Cam Belt

{mosgoogle} This is a very important issue that 928 owners ignore at their peril. If a cam belt fails (on all but the early 4.5 litre models) a 928 will suffer catastrophic engine damage and leave its owner with a bill in the £5000 region! The good news is that with regular servicing (for checks), and cam belt replacement, the chances of cam belt failure can be reduced to almost nil.

What Is A Cam Belt?

A cam belt is a long belt that drives the cam shafts with power from the crankshaft. The cam shafts in turn open and shut the valves (which allow air and fuel in, and exhaust gases out of; each cylinder). The valves must also be driven in sequence with the pistons in the cylinders so that air anf fuel is injection in at the correct moment and exhaust gases expelled at the correct time.

On a 928 there are 8 cylinders, and either 2 camshafts with 16 valves or 4 camshafts with 32 valves, depending on the model.

The Danger

The danger arises because most 928 engines (except the early 4.5litre - but especially all 32v variants) are designed so that when the engine is running the pistons and valves move in and out of the same space at the top of the cylinder. Therefore if the valves do not move in sequence with the crankshaft (bacause of cam belt failure) the pistons and valves will hit each other. With the huge forces involved (especially at high rpms) this can lead to severe damage of pistons, rods, valves, cam shafts, cylinder heads and maybe even the engine block and crankshaft themselves.

Porsche recommendation

Porsche recommend the following:

• check the condition and tension of the belt at each service.
• change the belt at every 60,000 miles or 5 years, whichever comes sooner.

928.org.uk recommendation

Experience from 928 owners and 928 mechanics suggests that the Porsche recommendation is not conservative enough - especially if you drive your car hard (where the abrupt changes in engine speed put extra stresses on the belt). Experts like David Roberts of 928 Specialists in the USA recommend the following:

• check the condition and tension of the belt at each service (ie: at least once a year)
• change the belt at every 36,000 miles or 3 years, whichever comes sooner.
• check the cam belt rollers and replace any showing wear.
• check the cam belt tensioner and rebuild every 100,000 miles. Further Recommendation - Also Change The Water Pump Experience shows that a new cam belt will greatly increase the chances of water pump failure. Replacing the water pump involves the removal and refitting of the cam belt (and thus is a long and expensive job). As a new pump is only £130 or so it is recommend that this is changed with the cam belt. The extra labour time will be almost nil and the chances of a water pump failure (and subsequent large bill) will be almost eliminated.