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the very first signs of saab headgasket failure.. long.
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Posted by trog100 [Email] (more from trog100) on Sat, 23 Nov 2002 22:42:12 Share Post by Email
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basically my theory goes thus.. all saab headgaskets seem to fail sooner or later.. i think they begin to fail long before most folks think "headgasket".. as the seemingly inevitable erosion of the gasket material between waterway and cylinder progresses.. a very small pinhole type leak occurs.. in its first stages this leak only occurs when there is high cylinder pressure.. ie when the engine is under heavy load.. lets say 80 mph or so highway driving.. so a high pressure but very small quantity of exhaust gas starts to enter the cooling system.. over say ten minutes or so this gas builds up around the T-stat housing and (possibly) water pump area.. it builds up there because of the height factor.. the t-stat housing makes a perfect airtrap.. hence the need for a bleed valve on top.. in essence it forms an airpocket.. the pressure between the t-stat airpocket will slowly equalize with that of the normal airpocket that lives at the top of the expansion tank.. when this slow high pressure leak (build up) reaches the 15psi or whatever the expansion tank cap is set at it slowly leaks out whilst maintaining a constant 15psi pressure in the cooling system..

there are no air bubbles.. no water coming out of the cap just air (and then only during the hard drive period) and not even enough of that to cause any noticable hiss.. no froth.. no oil in water or the other way around that most folks think is assocciated with head gasket failure.. an exhaust sniffer would show up this condition but at this stage to the unknowing there isnt a (real) problem.. so the sniffer would not be used.. all other normal tests would show okay..

basically the only obvious symptom of this very early stage of head gasket failure is the car runs hotter when driven hard on the highway.. hot enough to bring the cooling fans into operation when in actual fact they should not be needed.. lets say the temp gauge reads 3/4 instead of the 1/2 or slighly less it should read with highway driving..

why the running hotter.. i think the air trapped in the t-stat housing is impeding the coolant flow.. or air arould the actual t-stat is fooling it into thinking the coolant is cooler than it really is so it tries to close prematurely.. not much.. but enough to cause the slight rise in coolant temp..

being as there seems to a be a consistancy of around the 3/4 temp gauge reading i tend to favour the air pocket around the t-stat fooling it as being the likely cause of the temp increase when this condition is present.. those fans coming on are not doing much at all.. the airflow thru the rad at 80 mph is far far greater than those pissy little fans can produce.. dont make the mistake of thinking that they are actually cooling things down and maintaining that 3/4 gauge temp because they are not..

anyway this condition can last for a long time and i think its responsible for the prevailing theories that running your saab a little warm will cause head gasket failure.. so what i am saying is this.. it isnt the running warm thats causing the gasket failure.. its the other way around.. its gasket falure in its very early stages that is causing your saab to run hotter than it should do..

if am correct.. a prolonged period of running hotter will almost always precede the more recognizable headgasket failure symptoms.. so advice along the lines of try the cheapest remedies first is bad advice.. if your saab has done over 75k at the first signs of running hot under heavy (high speed) loads the headgasket is the first thing to check not the last.. he he ..

how to spot this condition.. first give your car some 80 mph highway driving.. if the temp does go up to the same as stop-go traffic driving suspect the beginning of gasket failure..

stop the car.. the fans should be running... if u crack the bleed valve air not water will come out of the bleed valve.. the coolant level in the tank will go down slighly.. this is because the pressure in the top of the tank is now higher than the other air pockets in the system..

cracking the expansion cap will have the opposite effect.. air will hiss out and the coolant in the tank will rise.. take the tank cap right off and it will actually start to overfow.. this is not the sudden gush that u get when the coolant is boiling but a far more gentle effect.. basically in real terms the car is not actually over hot just hotter than it should be.. also if your car still has pressure in the cooling system after being stood over night this can be seen as a symptom of the pin-hole high pressure leak..

one other sign of this condition is that after the hard driving makes the temp go up.. slowing down will not make it go back down to the normal halfway or one third mark.. once that gas pocket has formed it will stay there until the car is stood for some time.. basically the gas pocket only gets so big.. any extra gas finds its way to the expansion tank and disappears without trace thru the pressure release cap.. this also explains why the condition stabilizes at a certain temp.. ie.. it dosnt keep getting worse until the car really does over heat.. the very slow pressure release also explains why the cooling system just dosnt explode when subjected to the very high cylinder pressures..

dont forget we are talking a very small leak.. unnoticable except for the condition i have just described.. however small this leak is thow.. if it wasnt for that pressure release cap the system would blow at it weakest point pretty quickly..

this condition will not cause problems in its own right (assuming your pressure release cap isnt blocked.. he he) and it can last for several (perhaps many) thousands of miles or many months.. but it is the first sign of gasket failure .. once u know what to look for its also easy to spot..

any guru type comments on this one.. happy to hear em if there is.. having had this just happen to me and after giving the matter much thought i am reasonably sure of my theory.. alway happy to hear any obvious holes in it thow.. he he

a new head gasket now has my car running at just under 1/3 up the temp gauge (a little too cool by my standards) however hard i drive it on the highway.. the 2/3 stop go trafic temp hasnt change thats as it should be.. i emphasise i only replaced the head gasket.. i did think i would have to replace the rad.. but.. !!!

now if i had have replaced other things.. i would not know the real culprit and would be thinking the new rad had fixed things.. ie.. the over heating at speed which had then led to the head gasket failure and so on.. see what i am getting at here.. he he

trog100

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