Filter Worries

What could be causing my fluval u4 to delay coming on after plugging it in? It's not new and I've never took it apart except for occasional cleaning of sponges.

how long is the delay and does it sound like its pumping air in this time, if its anything like one of my eheim they suffer with a lot of traped air inside the filter canister , i found easiest way is to fill the canister almost to the top useing my siphon tube from the tank then put the filter head back on a fill the pipe work that leads from the tank into the filter also with water from the tank , i found this to get everything to start up pretty good and then justa few shakes of filter to remove the last of any trapped air.

Many years ago I had a Hagen Trio filter, and Fluvals are made by Hagen. It had a bad habit of not starting when switched back on, ie at every water change. I had to remove the pump, hold it under water with the outflow pointing down, plug it in (get someone else to do that since I had wet hands) then flick the impeller. And carefully put the pump back on the filter without soaking myself.

If the posts above don't work, you could try this as a last resort.