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Car won't accelerate with AC on?

I drive a manual 2001 Hyundai Tiburon. I tried to take it on the expressway today and it took a long time for the RPMs to get high enough that I could shift gears. It seems to want to hover at about 2000. When I finally got into 5th gear, it would only go about 50-55mph with the pedal to the floor. When I turned off the AC, though, it would go about 70mph, but it lost speed quickly on hills and took a long time to regain it. I am not sure, but it seems like the gas mileage was substantially worse than it normally is. I didn’t notice any new noises or smells or shakes or anything like that. The car is just really lagging and my mechanic is closed for the night. I would like to at least have a guess what it is so I don’t have to worry about it all night. Thanks.

Did your check engine light come on?

No, but it never has and neither does my gas light, so it might not work.

Two things come to mind. A tank of bad gas or a partially plugged catalytic converter. I’ve had both of these with 2 different vehicles and your description fits those problems to a “t”. It may be something entirely different though but they are worth checking out.

How many miles are on the car?

Describe your general approach to basic maintenance and then all the specifics you can. E.g. How old are your spark plugs & wires? Fuel filter? Air filter?

The thing about the AC is that it puts a pretty heavy load on the engine since the engine has to run the compressor. If the car isn’t running quite right - for any large number of reasons - then the AC load will knock it down quite a bit.

The CEL (check engine light) should come on when you first turn the key on, then go out a few seconds after the engine starts…Is this a 4 or 6 cylinder? Mileage?