I know somebody who has installed a 5th injector on a mk 3 golf gti 2.0 8v N/A engine, it is located in the induction manifold and believe it or not is manually operated via a switch on the dash board. Other mods to the car are a set of mild cams, exhaust manifold and full exhaust system and an induction kit.

Surely by my basic mechanics, This will wash the bore out through overfuelling as surely there isn't enough airflow.
I thought 5th injectors running off a computer control type system were used on home made turbo cars or cars that have an unsafe air mix ratio as a result through tuning.

What will this extra injector be doing?

Sometimes you'd run it with a nitrous system as the nitrous changes the fuel air balance and the extra fuel would compensate.

Otherwise I suspect they are trying to get more fuel in but what they will end up doing is knackering their engine by overfuelling. Seems very odd though.

I've seen a 5th injector used on turbo systems in the exhaust manifold that keeps the turbo spinning during gearchanges (flames out of the exhaust as well).

5th injector is a slightly crude way of increasing the fuelling in general. If the standard injectors were near the limit the best thing to do is A. Bigger injectors or B. Dual injectors (2 x small or 1 small & 1 big per cylinder).

Both A and B will need some way of being controlled though i.e aftermarket ECU £££s or remap at minimum.

1 extra injector split between all the cylinder will increase the fueling, but unless the design is spot on then some cylinders will get more/less fuel (so inconsistent across all cylinders).

And manually controlling the injector via a switch

One word - bodge!

What would this actually do though? Would it cause engine damage through washing the bore out? Could it improve performance by reducing engine temps?

Has anyone done this to there coupe ??

I sincerely hope not

If they have and I find them, they're in trouble.

My coupé has a fifth injector...right next to the forth as it should be

I think the extra injector is old news from back in the day when an ecu couldn't manage such a large % change in injector timing. Iirc the turbo technics 205 had a mechanically controlled fifth injector.

Would be interesting to see if it adds anything to the Golf, other than the obvious extra fuel.

You would still need to run the injector off some sort of piggyback ECU, you simply can't just run it off a switch. The Aquamist water injection ECUs were popular on 205 turbo conversions but anything that will give you an adjustable pulse width will do it.

It is a bodge though, I believe the true turbo technics conversion added larger injectors.