It would be a lot cheaper to have them done without the head being removed.

This would mean finding a shop willing to do "the air compressor trick" where you hook up a a air compressor hose (with a home made adapter) to the spark plug hole to keep the cyl pressurized and the valve in place while the spring is removed and the seal is changed out.

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1988 Mercedes-Benz 300E W124 M103 California Emissions/Controller