I have 2000 SES with 24V Duratech. I have owned since new. Great care. Has 75K on it. In the past I have replaced, fuel filter, new motorcraft plugs, air filter, etc. I live in Michigan. Just started to use the air. I notice when I am at a stop sign with the air on, the engine kinda jerks. I look at the tach and I can see it jump a little. Didn't do this last fall. Any ideas? The check engine light is not on. Would it store a code without turing on the light? When look at the engine in the driveway with air on, in drive, with foot on brake, I see the engine jerk back and fourth a little. Any thoughts?