When I attach a combustion engine to my pump and give redstone signal to the engine, the pipe of the pump that goes into the liqud goes back in. Is this a bug or am I doing something wrong?

is it possible your redstone signal is also traversing to the pump? a redstone signal will dissable the pump and cause it to retract it's hose

I uploaded a screenshot of how i try to do it. I believe it is not reaching to the pump.

Have you any fluid source blocks under pump?

can you replace that torch with a block and put a lever on top of that? the engine might be carrying a weak redstone signal to the pump

Still nothing, sir.

Replace the iron pipes with either cobble/stone/quartz or gold fluid pipes. Iron fluid pipes 'as stated on their tooltip' are for piping the fluids in a certain direction so the water is flowing into the pipe then back into the pump as the iron pipe is facing that direction.
You can also use a wrench to change the fluid pipes rotation

Thank you sir, problem solved!

alright, that's a note for the future manual: pumps turn off and retract hose when internal tank is full now

No problem! Here is a playlist of tutorials for if you get stuck again for BuildCraft which are fairly upto date:
https://www.youtube.com/playlist?list=PLPCqiTnvx2GsE8t96c7CAepx9tDo7TgzN

Bare in mind that BC for 1.11.2 is in alpha so there will be alot of missing features/bugs compared to these but they should help :)