Ford Focus shuddering when turning the wheel

Probably power steering pump as it's under load .

Thanks for the reply but what do you mean under the load? this may be the case as I have had a small Power steering fluid leak as well

Not 100% sure on that year but on some the power steering has a pressure switch, which is designed to increase engine revs at low RPM (while parking for example), to prevent the car from stalling due to the load on power steering pump .

Mashbender, when you turn the steering wheel with the engine running, the power steering pump starts operating and pushes fluid under pressure to force the wheels to change direction. This puts the PS system "under load".

If the system has a leak, this load builds and then - suddenly - releases as the leak allows fluid to escape from the pressure it is under at that moment. This reduces the pressure in the system, resulting in a jerk, as the power assistance drops.

Then, the whole procedure is repeated again. This can result in a judder. There are other reasons for judder ocurring, but, as Whittinghame points out, overall wear on high-mileage cars may mean that general wear and leakage may be a result of the system and rack being simply worn out. Can you see where the leak is coming from?

It may be the switch where the leak is coming from also .When the switch detects the high pressure, it sends a signal to the ECU. The ECU then increases engine speed in order to stop the
engine from stalling if you are going slowly.

My apologies to Whittingehame for misquoting him (Its Sunday night and er...)

No problem , we all make mistakes .

I have looked underneath loads of times but cant trace where the leak is coming from also I've tried some Stop Leak which is meant to guarantee to fix it but obviously it hasn't I even took it into a garage but at that point when they jacked it up onto the lift it decided to stop leaking!!! It is a high mileage car well its done 130 K but I thought these days most cars can do that and a lot more. Obviously it no longer has regular servicing so thats not helped as Im the 6th owner and don't know when the previous owners decided to stop having it seen to. It's not a drastic leak but I want to stop it before that happens. Does anyone know the name of some Stop Leak That Will Genuinely Do The Job ???

Never found any of those products succesful and always preferred to wash down and locate / recify any leak by replacing faulty seal or whatever part needed .Surprised leak doesn't show as the pressure the rack is under and the distinctive oil / colour are usually easy to find .