I want to use uv dye in my car's cooling system to detect a leak. Will an led flashlight show the uv dye that leaks from the engine?

Yes, but you need a UV LED light to detect any UV dye that you have put in whatever fluid you are leak-checking.

My favorite UV LED light is the http://dx.com/p/1w-390nm-450nm...like-3-6v-4-2v-16694 in the http://dx.com/p/ultrafire-flas...t=1&s=16694&u=102367 body.

We keep a couple around for scorpion detection.

If you want to spend a little more for an all in one package that uses C cells, the Streamlight http://www.amazon.com/Streamli...ight+Twin+Task+3C+UV is dimmer, but simple to buy and use, and will work for leak detection in a low light situation.