Layers not fully fused (ABS)

I'm just starting to do some printing with ABS and getting parameters tweaked. One of my test cases is a 3 perimeter hollow shell, about 3" tall with a 0.25mm layers. The prints look good, but the layers are not fully fused - when I stress test the part (ie, break it) it always breaks cleanly at the layer lines. Also, it looks like the perimeters are not always fused. I'm using an extruder temp of 230 and an bed temp of 110. The part is stronger at the base than at the top. It does take some force to break it, but it seems like it could be a lot stronger.

Should I increase the the extruder temp? Increase the bed temp? Increase the extrusion multiplier? Any suggestions to improve the fusing would be most appreciated.
-Jeff

I had simular problems with black abs, i don't know is if its specific to black but with natural abs i had better results, i solved this in two ways aceton fuses the layer together and i switch to 0.2mm layer height

I've never printed with black ABS. I printed with green & natural ABS. When I increase temperature from 210 to 230, I see significantly stronger bonding between layer. I could be wrong, but you might have wrong temperature reading or maybe your fan is making it too cool. That might explain why your bottom layers are bonding stronger than the higher layers.

Thanks - good to know that black is a little more difficult to work with. I'll try running at 240c and see if that helps. Also sounds like a spool of a different color is in order.
-Jeff