Porsche CARRERA 4 VOLUME 2, 968, CARRERA 2 VOLUME 2 User Manual

Porsche 968 Variocam Assembly: Inspection Procedure and
Maintenance Schedule

Background

There have been a number of documented failures relating to the Variocam
mechanism fitted to the Porsche 968. Porsche has not published any periodic
inspection or maintenance requirements for the Variocam. Consequently even cars
with a full Porsche main dealer service history may suffer undetected excessive wear
or even catastrophic failures. In the worst case this will result in bent valves and
major engine damage. Cars that have not yet reached the stage of catastrophic failure
may still be suffering serious wear to the camshaft sprockets, requiring camshaft
replacement.

This document was produced by Derek Holliday and Ugo Manfredi. Both of us have
first hand personal experience of Variocam problems, which in each case required
replacement of both camshafts, chain, and tensioner pads. The information contained
here is based on our own experiences, and additional information obtained from the
“968.net” community and other sources.

Purpose and Scope

The purpose of this procedure is to describe a method for home inspection of the
camshaft sprockets, cam chain, and Variocam chain tensioner pads. The procedure
does not include any necessary repair work that may be necessary as a result of the
inspection findings. This procedure should be suitable for all normally aspirated 968
models. Also included is a suggested schedule with recommended inspection
intervals. These periodic inspections will provide regular checks of the health of your
Variocam and its related parts and should ensure that wear or other problems are
detected before they result in catastrophic failure.

Overview of Variocam Operation

Due to the apparent lack of detailed information on Variocam operation from
Porsche, the following section is based on the authors’ understanding, derived from
various articles and personal observations.

Variocam is a mechanism that allows the intake camshaft timing to be varied for
different engine operating conditions, thereby maximizing torque. In the 968 engine,
the exhaust camshaft is driven by a toothed rubber belt from the crankshaft. Halfway
along the length of each camshaft is a toothed sprocket and a connecting chain that
transfers drive to the intake camshaft from the engine driven exhaust camshaft.
Earlier variants of Porsche’s 16 valve engines (2.5 liter 944S and 3.0 liter 944S2) used
a similar setup but without the Variocam mechanism. Instead, they used a simple
spring and hydraulic operated chain tensioner whose purpose was simply to maintain
constant chain tension. A fundamental difference between these earlier engines and

the 968 engine is the ability for the position of the upper and lower pistons in the
chain tensioner to be adjusted in a vertical plane. This vertical movement, actuated by
controlled flow of engine oil, causes the lengths of the top and bottom runs of the
chain between the camshaft sprockets to be varied. Since the exhaust camshaft
position is fixed in relation to the crankshaft, this movement of the Variocam
tensioner pistons causes the intake camshaft’s relationship to the crankshaft to be
changed by 15 degrees.

Cross Section Through Variocam Assembly

The diagram above shows a cross-section through the Variocam. The actuation
mechanism comprises a housing that is bolted to the engine. The housing
incorporates separate moving upper and lower tensioner pistons, each having a plastic
rubbing pad that acts directly onto the upper and lower chain runs. The distance
between the upper and lower tensioner pads is maintained by an internal spring,
supplemented by constant engine oil pressure when the engine is running. This spring
and oil pressure maintains correct chain tension, in a similar fashion to the earlier 944
series cars. The vertical position of the pistons is controlled by engine oil pressure.
When the engine is running, engine oil pressure, controlled by a shuttle valve actuated
by an electrical solenoid attached to the Variocam body, is directed to either lift or
lower the pistons, depending on engine rpm. Below 1500 rpm, the DME unit (main
control computer) energises the solenoid, moving the position of the hydraulic shuttle
valve in the Variocam, thereby directing oil to lift the pistons. As the pistons lift, so
the plastic pad on the rising top piston pushes upwards on the upper run of the chain,

causing it to extend. At the same time, the rising lower piston allows the lower chain
run to be shortened. The overall effect is that the intake camshaft is rotated
anticlockwise in relation to the exhaust camshaft, thereby retarding the intake timing
back to its nominal setting. Above 1500 rpm, the DME de-energises the Variocam
solenoid, causing the shuttle valve to move to a different position. Engine oil pressure
is then redirected to lower the pistons, extending the lower chain run and shortening
the upper chain run. This causes the intake camshaft to rotate clockwise in relation to
the exhaust camshaft, advancing the intake timing by 15 degrees. Above 5500 rpm,
the solenoid is again energized, and the intake cam is retarded back to its nominal
position.

Possible Causes of Variocam Failure

Although it is impossible to reach a general consensus on this, from the public
discussion in the “968.net” community and actual examination of failed Variocams,
the following appear relevant reasons for their failure:

1) Aging of the plastic pads (rails). With time and miles, the pads became much
more brittle than when new, leading to an increased chance of breakage, and
in any case to an increased rate of wear. In failed 968 Variocams, the pads
appear very worn with deep tracks, or partially broken, or brittle with some
“craters” on them, meaning the material has lost its original properties.

2) Stretching of the camshaft chain. Some stretching is inevitable with use, but
the stretching increases the effective pitch of the chain links, accelerating wear
of the camshaft sprockets.

3) Variocam on/off action. As described above, the Variocam is “on” (intake
timing advanced) in the 1,500 to 5,500 rpm range, and “off” outside this
range. The engagement of the chain with the intake/exhaust camshaft
sprockets differs between these two positions, and this may affect the wear
patterns on the sprocket teeth. The wearing pattern is different with respect to
a non-Variocam car (eg 944 S2), and apparently reduces sprocket life. The
driving style (average RPM), can also produce an asymmetric wear of
upper/lower pads, and sometimes the lower pad wears more than the upper
one: this is a problem, because the lower pad is more difficult to inspect.

4) Camshaft slope. To support the timing advance, the slope is “aggressive”, that
is has an elongated form, to open the valves very quickly. This means
additional stress on chain, and when the chain gets stretched, a “hammering”
effect on sprocket teeth, particularly at high RPM.

5) Given all the stress on cam sprockets, there has also been speculation that the
thermal treatment during manufacture of the cams was inadequate or
inconsistent.

It appears that any 968 car with more than 60,000 miles (100,000 km) can potentially
develop a Variocam failure, even if luckily only a small percentage will effectively
experience it. Whether problems develop or not, depends on many factors: driving
style (high RPM means asymmetric and accelerated wear of Variocam parts), age of
the car and the type of oil used (influencing pad/chain wear), car abuse or poor
maintenance (high engine temperature and insufficient lubrication mean accelerated
wear), previous Variocam maintenance, and total mileage.

Variocam Inspection

Disclaimer and Precautions:

The information contained in this document is offered in good faith but the authors
take no responsibility for any errors or omissions. In the absence of any documented
recommendations from Porsche, we believe that by following these steps, you should
have better protection against catastrophic failures. However, we cannot guarantee
this, and recommend that you seek expert advice if you have any doubts about the
condition of your Variocam components.

Certain basic safety principles should be followed. These include:

• Before starting work, disconnect the battery

• Wait until the engine has fully cooled before starting work

• Work in a well ventilated area and avoid naked flames or other combustion

sources

Also, when working on the engine, it’s very easy to dent the front wings (fenders) by
leaning on them. Place soft covers (old blankets are ideal) over them to prevent
scratches and don’t lean on them.

Tools Required:

• 17mm and 19mm good quality open ended spanners (wrenches)

• Strong light source

• Old rags

• Inspection mirror (small dentists-type is ideal)

• Hexagon drive bits, sizes 4mm and 5mm, and suitable driver

• Spark plug socket, drive extension, and handle

• 24mm deep 6 point socket and ratchet drive handle

• Torque wrench with range to cover 10 Nm / 7 ft./lbs (desirable)

Parts Required: (assuming no repairs needed as a result of inspection)

The following parts may be required if the existing items are damaged but it is usually
possible to re-use the old parts:

• Cam cover gasket (quantity 1)

o Part number 928 104 447 09

• Sealing rings/grommets (quantity 4) for sealing the spark plug recesses

o Part number 928 104 443 08

Inspection Procedure:

1. Disconnect battery.

2. Remove four hexagon socket head screws securing aluminium trim panel over
cam cover/fuel rail, remove “Variocam” badge and trim panel.

3. Disconnect electrical connector for Variocam solenoid. Note that spring clip
on connector should be pushed in to release. (See figure 1).

4. Remove two hexagon socket head screws securing seal retainer around
Variocam solenoid body and gently pull off seal retainer and gasket.

5. Place rags beneath connections on two fuel hoses at rear of cam cover (see
figure 1).

6. Undo both fuel line connections using 17mm and 19mm spanners. Take care
to ensure that the fittings nearest brake master cylinder are held stationary and
not twisted. Note: a few ml’s of fuel will be spilled during the process, hence
the rags.

7. Gently pull fuel lines clear of the cam cover, taking care not to kink the lines.

8. Pull out spark plug caps/leads and remove spark plugs.

9. Insert clean rags into spark plug recesses to avoid risk of dropping loose
objects into cylinders.

10. Unscrew 13 hexagon socket head screws securing cam cover. Note that these
screws do not need to be removed from the cover once released but be aware
that they are not held captive in the cam cover and may drop out when the
cover is removed.

11. Gently pull on the cam cover to release its seal. It may be necessary to apply
leverage but be careful and do not insert any sharp objects between the sealing
faces.

12. When the cover is loose, gently pull away from the engine. The black rubber
seal can either stay in place on the engine, or stay attached to the cam cover.
When withdrawing the cover, it’s often needed to peel the seal away from
parts of the engine or cover. IF COVER SCREWS ARE STILL IN THE
COVER, TAKE CARE THAT THEY DON’T FALL OUT AND DROP
INTO THE EXPOSED ENGINE!

13. Note that there are four black circular rubber seals around the spark plug
recesses. These will hopefully stay attached to the engine when the cover is
removed but may pull away with the cover.

14. With the cover removed, the inspection can begin. Take great care not to drop
anything into the engine, as there are oil drain galleries below the exhaust
camshaft and screws, dirt, or small tools can easily find their way into the
engine. If this happens, you will need to remove the oil pan to recover them!

15. Using the light source and mirror, carefully and methodically inspect the upper
and lower tensioner pads, both camshaft sprockets, the metal oil pipe feeding
oil to the tensioner body, and the shiny piston rod of the upper tensioner for
wear or scoring (see figures 2 & 3).

16. These checks are very subjective, there is no test gauge or tool to measure
anything. The tensioner pads can be expected to have visible wear grooves
(the lower tensioner pad is very difficult to inspect without removing the
tensioner assembly and this requires removal of the cams). Check as far as
possible for any signs of cracking in the tensioner pads, missing pieces, or
excessive wear (see figure 4). Cracks tend to form in the corners of the pads.
Very close inspection with a strong light is required for this. Any cracks or
missing pieces will require that the pads are replaced (out of scope for this
procedure). See also the section below for further information on suggested
wear limits.