I have a 1999 Honda Civic EX. The mileage is 171,000+ miles.

When I turn on my AC my car shakes while I'm idle and even when I'm in park.

So I haven't been using my AC. If I use my fan (without AC) then my car seems fine. I used my heater today to test it out and it works fine while idle and in park. But if I'm at a red light for a long time, I noticed that my car shakes/jerks a little.

What's wrong with my car?

 

Check your mounts, specifically your A/C mount.. Bet it's internals are non existent haha..

 

Only with A/C on?

Does the A/C work fine? Any recent A/C work done?

Any low idle problems without A/C on?

 

Without the AC on, while on idle I noticed that the car jerks a little. So when I'm at a red light, I put my car in park so that it won't shake.

With the AC on, my car shakes a lot.

Nothing new done with the AC but I recently had my engine switched because the old one died out. Not sure if it's because of that.

But the other week, just after I turned on my car, my engine was revving up and my foot wasn't even on the gas petal or the brake petal. It didn't do it again after that.

My car's been acting weird after the engine was switched.

 

Clean the IACV.

 

Thanks! I'll try that.

 

 

After checking the usual things
You should check one or booth of these
1-crank shaft sensor
2-throttle sensor
Trust me on that one