I need an impact driver that goes in reverse
 

Not just reverse, but IMPACT reverse for small difficult nuts and bolt on automotive work. Does such a thing exist? I am under the impression that the current impact drivers only impact in tighten mode, not loosen.

Yup, they sure do.....just look up "impact wrench" instead of "impact driver". Pneumatics are best if you have a big enough compressor, but you can get both corded and cordless electrics in most of the major tool brands.:thumbsup:

All of my 12V+18V impact drivers impact forward + reverse. I have used my 12V+18V impact drivers for small rusty bolt removal in auto work..

My corded impact wrench impacts forward + reverse.

(DW Makita Ridgid Bosch)

I am familiar with both electric and pneumatic impact wrenches, but I would *greatly* prefer a small, lightweight, completely portable tool that I could use for the same thing, just smaller tasks.

I really like the Bosch impact drivers. I just haven't been able to test one in reverse.

I have both a cordless dewalt impact wrench and a cordless one from Harbor Freight. Both are @ the same size as my big cordless drill and I use them in the house and in the garage.

Technically impact guns are not supposed to tighten things such as lug nuts and wheel bolts unless you know that the max torque of that wrench is. Just about every impact wrench in the industry has more power in reverse than in foward. Also for automotive applications you should get a wrench with an anvil rather than a hex 1/4". works much better.