Porsche 968 - ASSEMBLY INSPECTION PROCEDURE AND MAINTENANCE SCHEDULE Maintenance Manual
Inspection procedure and maintenance schedule
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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
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Summary of Contents for Porsche 968 - ASSEMBLY INSPECTION PROCEDURE AND MAINTENANCE SCHEDULE
- Page 1 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.
- Page 2 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.
- Page 3 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.
- Page 4 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.
- Page 5 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.
- Page 6 24. With the inspection complete, reassembly is the reverse of strip down. If the cover seal appears in perfect condition, it’s possible to re-use it. Porsche use a liquid sealant in addition to the moulded seal at the corners of the cover mating face.
- Page 7 As stated above, we are in a very subjective evaluation territory here, and there is no official data on which to rely available from Porsche. Moreover, the “wear limits” can be precisely checked only by dismounting the Variocam itself, and at this point you have done all the work, and you can substitute all the worn parts without significant extra work.
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Page 8: Suggested Maintenance Schedule
camshaft. Even if there are few or no missing teeths, a cam has to be changed when the central teeth part is too thin, hence prone to break. Compare teeth profile of fig. 3 (bad) with that of fig. 6 (new), and you have a good idea of how to judge your cam. If you change one camshaft, do not mix used and new ones: via the new chain, the wear pattern is rapidly transferred from the used cam to the new one, and in a few thousand miles, you will have two badly used cams. - Page 9 The authors are happy for this document to be freely used by individuals for non- profit making purposes only, subject to the user accepting full responsibility for any consequences resulting from use of the information contained herein. Constructive feedback, comments, or questions are most welcome: Derek Holliday: drholliday@compuserve.com Ugo Manfredi:...