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Q: Extreme power loss, engine rattling.

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Hi, my post is about a 2008 BMW 523i... The car is very low mileage (43000km/26000mi). I didn't have it at the time, however the oil level was showing low and hence my father who had it borrowed at the time filled it up with one litre, just to last until it got its oil change. About one week later, the car lost power (SUPER slow acceleration, the engine light is on;), also when it revs over 2.5k rpm this weird, rattling sound can be heard. (it didn't do this at first) Since, I've had the ECU repaired as apparently it was faulty, and one injector replaced as the diagnostics were throwing these codes. These have disappeared after the fix, but the problem persists. Mechanic told me that it's likely a loose timing chain. Could adding 15W-40 instead of 5W30 create this issue? Does my (brief) description sound like that could really be it, and what needs to be repaired if it really is a loose timing chain? Does it need to be completely replaced? Thanks... :/

My car has 26000 miles.
My car has a manual transmission.

Using 15w-40 instead of 5w-30 could be the cause of your problem. Your engine has very small passages that lead to the valve train. One components in particular, hydraulic lifters, require a good flow of oil through a small passage to be able to properly function. 15/w-30 is a thicker oil than the engine is designed to use.

By using this thicker oil, oil flow through the small passages slows down. With the flow of oil slower, the lifter can’t properly operate. The result is that your intake and exahust valves will not open fully and at higher RPM’s, there is not enough flow to maintain the designed clearances between certain parts of the valve train.

The rattle you hear may be caused by this lack of oil flow, and your loss of power may be a by-product of your valves not fully opening. I would suggest a full oil change as a first step, making sure to use the oil specified by BMW. If you are still having problems, a complete engine diagnosis will be in order to determine the exact cause of the loss of power.

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