The MK3 Jetta and the MK4 Jetta share the same platform, which means that the engines are interchangeable. However, there are some differences between the two engines that you should be aware of before making the swap.
The MK3 Jetta came with a 2.0L 8V engine, while the MK4 Jetta came with a 2.0L 16V engine. The 16V engine produces more power and torque than the 8V engine, so you can expect a slight performance boost if you swap the engines.
In addition, the MK4 Jetta engine uses a different engine management system than the MK3 Jetta. This means that you will need to swap the engine control unit (ECU) and the wiring harness in order to make the engine work properly.
Finally, the MK4 Jetta engine is slightly longer than the MK3 Jetta engine. This means that you may need to make some modifications to the engine bay in order to fit the engine.
Overall, swapping a MK4 engine into a MK3 Jetta is possible, but it is not a simple task. You will need to have some mechanical experience and you will need to be prepared to make some modifications to the car in order to make it work.
