968CS / winter mods & repairs / Camshaft Inspection
(+ injector cleaning)
The 16v engines seen in the 944S, S2 and 968 have only their exhaust camshaft driven by the crankshaft. In the middle of each camshaft is an integral sprocket and as these are linked together by a chain under tension, the intake camshaft is in turn also driven. In the 968 this is made more complex by the presence of the variocam system which allows vertical movement of the chain by moving the upper and lower tensioner pads that the chain sits on. This provides a resultant advance in cam timing of 15 degrees (between 1500-5500 rpm).
The truth of the variocam system is that it needs regular maintenance. Porsche have never published any inspection or maintenance interval so over the years many owners have found out the hard way. The mechanism can fail at all sorts of mileages leading to very serious and expensive engine failure and it is now commonly advised amongst enthusiast circles that any acquired 968 that has not been checked, should have its variocam system inspected at the earliest opportunity. Although some specialists advise that you will hear a rattling of the system as a warning before it gives up the ghost, if you don't intend selling the car any time soon then its wise to take a look as soon as you can and perform any required maintenance to get your engine as safe as it can be from the outset.
Cam cover off ... a 5 minute job, just undo the 2 main fuel lines, unscrew 'PORSCHE' fuel rail cover, unplug the variocam electrical connector, remove variocam badge and oval shaped seal retainer and gasket, take out spark plugs, unscrew all cam cover screws and gently prize away carefully taking the gasket and spark plug seals with it.
Whilst there is considerable debate on this, the main theory centres around the loosening of the chain. Over time the cam chain will stretch and then its possible that a solid chain link at some point will strike down on the point of a sprocket tooth as opposed to an indentation. This can then snap the tooth off. I understand that these sprockets are hardened with the cams and the chain is also quite hard so you have a fairly brittle unforgiving system. The sprockets also wear and become pitted. When several teeth go this will naturally lead to a complete failure at some point.
Now all of this wear can vary considerably though with oil, driving style, car age and mileage but the best rule of thumb is to inspect any vehicle you purchase especially if it is over 60 or 70k miles and has no documented history concerning the cam chain. If the cams are good and have no missing teeth and the tensioner pads do not have excessive wear then at the very least I would look to fit a new chain and pads. That way you can prolong the life of your existing cams. If however you have to fit new cams then there are 3 choices. Replace both with OEM porsche cams. Note that Porsche changed the design in 1994 I believe with sharper, pointier teeth to try and negate the chain engaging poorly. Second choice is to just replace one of the camshafts (if only one looks damaged with missing teeth). Some specialists advise this but only with a thorough inspection of the camshaft that appears sound. I know that Hartech have over the years devised a graph and have some critical measurements by which they assess whether a cam should be left in the car or not. If you are trying to save money on just the one camshaft then maybe they would be your best bet to carry out the work. The third option is to use rebuilt camshafts. These are cams that have had new sprockets welded in (in two halves) and the old ones removed. Some outfits like RSBarn have been selling these for 4 years or more without failures and also offer a parts warranty on them. If you are happy with this route then it can mean a significant saving over the 780+vat it costs for new camshafts.
The procedure to inspect your cams has been very well documented by 2 968 owners. You can download the exhaustive PDF which they wrote a number of years back here.
Plugs in good condition. A bit of oil on the threads from a less than prefect gasket seal.
Intake camshaft clearly showing 2 missing teeth. You will need to rotate the engine with a 24mm deep socket (rotate clockwise only when looking at the engine from the front in the direction of the back of the car) so you can see right round both sprockets.
A closer view showing the wear.
As you can see i've been unlucky. The car has only done 65k miles but then in a way I'd rather do the job now and have piece of mind for years to come, knowing that the variocam is sorted with new cams and a new tight chain which can then be replaced every 20k miles or so.
Its common for 968 owners to get their injectors cleaned. I sent mine to HGL and for 12+vat they will clean and renew O rings and provide you with before and after test results.