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Renault scenic knocking

Hi i have a renault rx4 scenic 2002 when car is in idle i get a knocking noise from the engine,but when revved the knocking noise goes ....can you help please

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Well,
There could be a few causes. The rap really could be the bottom end, or number 3 cylinder, or piston slap (when cold), a belt-driven component or cam drive components or even a nerfed valve. The best way to find out is to listen, but I illustrated the Timing Light Trick first if your ears aren't trained (and while you are hooking up that light with the engine running, take the time to listen to it and 'train' your ears to finding out what the heck that noise REALLY could be and what it certainly ISN'T (such as a bubble in the tire thumping the fender, but hey, you are stopped with the car in park while you're hooking up that light, right? (Always work SAFELY).
Here's the breakdown before you declare it's the bottom end:
1) Connect a timing strobe in accordance with the manufacturer instructions to the main secondary coil wire. If you have a coil over plug installation, there may be an adapter you would need to get.
2) In idle, flash the strobe. If the knocking appears to be in time with the timing light, it's the upper half of the engine (cam, tappets, valves, etc.)
3) In idle, flash the strobe. If the knocking appears to be twice that of the strobe, you can conclude it's something in the lower half of the engine. Generally, crank journal main bearings make a worse metallic ringing under load. Connecting rod bearings usually quiet down under load if they're worn as the extra slack is taken up by cylinder pressure.
4) The most accurate test is to use your ears. Listen with a stethoscope all over the engine until you can tell me whether that knock is really a faulty power steering pump bearing, an alternator bearing, the number 3 cylinder or the camshaft. Take the time - you are training your ears to listen to what the engine is telling you.

Of course, if you have gas in the oil, it'll knock. If you did an engine flush, it may knock - all that gunk overloads the filter to the point the bypass opens in the oil pump - you're SOL if that happened. If you're out of oil, it WILL knock. If you changed heads recently and now you have a knock, the most likely cause for that is coolant in the oil; whoever changed the head burped the old one off without draining the coolant from the block. Even a small amount of it running down into the crankcase from that operation will create an acid that will destroy bearing surface material in a very short period of time. Even a complete oil change will not get all the contaminate out of the engine.
Lastly, Stop Leak may turn into Stop Engine - so if you use oil additives like these that can possibly clog the journal oil holes and cause a knock.

Bottom line - do your research, listen to what that mill is telling you, rule out belt driven accessories and oil levels (and condition) and when this knock started. Finally, tell me where that knock is coming from - a belt driven component, number 3 cylinder or the flexplate (auto slush boxes) or the flywheel (gearboxes).

Posted on Oct 21, 2013

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