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If you've got a light spark knock on acceleration in a R171 280 or 350, there's something to check if the typical fixes don't work.

Techron is nice and of course you should run correct fuel, replace spark plugs when recommended, check engine air filters, etc. but on both my 350 and 280 I've had the same knock ("pinging") on brisk acceleration, and both times the problem was cured by replacing the battery.

It doesn't seem to make sense at first, but I theorize that low voltage from the battery is having some effect on the ECU or other control unit that manages the engine ignition timing.

Add this to the list of weird problems that can happen to an SLK with a bad battery.

Note that the battery doesn't have to be "dead" for it to be a problem. If you turn on the voltage display (turn key to ON position and tap R button 3x) you can see the voltage on a weak battery drop to 8 or 9 volts briefly during startup. A good battery will drop to 11-ish volts while the starter is turning.
 

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left side of your console outside left of the speedometer. ahhhh the owner's manual! :D :D :D
 

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