Swap a Golf MK5 GTI TFSI engine into a Golf MK3#

This website explains the full project to swap a Golf MK5 GTI engine (engine code BWA) into a Golf MK3 GTI. In addition to that it also explains how to make it road legal (in Spain) and all the challenges to overcome to make it work. There is plenty of custom fabrication to fit the engine and all the pipes around, but by far the most complex part is the electrics.

Two different wiring harnesses must work together. From an engine point of view a Golf MK5 GTI (BWA) wiring harness must be adapted to work within a Golf MK3; different ABS, speed sensors, etc. In addition the original Golf MK3 instrument cluster is kept, so engine speed, VSS and other signals must be adapted to make sure the original cluster continues displaying the right information.

This means that the original Golf MK5 engine Wiring Harness must be modified to be able to start the engine in a Golf Mk3. The modifications into the engine wiring harness will depend on the approach followed.

In that case no fuse boxes from Golf Mk5 were used and the wiring harness contained 3 fuses (in replacement of the Golf MK5 fuse box). That approach allowed for a clean separation between the engine and body electrics. As mentioned before several signals must still be adapted to be able to continue using the original instrument cluster; engine speed, VSS and oil temperature. Other signals as coolant temperature can be connected directly.

Some of the Engine Wiring Harnesses sold for this type of swaps remove the support for the V50 (coolant circulation pump), as this pump is only required when the engine is shut and the turbocompressor is still hot. In this swap the V50 pump is retained and properly used to cool down the turbocompressor when the engine is switched off.

Another aspect to consider is the exhaust, the long short, a Golf MK5 BWA exhaust must be adapted to a Golf MK3, and these changes must be done in a way that the new exhaust that fits in a Golf MK3 is certified for a Golf MK5 BWA engine. As you can imagine the exhaust is custom made and requires a lot of fabrication.

The last but not the least is the certification process. As it will be explained all those changes must be made road legal; so they have to be certified by an engineer, verified by a certification body and finally approved in a MOT like inspection.