I had this on mine for months.... blagged the cr*p out of me! Turns out it was the dump valve. If you have a valve fitted, try kinking (finger/thumb) the vacumn hose that comes out the top it. You may find that thats your issue too.

If the car continues to idle, just remove the vacumn hose from the valve and bung it with something - then replace your DV.

Try it