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Quote:
Originally Posted by JET951
Yes we do agree some more regular servicing would have been a great thing but what is done is done and we are just glad this block has no damage. We actually knew the previous owner of the car before the current owner who does want to bring the car back up to a drivable standard. If I can offer one statement is that the current owner just didn't know what was needed to keep them up to drivable standards and may have been under the impression that if it wasn't being driven then it may not have needed the regular maintenance. Its a tough lesson to learn.
Hi Greg,
Im quite surprised by the lack of corrosion on the heads, I thought there was going to be more. I don't believe that the pitting of the aluminum is either completely caused by galvanic corrosion, crevice corrosion, bad coolant, or even pitting edema.
The fact that there are different levels of damage, from one engine to the next, makes me think that the aluminum deterioration is from a combination of factors.
And I don't care what it is called....I just know that I have to deal with whatever is there.
Absolutely Fred,
Here are the pics from one gasket, the other gasket had just deteriorated into pieces as seen in the pics above
What's interesting is that you can see the Block side of the gasket had deteriorated quite a lot where the coolant can flow along it as you can see the brown outline of the block imposed on the gasket as seen in pic 1,2 and 3.
Pictures 4,5 and 6 are from the head side of the gasket.
Again this really is an indication that the gaskets should be changed between the 15-20 years time frame somewhat irrespective of KM's travelled.
I agree completely!
At this point in time, owners with late model 928 engines (which have not had the head gaskets changed) are playing a "poor odds" game of "Titan Submarine" (Formerly known as Russian Roulette, until a couple of weeks ago.) One thing you may have not checked is the size of the bores....especially on the back cylinders.
We've had several of these engines, with head gaskets as bad as these are, with too much wear on the rear most cylinders.
The "coolant flow" gets "short circuited" though the holes in the head gaskets and the rear cylinders simply run too hot and have increased wear.
This is a huge problem to repair, once you find out that Porsche doesn't have oversize pistons..... Notes, in your text, above.
Originally Posted by GregBBRD
Notes, in your text, above.
Good info Greg and I totally agree here also regarding gasketchange no matter the mileage. This is a complex problem and we have many variable factors in play. Even the composition of the castings themselves would have a play here. You cannot subtract this to a precise chain of events that leads to an exact result. Period. It's like a long no/yes/and/or schematic that has 100 different outcomes...
Good to see you putting some thought into what you are seeing in front of you and your logic is apparent however I would politely suggest the logic is flawed and I will expand on that.
The major logic flaw in your assessment is that the main body of the coolant can eat away in discrete locations on the block side but if such really were happening the entire wetted surface of the exposed surfaces would be chewed away and that is just not the case. Just take a look at the first photo you posted of the block side - one can see very clearly that the gasket has been delaminated because of what was going on inside the gasket. As I stated in point 13 of my analysis once the gasket is breached the gunge can migrate along the tiny gaps formed by the sheet material where there is no compression, the reinforcing mesh corrodes, the resulting volume of corroded product increases, the gasket delaminates and the gunge can and does migrate along the inside of the gasket. When the gasket is compressed either side by the head and the block this does not seem to happen and thus why the periphery of the gasket is generally not breached and comes out intact with the original print colouration intact as can be seen in your photos. Now perhaps I can suggest a simple test- take a section of the peripheral gasket residue that is still intact and try flexing it gently- does it remain intact or does it crack?
The photos you published show without a shadow of doubt that coolant is penetrating between the gasket and the head and this is indicated by the bleaching of the ink in areas where there is no obvious surface damage head side and one can also see signs of the orangy discolouration from the mesh underneath being corroded and thus breaking through to some extent. When one sees serious damage to the head like the path that cuases the leak one invariably sees the gasket damaged adjacent to that location. The problem in these locations is that once it starts there is nothing to stop it short of dropping the coolant and thus disrupting the electrolytic curent flow. On the other hand is the requisite potential is not reached there will be no localised severe damage from crevice corrosion.
Indeed if one looks carefully at the undamaged surfaces on the head side of the gasket one can see lots of little dots which is an indication of something going on below the surface- the dots being the location of the barbs. This of course tells one that once the degraded coolant has breached the gasket material it can and does migrate along the length and breadth of the gasket and once that happens it is somewhat random where it breaks through. In the case of the 5/8 gasket this process was extensive thus why for cylinders 5, 6 and 7 there was literally nothing left of the gasket material other than the outer periphery and the fire rings.
I also believe there is another issue with this design in that where there are openings in the gasket for coolant to flow through, sooner or later the coolant simply seeps between the two gasket sheet layers that are not in compression, stagnates and eventually acidises thus why for instance we sometimes see the gasket material eaten away local to those metering holes in the gasket .
As I indicated in a previous discussion modern Porsches post the 928 generation have the coolant passages in compression from both sides of the flow conduit and this I suspect is no coincidence.
I truely understand what you are saying Fred and I completely agree that coolant does get behind the head gasket on the head surface where it shouldn't be which causes havoc. But I also truely believe due to degradation of the head gasket by means of age/coolant health/ heat cycles/ overheating events etc that at some stage and maybe even before the coolant gets behind the gasket that it is also able to be soaked into the block side surface of the head gasket. The gasket is prone to failure with age or Km's/Miles which is my original point.
997/997 are open deck design line the 928/944. We have seen corrosion happen on these heads as well but they are not as old as the 944/928 so not as much. We have used the Cometic MLS guilty layer steel gaskets on the 944/928 series for many years although we do prefer the original gasket for street use as they never leak externally
Regards
Sean
I truely understand what you are saying Fred and I completely agree that coolant does get behind the head gasket on the head surface where it shouldn't be which causes havoc. But I also truely believe due to degradation of the head gasket by means of age/coolant health/ heat cycles/ overheating events etc that at some stage and maybe even before the coolant gets behind the gasket that it is also able to be soaked into the block side surface of the head gasket. The gasket is prone to failure with age or Km's/Miles which is my original point.
997/997 are open deck design line the 928/944. We have seen corrosion happen on these heads as well but they are not as old as the 944/928 so not as much. We have used the Cometic MLS guilty layer steel gaskets on the 944/928 series for many years although we do prefer the original gasket for street use as they never leak externally
Regards
Sean
Sean,
Any gasket that loses its elasticity will fail and age/heat cycling will do nothing to improve that but the designers would have known this full well. As I stated earlier, it is obvious to me that the designers knew full well that coolant was going to get behind the gasket and where they failed was in not understanding the potential consequences of this. I have yet to see any signs of the block side of the gasket allowing coolant to permeate through it and from my point of view this example is compliant with that logic thus why one can clearly see that the gasket has deformed from the inside pushing outwards to the block side where there is nothing to restrain it- at some point when there was little to nothing left of the gasket integrity the residual gasket material gave way but basically there would have been nothing left of the binder material at that point. This is why I suggested peeling back the residual material- I suspect you may well find no trace whatsoever of the reinforcing mesh that probably disappeared some time ago. Obviously such has happened on the 5/8 bank gasket thus why it disintegrated around cylinders 5,6 and 7.
Ergo- the gasket is not failing due to age as is the "common perception" rather it is failing due to an acidised chemical attack and no "coolant" in its natrural state is going to cause the problems like those seen in this example. If the problem was solely due to ageing degradation every gasket would show the same behaviour pattern over the entire wetted surface sooner or later and that is just not the case. The randomness of the attack is a dead giveaway as to what is going on.
If the system can survive for 20 years without any corrosion issues then the gasket owes the owner nothing in my opinion. Unfortunately there have been way too many examples that have caused their owners serious problems prematurely as I saw over here where 10% of the S4/GTS fleet suffered major problems prematurely [12 to 15 years of age] but the small fleet size [about 35 in total] cannot be considered statistically relevant.
The crevice corrosion attack of the head is something that appears to follow as a consequence of these attacks and once initiated can be deadly. In this instance the only serious corrosion location [that caused the presumably external signs of distress] can be considered fortuitous but such is the randomness of this issue.
The 996 and 997.1 non turbo models are indeed open deck design - it is my understanding that coolant crossover passages are not like those in the 928/944 design but then I am not overly familiar with them either. My Cayenne Turbo S vee 8 is a closed deck construction so hopefully not a problem from that perspective. Not really relevant but the Hartege [a UK based independent Porsche specialist] solution to the 996/997.1 bore score problem is to convert the motor to a pseudo closed deck construction as I learned from an episode of Wheeler Dealers a short while ago- interesting in and of itself.
Very lucky owner...... The $300 used engine I put in the 928 Estate lemons racer turned out to have a bad headgasket and "extra" cooling from a corroded head, found AFTER it blew up a rod bearing.....300ish hours on track with this headgasket"
07-04-2023, 08:19 PM
