil Separator).
Oil Change Intervals: The best recommendation to any modern Porsche owner is to change their oil more often, say every 5,000 miles, as well as use a minimum 5w40 viscosity oil. On cars not equipped with Variocam Plus (2006 and earlier), use of a 15w50 or 20w50 provides a much higher film strength, which will improve internal bearing life considerably.
Low-Temperature Theromostat: Also useful is the LN Engineering low-temperature thermostat (see Pelican Technical Article: Porsche 911 Carrera Water Pump and Thermostat Replacement). This specially designed thermostat starts to opens at 160 degrees F instead of 187 degrees F for the factory unit. Keeping the oil cool is key to keeping it fresh, and the lower operating temperatures also help give you a bit more horsepower too.
This photo shows the flywheel end of an M96 motor. This particular motor is a 3.4-liter crate motor from Porsche that has been updated with the latest and greatest improvements from the factory. The intermediate shaft has the updated bolt (yellow arrow), but the issue of the intermediate shaft failures has still not been completely addressed in this redesign. Although these updated engines tend to suffer from fewer problems than do the early ones, there is still a risk of IMS bearing failure. The rear main seal shown here is the updated and improved one that should not leak.
This is the infamous "D-Chunk" problem that happens sometimes on these motors. Occurring almost exclusively with the two cylinders in the middle, it is theorized that excess twisting and vibration causes cracks to occur in the cylinder walls. Found mostly on the 2.5 and the 3.4 engines, this problem is almost non-existent on the 3.2-liter engines because the 3.2 has thicker cylinder walls. The solution is to machine the case to accept cylinder liners (next photo).
Here is a fully repaired M96 engine with the LN Engineering Nickies liners installed. The case is prepared by machining out the old liner and completely removing the section that floats in the water jackets. Then, a Nikasil cylinder is press-fit into the case in its place. It's a clever solution to the irreplaceable case problem. The case itself must be a good rebuildable core and have no major issues (no major damage, no case porosity issues). In addition to replacing the liners with new and improved ones, the process allows you to increase the displacement of the engine at the same time.
Although it may not look like much, this crack is enough to cause a lot of problems. Cracks like these can cause oil/coolant to mix, resulting in contamination of the coolant and/or oil. Cylinder head cracks are common among water-cooled cars and can sometimes be repaired by grinding down the head and then re-welding it.