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I've had my '86 hand washed twice recently and on both occasions, when I pick up the car and drive away the Service Engine light is on. It lasts for a few hours then it won't come back. While the light was on, the engine start was tougher and the idle was rough. Any idea what may be causing this? I assume water is getting somewhere and causing this...
You're getting water on/in the ignition system causing a misfire. Once it burns off you're OK.
What exactly are you having hand washed? Are you doing an engine cleaning? If so, you have to keep water away from the distributor. Also, what is the condition of your cap and plug wires? If they are getting old the water may be causing the cap or wires to short. Spray a light mist of water on the engine at night and see if you observe any arcing. If so, the wires need replacement.