83 300SD just started being hard to cut off. turn switch off
car keeps running where is the best place to start looking?

vacuum lines

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

If it hesitates a few seconds, then shuts off it is the vacuum box.
It is located on the rear of the injection pump. If the car will not shut off at all unless you move the shut off under the hood, it could be a vacuum line loose. The vacuum box should have a brown vacuum line attached to it. Pull it off and see if there is any oil in in then reattach it.

When the vacuum box goes bad oil will find it's way to the switch behind the dash and it can get quite ugly.

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1980 300SD Manganese Brown Metallic Clearcoat 248k
1979 300SD White 148 k
1984 300D Silver project car for daughter
1982 300D wrecked
1980 300SD 310K not running

Put a piece of vacuum hose on the shut off switch and with the car running apply vacuum (suck on it), If the car does not shut off, its the vacuum switch. If it does shut off, take a glass or two of beer and call us back.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"