There's an '89 190E 2.6 around here (not mine) that needs ether to start, but runs fine once it's started. If you don't use ether, it just cranks and won't even cough. Use a little snort of ether, and it starts right up and runs fine after that. The car has 170-some thousand miles and looks to be in good shape generally, but the history of it is unknown.

Any idea what could cause this problem?

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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 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.