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Cracked cylinder head - best place to repair?

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My 2001 996 has been depositing oil into the coolant for some time now. After reading everything I can find on the subject, I suspect that it has developed a crack in one of the cylinder heads which allows oil to enter the cooling system.  I am going to get a local shop to help diagnose which head is cracked, any suggestions other than removing and examining both? Car runs perfectly otherwise, so I suspect coolant is not getting into any combustion chamber (a test for combustion gas in the coolant was negative).


While not terminal for the car, it seems like a real pain to drop the engine, remove a cylinder head and send it out for welding.  Does anyone have a suggestion as to who does this repair well?  Ideally, I would be able to swap heads on a core exchange basis. 



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I've heard of several folks who have had 996 head welding and repair by 


Motorcast Components Ltd in Shrewsbury, Shropshire, is one of the uks leading automotive cylinder head suppliers. cylinder head repairs, extensive parts supply service, competitive prices and...

 with success.  Not exactly local though......

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"While not terminal for the car, it seems like a real pain to drop the engine, remove a cylinder head and send it out for welding."


 Well now, that depends on how long "for some time now" refers to.  Is there any coolant in the oil sump? If so, there's a good chance that every time you start or drive the car, you're doing damage to other engine components. Yes, it's a real pain to drop the engine and remove the cylinder heads - but what's the alternative? Drive it as is? - Not a good idea. You'll also need to flush the entire cooling system, as you've now got a oil/coolant mixture in all the rubber hoses. The oil will cause the hoses to swell and become soft - not good for a pressurized system. If you flush them with detergent asap (to break down the oil) you may be able to save them. Good Luck

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  • 2 weeks later...

No coolant in the sump.  Driving also does not seem to consume any noticeable oil. The car did have a failed oil cooler when I got it - there is no question oil got into the cooling system, I replaced it as well as all the hoses.   As oil circulates  at a much higher pressure than the coolant, there is no question that oil would get into the cooling system. 


One idea of a local technician I know is that the oil in the cooling system may be significant in amount, but residual from the time spent with the broken oil cooler. He suggested flushing it, repeatedly.  


I have now flushed it 7 times over 2 weekends, using 4 cans of rad flushing fluid in the process (last 2 flushes with water alone).  The drained fluid has gone from red to pink to light beige, with diminishing amounts of what looks like old, dark oil in it (none in the last flush) although a small layer of white foamy scum always settles on the top of each sample. I drive it with the heater on for about an hour between flushes and unplug various hoses to push excess water out, using a compressor each time. 


I may just try the garden hose until it runs clear and try my luck. All else fails, I think I can pull the cylinder head myself (if I can figure out which one is cracked).  



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Have you had the head(s) diagnosed & repaired yet? If not, I think you're chasing your tail...Putting your cart before the horse etc... etc... etc... Just because  you haven't gotten coolant in the oil YET, doesn't mean it's not going to happen. Every time you heat cycle the engine, the chances of that head crack opening up/getting worse increase. You have to fix the problem, not just treat the symptom - it's not going to get better by itself. Good luck

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How can I diagnose the crack in a head without removing the engine, both heads, etc.? What percentage of 996 heads crack?  If it hasn't yet in 19 years it may not.  Even if I have them both welded, they can just crack elsewhere, no? Can I check it with a cooling system pressure check to see if it holds pressure?


My current strategy is to flush the system completely until no oil can be found, drive it for a while longer and wait to see if the issue reappears.

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You are correct sir! You'll need to remove the heads - I'm pretty sure (but not positive) you'd need to drop the engine for that. Head cracks are somewhat common for the M96 - much more common than your oil cooler/heat exchanger. I didn't realize you were in Dubai until now... That could have a real effect on how to deal with this problem. When properly repaired, the chances of the head cracking again are extremely thin. Do some reading here


M96 cracked cylinder head repair

 ^^^ These guys are the tops in the USA - not sure who you can find over there. I think a pressure test could be useful to you if done properly... 

Better yet, see here


Click here to see video of a  996 3.4 M96 head being pressure tested, revealing a microscopic crack that was not visible to the naked eye. This particular head was a prime example that a head can not be judged to be crack free without being pressure tested. I know where to look, but I…


Edited by dporto
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  • 2 weeks later...

OK, so replacing NPG coolant (which runs under no pressure) with straight water in an attempt to flush the cooling system proved beyond any doubt that there was a crack in the head.  In short order the 4-6 valve cover developed some oil water mix.  So the engine is out and I am about to remove the cylinder heads and send them to repair at Hoffman. Thanks for the tip. 


Along the way I will swap out the old coolant hoses which were exposed to oil.  Should I replace the (1 year old) AOS and oil cooler as well?  I want this oil in the coolant issue behind me. 

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Always a good idea to replace the AOS if you've got the engine out (fairly cheap part and extremely difficult to do with the engine in). There is a new product available from a guy on Rennlist called "the Ultimate AOS" (UAOS). 


996 Forum - The "Ultimate AOS Solution" - OK, I promised the Ultimate AOS Solution that would eliminate replacing the Factory AOS, but still function as designed. I will show the process here. I have even enhanced it further with a couple more small but vital components/fail-safes that are even way cooler...



Perfect timing! Good luck with the head repairs 👍

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I did get both heads off yesterday, pretty easy, actually. No visible cracks, but the AOS vent line to the 4-6 bank is filled with oil / water intermix.  Weird. I cannot pressure test the head myself, so will see what Hoffman says. Actually, cam journals, bores, combustion chambers, etc all look really good for a 130,000 km 2001 car. No scoring, all pretty clean, not much carbon build up.  Now to build those shipping boxes...

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Yes, there is an actual product - should be near the end of the thread. It is developed by a guy on Rennlist (Porschetech3) he is having them made and marketing them himself. It utilizes the stock AOS but moves the diaphragm (the part that actually separates the oil from the air - hence having to seal the stock unit) to a location in the engine bay which makes it a easily serviceable/affordable part. The diaphragm itself retails for between $20 - $75 - depending on brand. He's bought and tested several different units and done comparisons. 

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