I'm getting a loud knock from my 1991 Polo 1300 engine top end. It is rhythmical, matching the engine speed. Strangely it is not always there and though it happens more when the engine is cold that is not always the case. If I listen with a "stethoscope tube" with the rocker cover off and the engine running it seems to be coming from the region of the cams or hydraulic tappets of cylinder no.1 yet the cams are smooth and intact and I can find no excessive clearance between cam and tappet. In fact I can't find any clearance between cam and tappet on any of the cams with the cylinders at TDC (Haynes says there should be 0.1 mm but I can't find any). There is no blue smoke from the exhaust when I do the MOT type blue smoke test (rev to 2k then release) so I can't think it could be a broken piston ring. The car has done 142k miles. Would a broken valve spring or a burnt valve do this. Has anyone got any clues please?
Philter
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sounds like: wear on a cam tappet or cam: taken up by hydraulic tappets.
As it comes and goes suggests irregular wear on a hydraulic tappet.- goes with the miles. Either ignore it or strip off camshaft and examine all tappets...
Or you could have worn cam bearings - unlikely to give those symptons.
Is the oil feed to the cam ok? If blocked you might get those symptons. Try an oil change/new filter and possibly an engin flush. Are you using the recommended oil specification?
madf
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This may be a failing cam follower i.e. occasionally failing to refill with oil. They have a spring in them to assist opening and a hole in the side which is force fed with oil and if you gat muck in the small valve the follower deflates and the next time round on the cam it clunks. However you describe this as a knock, may be through a tube thats what it sounds like. If you rest a plastic knitting needle on the follower one at a time you may be able to detect the miss in the rythm. An engine flush may well clear the problem if you do not want to remove the cam. Regards Peter
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Thanks to both who have replied. You have explained the mysteries of the hydraulic tappet well. I therefore came to start dismantling the cam and fortunately started at the camshaft sprocket. Instead of being torqued up to the specification 52 ft lb it was actually loose! So I thought I'd try tightening it before going any further, just in case it was the cause, though it seemed highly unlikely. Lo and behold the knock has gone! I shall bear your suggestions in mind in case it returns with the sprocket still tight but meanwhile I'm breathing a big sigh of relief. Thanks gents.
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