The illegal act of bidding up the value of a fund's holdings right before the end of a quarter, when the fund's performance is measured. This is done by placing a large number of orders on existing holdings, which drives up the value of the fund.

Also known as "marking the close".
Portfolio pumping can be highly destructive for investors in the fund because it is a temporary gain and the stocks will generally fall back to previous levels once the price manipulation is over. For example, if a fund has 1,000 shares of ABC purchased for $10 per share, if the shares are trading at $9 right before the managers’ performance is measured, they will have performed poorly. As a result, the managers may resort to portfolio pumping and place enough orders to bid the price to $14, dramatically increasing the fund’s performance. However, it is likely that the shares will fall back towards $9, leaving investors with a $9 stock that was made to look like a $14 stock.