Need help diagnosing a problem.
 

This is for a friends car. he has a 2003 SVT Focus. It has a vortech supercharger with the aftercooler on it. While cruising, the car seems to surge every once in awhile. He's tried changing plugs, coil, bypass valve, and the car still does it. He says it doesn't seem to do it as often or as much when the AC is on. He is starting to notice that it doesn't really do it when the car has a full tank. Fuel pump maybe? Anyone got any ideas?

My focus did the same thing and I could never get it fixed. It would do it idling too, like it would lose fuel for a second then cut back in. We replaced the MAF sensor, throttle sensor and a whole bunch of other junk.

A random ass mechanic told me the intake had messed up and was stuck on long flow and wouldn't switch back to short, but that was a lie because I could hear it on the turbo...

It always seemed better when the A/C was on too. Like it equaled out the surge issue.

Long story short, I sold the car and the guy driving it now says it still does it.

Well I guess it sounds like it's going to be a neverending problem.

I'm going to bet that there's too much timing at idle and part throttle. You see it on NA cars too... bucking and surging if the car isn't under a consistent load (like idling through traffic in 1st gear) especially at low/no load and low engine speeds. When the A/C is on the engine is under a constant parasitic load and the extra timing is not a problem.

Thanks R3D, I'll have him check with his tuner to see if they can tweak some things before he takes it somewhere to get checked out.