I very recently purchased a 1990 560 SEL with 48k - this car most likely sat for a while. This car runs a little rough at idle and has a significant hesitation when driving at lower speeds (around town), most noticeably from stop or a near stop, when attempting to accelerate. So far, I have run a tank with Techron, changed sparkplugs and the fuel filter, however there has been only a limited improvement. I'd like to get an idea how to diagnose the problem before spending thousands on random parts.

As you can see below, I also own and plan to keep for now, an 88 420SEL (75k) which runs like new. If needed, I can borrow parts from this car (to the extent they're the same) for purposes of diagnosing the problem with the 560.

Any suggestions would be appreciated.

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2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey

SOLD:
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino