|Mechanical and Technical Mechanical and technical topics, help, and discussions|
|August 28th, 2005, 09:03 AM||#1|
Join Date: Feb 2005
RMS leaks and you...
Originally posted on the PCA website... the part in bold is very interesting.
I have just noticed an oil leak from the rear of my engine by the transmission. Would this be a rear main seal leak? If so can it be fixed?
The Rear engine oil leak seems to affect close to 100% of all the Boxsters and 996's at one time in its life. Most of these leaks are viewed as a rear main seal when in fact they are several different ones. Do to the reputation of the rear main seals any leak in the bell housing are has been considered one. They include the bearing cover, rear case bolts, porous case and of course the rear main seal.
The porous cases mainly affected the early Boxsters MY97 and some MY98's. This was do to trapped air in the die at the time of manufacturing.
The bearing covers affected primarily all the Boxsters up to MY99 and to include the 996 for MY99. They utilized an O-ring for sealing the bearing cover and standard 6mm bolts. These O-rings tended to fail at a high rate. These O-rings were replaced in production with another bearing cover that incorporated a 3 way sealing ring. Basically it was like getting the sealing capabilities of 3 O-rings side by side. Should the sealing surface area of the first one fail you still had the second and third wiper to stop the oil. The 3 bolts that hold this bearing cover in were also replaced with micro encapsulated ones. These would seal the threads on installation and stop the oil from wicking through the case.
Now the number one leak that continues to be an issue to all of the 996's and the Boxster's (986) is the rear main seal leak. This issue has been more problematic with the manual transmissions than the automatics. This makes sense when you understand the actual design of the engine. The crankshaft is mounted in a cradle that is intern bolted between the case halves. The tail of the crankshaft is also unsupported do to the chain drive for the intermediate shaft. Now at this point add the weight of the clutch and flywheel to the end of the crankshaft. This now can either cause the crankshaft cradle assemble to slowly move in a downward position or allow the seal to move and fatigue. A seal can be replaced and work effectively should the crank assemble still be ok when checked. Porsche has a tool number 9699/1 that slides around the crankshaft and into the case. Should the tool not insert then the engine would be replaced. This replacement was at a high rate do to the design of the tool. Since the tolerance was so tight you could not insert the tool at times. This was do to the trapped air at the end of the crankshaft. The problem was easily cured by drilling a 1/8" hole in the face of the tool , thus allowing a proper check.
Now to get to why the Tiptronics (automatics) leak less. Remember that unsupported weight at the end of the crankshaft with the manual transmission earlier? This is supported now by the Tiptronic. Granted the torque converter is also bolted to the crankshaft, however it is supported by the oil pump bearing. This extra support takes the load off of the crankshaft and reduces the oscillations to the seal and the crankshaft carrier.
The next question is what to do about an actual rear main seal leak. One is to replace the engine should the crankshaft fail its location with the 9699/1 tool. Lets say the crankshaft checks out ok then what do you do besides installing another seal. Replace the rear case bolts and intermediate shaft bearing if it falls in the early range of cars. Another well proven repair is to epoxy the threads on the back of the case bolts and the case halves just above and below the seal. This seams to retain the position of the seal to a very high success rate. The epoxy will not stop the possibility of the carrier from moving, however it will in large keep another seal from leaking.
Another option is to use the Cayenne V8 rear main seal. This seal is made to the same dimensions as the currently listed seal for the 986,987,996 and 997. These are the same type of construction as used in the GT. When installing this type of seal it is extremely important not to damage the sealing lip that contacts the crankshaft. This seal is not as pliable as the stock rubber seal that is currently used in all of the Boxsters ,996's and 997's. The benefits have been so far very good. None of the cars that have had these installed have had a return of this leak either.
Porsche has lightened the 987 and 997 crankshaft in the rear by counter drilling Ѕ a pound off of the flywheel side. Granted this would lighten the mass in motion for quicker revving but do little to keep the rear main seal from leaking given the causes. The lightening of the clutch or the flywheel assemble would greatly reduce the leaks but not fix them. Unfortunately to lower the RMS issue down to an acceptable failure rate that earlier cars had such as the 911,964,993,928,944 models, one would have to redesign the engine. Then again all of these had there own gremlins to deal with.
Scott Slauson - PCA WebSite - 7/24/2005