My Jetta GL 2.0 2005 automatic transmission, struggles to start and it wont go past 20 mph when it does. It only starts if I step on the gas pedal while starting. It makes a weird sound just after the motor starts and sometimes it just turns off when I'm at a stop light.
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check your mass air flow sensor(MAF). It monitors the amount of air entering the engine. They often get dirty or simply begin to fail and the ecm can't correctly adjust the air/fuel ratio causing it to go into a limp mode in hopes of protecting the engine. Cleaning the throttle body wouldn't be a terrible idea either
I had a vehicle once that would barely make it up hills and also wouldn't go more than 40 MPH or so. The vehicle was much older (1980's), though.
The problem ended up being that 2 of the 4 cylinders weren't working due to the spark plug wires had gotten old and weren't transferring the charge from the plugs to the cylinders.
While this is a possibility for your case, you left out a lot of details about your car's behavior so for more specific assistance try to include additional details.
There are only three things that will reasonably cause this problem... (the fourth is very expensive... and very sad)
(Technically, there could also be something physically causing a very high load to be added to the engine's normal work. This would be something like a VERY bad transmission, a frozen wheel bearing, dragging the back seat behind the car, or something else very obvious, and very expensive. But enough of that.)
Why might the engine not be getting enough air?
Why might the engine not be getting enough fuel?
Without more details, it's kind of hard to guess which of these things might be the problem. I hope this helps you narrow it down!