944 S2 burning oil - one for the experts?
Discussion
My S2 has been drinking more oil recently, accompanied by a bit of smoke at revs under throttle - bad news I presumed, probably rings on their way out (has done 236k though!). Quick compression test didn't show up anything too concerning however, then came across some posts on Rennlist that suggests when the seals in the oil / air seperator go bad, oil vapour gets drawn up in large quantities & dumped into the throttle body via the breather line. (e.g. see http://forums.rennlist.com/rennforums and http://forums.rennlist.com/rennforums ) Anyone have any views / experience on this?
stevep2000 said:
My S2 has been drinking more oil recently, accompanied by a bit of smoke at revs under throttle - bad news I presumed, probably rings on their way out (has done 236k though!). Quick compression test didn't show up anything too concerning however, then came across some posts on Rennlist that suggests when the seals in the oil / air seperator go bad, oil vapour gets drawn up in large quantities & dumped into the throttle body via the breather line. (e.g. see http://forums.rennlist.com/rennforums and http://forums.rennlist.com/rennforums ) Anyone have any views / experience on this?
If this is happening you should be able to see lots of fairly fresh oil gunge marking around the joins between the breather pipe sections. Pull of the breather hose and have a look. The reason I know this is that my other car (a turbo) is starting to dump some oil and one can see the gunge around the throttle body area. In that particular case it is caused by the turbo. For comparison this particular car (Saab 9-5) is only using about 3/4 litre every 12K miles so if you are loosing more then that and it is caused by oil dumped into the breather (perhaps when coming on or off throttle) rather then just from worn rings then I would expect to see one hell of a lot of mess in the breather lines.
Difficult one this. For these symptoms and mileage - it usually has seized and freed off and has one bad cylinder but this should show up on the plugs.
Similarly if a valve stem oil seal has been fitted slightly out of position.
Could be that the increase in compression after having the valves done (assuming they were) has encouraged blow by past the pistons into the breather system = new rings.
Check that the oil return hole in the crankcase is not blocked - it is seen by removing the plastic oil filler moulding, cleaning and re-fit with new seals.
Could the mixture be too rich because either the temp sensor is at fault or the diaphragm in the fuel pressure regulator has holed/perished?.
Is the performance the same (had to be re-timed after re-build?
Baz
Similarly if a valve stem oil seal has been fitted slightly out of position.
Could be that the increase in compression after having the valves done (assuming they were) has encouraged blow by past the pistons into the breather system = new rings.
Check that the oil return hole in the crankcase is not blocked - it is seen by removing the plastic oil filler moulding, cleaning and re-fit with new seals.
Could the mixture be too rich because either the temp sensor is at fault or the diaphragm in the fuel pressure regulator has holed/perished?.
Is the performance the same (had to be re-timed after re-build?
Baz
Certainly oil in the throttle body, but seems more old sludge than fresh oil. Replaced the temp sensor over the weekend, and had the fuel pressure checked today which was ok. Seems to be only when I use more than 4k revs for any extended period, either a fast motorway run or a back road blast, when it starts to drink the stuff, so assuming it's most likely rings, but no oil on plus & ok compression test made me hope there may be something more easily (& cheaply) addressed. Performance seems fine, just worried that something major was looming. Still, have a list as long as my arm of to-do jobs - part of the joys of life with the old girl I guess!
stevep2000 said:
Certainly oil in the throttle body, but seems more old sludge than fresh oil. Replaced the temp sensor over the weekend, and had the fuel pressure checked today which was ok. Seems to be only when I use more than 4k revs for any extended period, either a fast motorway run or a back road blast, when it starts to drink the stuff, so assuming it's most likely rings, but no oil on plus & ok compression test made me hope there may be something more easily (& cheaply) addressed. Performance seems fine, just worried that something major was looming. Still, have a list as long as my arm of to-do jobs - part of the joys of life with the old girl I guess!
This reminds me that I have often wondered about the relative merits of a compression test versus doing a leak down test. My understanding is that the compression test is more for finding problems on a single cylinder such as a blown valve or gasket, whereas the leak down test is needed to understand how worn the rings are across all the cylinders.
Any thoughts Baz?
Yes a compression test tells you - if nothing else - if one cylinder has a problem compared to the rest.
If they are all similar then low readings explain poor bottom end performance and blow by - confirmed from experience of mileage etc.
Most engines perform well when "on the cam" anyway so it is generally bottom end performance that can suffer if general wear is the problem.
Leak down confirms if there is leak by the valves or rings - but is IMHO less informative than a general compression test and comparison bewteen cylinders - but that may be just because I am more used to using compression tests more than leak down - perhaps others who use leak down more can interpret them just as well.
Baz
If they are all similar then low readings explain poor bottom end performance and blow by - confirmed from experience of mileage etc.
Most engines perform well when "on the cam" anyway so it is generally bottom end performance that can suffer if general wear is the problem.
Leak down confirms if there is leak by the valves or rings - but is IMHO less informative than a general compression test and comparison bewteen cylinders - but that may be just because I am more used to using compression tests more than leak down - perhaps others who use leak down more can interpret them just as well.
Baz
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