I'm not a Vx expert, but your comment about the plastic pulleys made me think of the following (from www.monoposto.co.uk):
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POSSIBLE CAM BELT FAILURE ON VAUXHALL ENGINES !
Some of you may have seen the item on "Watchdog" which alerted me to the possible problem with a faulty component in the Vauxhall 16v 2 l engine. I have investigated the matter with the help of the local Vauxhall dealer with the following results.
There are two variants of the Vauxhall 16v 2 l engine. The change point is at engine no 14142838. The later engines have a modified cam drive and cam belt arrangement, which appears to have been carried over onto the Ecotec engine. I have the later engine, which has two idler wheels on the cam drive train. The original wheels are made of a plastic material and carry the marking "para GF50 ". These are the wheels that are the subject of a Vauxhall recall, and should be replaced with wheels made of a rubberised material.
If you have a complete Vauxhall road car, Vauxhall will change these wheels FOC. They won,t exchange the wheels though of a buckshee engine - so the cost to Monoposto racers is around £30 - cheaper than a cam belt failure I suppose !
By the way Vauxhalls recall campaign is aimed only at '94 on (Ecotec engined ?) cars, they don’t cough to a problem with the earlier 16v engine, but mine had the rogue wheels fitted !
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Mike