MY 560SEL HAS BEEN SITTING IDLE FOR 2+ YEARS. Should I drain/replace gas, oil, cooling, replace hoses etc...before I begin to troubleshoot engine/electrical issues? It has sat in 120 degree weather for 3 summers.

At least drain the old gas and change the oil. Full fluid change would be preferable. Inspect belts and hoses and replace as needed.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

Drain the old gas. I would want to have some lubricant in the bores before turning the engine over with just the starter (no spark plugs) to prevent wear. At that time, check the compression in all the cylinders. I would be listening for the fuel pump to energize. New oil and filter Only then consider attaching the plug wires and attempt to start up for a short run at idle. Listen for timing chain slap at startup, which may signal a non functioning chain tensioner.

Ok I'm slow, drained all fluids...this is what I've found. I have a broken valve spring on #6 cylinder. I live in small town; ie no one wants to touch it. One mechanic kept car for 3+ weeks and charged $1200 for diagnostic and couldn't tell me the valve spring was broken. Any ideas would be appreciated; I have CD and am mechanically inclined; just a little OCD in that everything I move or touch on car is labeled and diagrammed.

For starters, don't use that "mechanic" again. If you have a clean and secure place to work, replace the valve spring yourself. Since one broke, you might want to replace them all. On many engines this can be done with the cylinder heads still on the car, not sure about the M117. See what the manual says.

__________________
Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

thanks!

How many miles on the car? The M117 engine are one of the best of the best but they do need some maintenance. Check the chain stretch and the color of the upper chain guides. If they are brown in color then they need to be changed. If they are white then they are new. The spring can be changed but make sure you have the cylinder with the broken spring at TDC so if something slips, the valve won't fall into the cylinder.
Do you know anything about the history of the car? Any work that was done to it in it's past life?

PaulM

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84 500 SEL (307,xxx miles)