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Q: the cost to fix oil leaking from the pistons. on 1990 Mercedes-Benz 420SEL

car cuts off sometimes when idling and it is losing rpm
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You description is not clear enough. If you mean your engine is burning oil to the point where it is fouling the spark plugs and making the engine run rough and stall then the piston rings are worn, the cylinder bores are damaged or the pistons are damaged. In any case the engine needs to be disassembled and inspected and or replaced with a rebuilt engine.
This idling is caused by the vacuum hoses. i have had preveously 3 mecedes my first mercedes was a 420sel and that was the problem i check a vacuum hose that goes to the engine and it was disconnected and thats what caused the engine to loose rpm, i also had a 300E and the same problem happen. Now i have a 420sel again bcuz i wecked the first 420sel, but any ways this sone is not giving me problems yet but it will in the future on that same problem
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