Twice the Bluebirds were ahead through Joe Ledley and a superb Koumas free-kick but the dogged visitors were never out of the hunt and goals from Tyrone Mears and a last-minute strike from Chris Sedgwick earned them a deserved draw.
The Bluebirds kept the side that won at Brighton on Saturday while Preston showed three changes after their home draw with QPR.
Graham Alexander flighted a free-kick across goal in the fifth minute and with no Cardiff defender attacking the ball it drifted wide of Neil Alexander's far post.
The conditions were proving difficult for both sides as the rain swept down but Koumas was still able to fire pinpoint passes down the line to Kevin Cooper and Cameron Jerome.
A free-kick by Koumas led to the Bluebirds taking the lead on the half hour. He hit the post with Carlo Nash beaten and when the ball came out stroked it firmly into the back of the net.
A superb ball from Michael Ricketts sent Jerome racing away 5five minutes later but his weak shot went straight to Nash.
Preston went back on the attack and Patrick Agyemang lifted a shot over the home bar when well placed.
Indecision in the Cardiff defence was giving the visitors hope but with only one front runner Preston failed to prise out any clear cut openings.
The Bluebirds rearguard seemed all at sea as Preston went looking for the equaliser but it was up front that City boss Dave Jones made a switch when Alan Lee replaced Jerome on the hour.
The goal Preston deserved came in the 62nd minute when a break by Adam Nowland left Mears in the clear to rifle a shot beyond Alexander and into the corner of the net.
Paul Parry replaced Cooper as the Bluebirds tried desperately to get back into the game they now looked more likely to lose so on top were the visitors.
Koumas rescued the Bluebirds with a trademark goal in the 80th minute after being brought down by Mears he flighted a superb 30-yard free-kick into the top corner, giving Nash no chance even though the keeper went the right way.
Preston brought on both remaining substitutes with five minutes remaining and it was one of those, Sedgwick, who fired home to level in the 89th minute after a goalmouth scramble.