I've got an odd issue that I've been dealing with since buying the boat last year. 02 35 Lightning and the hydraulic steering gets super stiff the hotter it is outside (as in the outside ambient temperature). When I first got it I just thought the steering went out but then the next morning it worked again. So morning it works fine but as the day goes on and gets hotter, the steering gets harder and harder and then in the late afternoon/early evening when the temp starts to drop it starts "working" better and better. Mind you I'm not running the boat any different in those late afternoon early evening runs (truth be said, I tend to run the boat harder at end of the day to either burn gas, make it in before dark, just enjoying the best part of the boating day, or all above). It seems to be at its worst when it reaches about 85 degrees and above outside. Anything below that it works just fine, so early in the season where temps don't even reach that high I have no issues all day. Hydraulic oil is at the proper level. I might be crazy in correlating the outside temp, but it's the only pattern I've been able to establish that reproduces the issue every time.

Any ideas? Feel free to laugh at me if there is no way in the world that the outside temp could have something to do with it.

Are the fluid lines run through the oil cooler or possibly been rerouted and deleted the cooler ?

It is a single owner boat before me and I was told that nothing has been changed from the factory. With that said I haven't looked at that to see. Thanks for the suggestion.

What engines and steering ? the hotter the day the hotter the bilge

500 EFI with factory "Fountain" steering, I'm not sure who actually makes it. So what would the extra heat in the bilge be doing to cause the steering to be stiff?

What fluid are you running in your steering? I assume you have a serpentine belt set-up. Is your belt set at the right tension? Maybe a kinked hose? Just trying to throw out some easy checks.

I reread your post. You said: Hydraulic oil. I run Dextron III

Not sure about the fluid, I recently bought the boat and I haven't replaced that. Yes serpentine belt setup, tension is good. I haven't crawled around yet to trace the hoses, would a kinked hose be affected by temperature? If kinked I would think the issue would always be present. I'll throw this on the list check next time I'm at the boat.

Its a weird one for sure, fluid might be breaking down, marginal ps pump ?

Edit: most ps systems have an inline filter, you may want to check that also

Are you running 1 or 2 pumps