Hi there, my focus st is having erratic boost issues. If I use the throttle very lightly, the engine is absolutely fine. If I use enough throttle to call on boost the car jumps around like a mad man. The boost gauge shoots up then down, up then down and literally throws you around in your seat. Safe to say I've been driving to work with barely any throttle. Perhaps this is the boost solenoid to blame? Or perhaps my turbo has had it, or maybe just the actuator? Any advice on how to narrow this down? Thanks, H

First guess would be the boost solenoid - think symptoms are car overshooting on boost or delay in boost dropping when you lift off. 
 

First result from googling

The Ford boost solenoid is a common "wear and tear" item, You can detect if you need a replacement when your car starts to loose response or hesitates when driving or if boost levels are dropping. On the other end of the scale, if you are getting boost surge issues, the solenoid can also be to blame

 

52 minutes ago, troy45 said:

First guess would be the boost solenoid - think symptoms are car overshooting on boost or delay in boost dropping when you lift off. 
 

First result from googling

The Ford boost solenoid is a common "wear and tear" item, You can detect if you need a replacement when your car starts to loose response or hesitates when driving or if boost levels are dropping. On the other end of the scale, if you are getting boost surge issues, the solenoid can also be to blame

 

Thanks. Yes I had seen that, my issue would definitely be on the more dramatic end of solenoid failure if that is to blame. Worth a try but I can see myself going full parts cannon if the solenoid doesn't fix it! 😬

If it’s not the solenoid try having a word with Gary at Collins Performance. 
 

He’s an absolute expert on the ST’s, I’ve known him for years and the only place I use for servicing and any jobs I can’t do myself