Engine rebuild 1999, advice

There is a 1999 Cab for sale that has an overheated engine that the owner says needs a rebuild. We have a 911SC, aircooled and have been looking for a CArrera aircooled and have the ability to rebuild these engines. The price on this car is tempting but I have zero information about how difficult this engine may be to rebuild or how many specialized tools are required. I can rebuild a piston aircraft engine but not a turbine engine. Would this be a similar leap in complex design? Another way to ask the question is--- should I keep to the known and find a 1984-1989 Carrera, or is the 1999 doable for the novice?

Jake, Beginning with the water-cooled engines, Porsche took the approach that is an engine went bad during the warranty period, they would replace it rather than rebuild it. Consequently, most 996 engines ('99 - '04) that went bad were required to be sent back to Porsche (core exchange) as part of the engine swap program. While it is possible to rebuild one, I don't think too many have been rebuilt. Mechanically, I am not knowledgeable enough to say it can't be done, but I have not heard of many that have been rebuilt. Maybe someone with more specific information will chime in here. Good luck.

I just did this last week in class (PTAP). I am sorry to say there is no way you are going to get the motor back together without some of the factory tools. Scott

There is an outfit that just started rebuilding them in the US on the east coast. PM me if you want the contact info. Special tools are required if you want to DIY and they make and sell them for shops that want to get into this. There is a very well documented DIY rebuild of a M96 motor (on a boxster) at 986fix.com w/ video instructions and all.

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Ideally, you want to rebuild an engine that just got tired, worn out. Engines that have suffered from overheating, loss of oil, oil pressure, other mechanical failures (IMS bearing failure, cam chain breakage with valves contacting pistons, etc) usually make a poor choice for rebuilding. The engine probably has suffered from some very high stresses or has metal debris throughout. Engine block, heads may not be straight or heads may be soft in places where localized overheating occurred. Then there's the possibility of cracks. I'd use the old engine for a core and buy a new engine from Porsche. If you can find a running but tired engine that has not suffered from overheating, etc. and pull that engine for rebuilding. Sincerely, Macster.

Buy a factory reman engine that will have updates. Replace the parts that also wear out or fail but are not included with the engine: coolant tank, cap, level sensor and updated lines, clutch, flywheel and release lever assembly, ac receiver/drier and other small parts and the vehicle can then be driven. Obviously, it depends on your budget.

The complexity resides in the valvetrain, it requires fixtures to hold the 2 camshafts on each side together and indexed.

Read the last Excellence magazine( Gray Ruf R50 on the cover), at the end there is an article about rebuilding an M96 motor.

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Ah, I kind of see it differently Mac. Overheating isnt that big of a deal, newer cars are running in the "overheat" range of the old, right? Parts can simply be flatness checked for warpage. Were talking temps far lower than melting point.

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not the overheating I was speaking of. I'm talking about an overheat condition from loss of coolant due to failed pump, blown hose, or cracked tank, etc. While the heads and other parts won't melt, the heads in places -- around the exhaust valves of course -- can get hot enough for the metal (alum.) to lose its temper and thus some of its strength. What happens then is the compressive forces holding the head down actually deform the metal slightly and the head bolt or stud/nut is no longer clamping the head in that area with the proper force. This is why a head gasket leak can occur after an overheating event. The gasket fails because the head to block holding force is lost, some of it, enough of it. Also, the heads are subjected to very uneven heating and this can cause heat cracks to form. The upper portions of the cylinders too will likely run "dry" and receive uneven heating, be subjected to extreme localized overheating. A crack may not be, probably will not be, severe enough to notice under cursory inspection though there are techniques engine machine shops use to help bring forth, highlight cracks in aluminum engine components, but the crack is there nonetheless and will only get worse over time as the head expands and contracts from normal heat cycling. You can of course check heads and block decks for warpage, flatness, straightness and within reason machine these back to spec (though any removal of material from the head/block deck of course affects engine compression and camchain slack), but you can't restore the strength of the head metal once it is lost to localized overheating. And you can't fix the crack or cracks that may be lurking inside water passages that you can't even see, get to from outside even with the head removed and on a bench. Not all engines that have overheated are scrap heap material, to be sure, but I wouldn't seek out an overheated engine unless there was nothing better to begin with. Sincerely, Macster.

I know exactly what you mean (we cant use localized heat on any jet engine part) but is it really that critical to the material? I mean, I would think cracking would occur from quickly quenching the (hot) material, not from having different temperatures subjected to the part. Just a thought.....I am no Engineer.....

Its not only cam timing that is going to be a pain in the *** but the 3 wrist pins and clips that you have you do inside the motor (deep inside).

Go here: http://www.lnengineering.com/boxster.html highlight "Cylinder Reconditioning" and look for "watercooled" then "996/997"

Here's an e-mail response I received after some questioning: Thanks for your email- I appreciate it. We have altered the oiling system of the 996 engine in many ways, and this has proven to lessen and in some instances eliminate the starvation issues that you mention. We have lots of possibilities for the 996 engine and hope to offer a spec engine for the 996 class once it�s created. Jake Raby Flat 6 Innovations Hope all this helps you out. Please continue to post on this forum what you find.

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You have two very hot exhaust ports/valves/seats and right next door two relatively cold instake ports/valves/seats, cooled by the incoming charge and fuel spray by the injector. Additionally in an overheating engine some coolant can flow over the hotter areas and cool the area enough to crack. With the coolant level going down coolant will continue to flow, in spurts, surges. Besides, with loss of pressure hot spots will develop and even if coolant flows over these the coolant flashes to steam and the steam acts as a barrier to the cooler fluid flowing past. The localized overheating as I touched upon can reduce the tensile strength of the head material. Even if head straight, flat, not cracked and "reusable", after torquing head down the area that was weakened from the overheating will compress and allow the clamping force to dissipate over time. A head gasket leak is the result. A hard to fix leak, cause even after gasket replaced and head bolted/torqued down again, the whole process starts all over again. The engine can go back together Ok even run Ok for a while but if the starting engine core was subjected to overheating it will almost certainly not run Ok for as long as one might hope for given the cost of an engine rebuild. Sincerely, Macster.

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To me, that is the environment ALL the time. You have ~700 degrees around the combustion chamber with ~1000 degrees EGT's. (Don't quote me on the #'s) Then, you have ~200 degrees of coolant above this area (normal temps). So overheating is what, another ~100 degrees tops in the coolant area and maybe ~100-200 more around the combustion chamber? ~100-200 degrees of EGT? I feel that overheating is kinda like splitting hairs when it comes to the integrity of the parts. Our jet engine parts are not scrapped due to overheating, just tested and reused if crack free. Lets be realistic, if an engine runs out of coolant, it will start to detonate so badly that it will not pull itself around anymore. I know this, I have done it to an '87 Camaro and many a times in old beater trucks off-roading. I highly doubt that a Porsche motor would even be subjected to this because the coolant warning light would of came on long in advance and the person with any brains would of stopped the engine in time. Just my thoughts.

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cooling system the temperature difference between various areas of the head (ignoring the surface of the combustion chamber and the exhaust valve seats and the alum. material immediately surrounding them), and block, are minimal. Coolant is say at 200 degs. F. And this is exit temperature. The heads are cross flow design so the cooler coolant incoming from its trip through the radiators flows to the hotter areas first and then to the cooler areas maybe even warming them up. While valves (especially exhaust valves which can run a dull red color from the heat) are hot this relatively small amount of heat is transferred to the valve seats which have much more mass to transfer their heat load to. And there is considerable coolant flow (cool coolant as I mentioned above) flowing past these areas. And all under pressure to avoid the fluid boiling. In the event of a coolant loss pressure is lost as well. What coolant remains in engine, at least the hotter areas, flashes to steam. These pockets of steam prevent whatever coolant there still remains in the engine from flowing over the hotter parts. While the surface of the combustion chamber may be quite a bit hotter than the coolant, the depth of the heat is minimal as the heat flows through the head material to the coolant passages and coolant just a fraction of an inch away. However, with flow blocked by steam pockets, that heat extends way into the head material. It extends deep into areas that were never intended to experience that kind of heat load. Uneven heating can lead to cracks forming. To add unsult to injury some cooler coolant may be forced over these hot spots. Thus these areas are subjected to sudden cooling. None of the above is any good for the engine. Additionally with coolant flow impeded and what superheated coolant there is may not even make it to the temperature sensor. The sensor can't register an overheating condition because it needs to be immersed in coolant and the water pump was never intended to pump vapor (steam). As for detonation if this is detected the ECU will back the timing down. The engine will almost certainly continue to run and the driver may not even be aware of this timing retardation. If the ECU has to go too far, it will set the CEL engine light. Or if the combustion process is so affected by localized overheating the engine develops a misfire, the CEL will be lit. In fact, in the case of a misfire the the CEL will flash because misfires feed raw (unburned) gas to the converters and they can't handle the additional heat for very long. (Word is, though I've never tested this and no one I know of has, but word is the ECU has a limp home mode that even in the event of say a loss of coolant it will alternate disabling cylinders by not injecting any gasoline to reduce the heat load the engine developes. However, I wonder how well this can work given the coolant temp. sensor may not reflect the temperature of the engine accurately. 'course, there's the oil temperature sensor and perhaps this is used to signal the ECU to enter this limp home mode, if it even really exists. Bosch documentation says it does... but.) I repeat, one can rebuild any engine he wants to, but using an engine, a modern engine, a Boxster engine in this case, for a rebuild that has experienced an overheat condition is just asking for it. Sincerely, Macster.