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Why does my Sierra have a metallic rattling sound at low revs?

I have a Ford Sierra 1.6 petrol - the "lean burn economy engine". Unfortunately it suffered from hardened valve seats last year as it turned out not be converted to unleaded. New valve seats were put it as part of the conversion to unleaded it had.

When the car was returned to me it ran brilliantly, far better than it ever did...funnily enough. The garage recommended I run it on 98 OCT fuel or higher - I have been using the locally available 99 OCT. However almost immediately after getting the car back, it began making a metallic rattling sound when driven at low revs (below 1500 RPM I believe - it doesn't have a rev counter) or uphill.

The garage adjusted the timing and solved this problem, however, it was then wobbling when idling. The car went back to the garage, they adjusted the timing again and gave it a carb clean with new jets but this has made no difference.

It never had this wobbling problem before the timing was altered. Can you please advise on how to achieve a 'rattle-free engine' which will not wobble on idle. Last week it also began spitting oil out, I think from the rocker cover gasket.

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It could be that your engine is suffering from a condition known as 'Pinking'. This is caused when the cylinder is hot enough to explode the compressed mixture of fuel and air without the spark plug. Left unchecked, it will cause damage. You'll need to check the engine properly - either by yourself or with a specialist. Make sure the mixture is correct and if the distributor has a vacuum advance make sure that's working because it adjusts the timing when the engine is at idle and under load. I'm not sure how much adjustment is available as I believe there is also a basic ECU at work.
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