25td revs problem

My son has a problem with his 25td that I'd appreciate some help/advice with. He says this problem only began a couple of weeks ago after driving through a heavy downpour - not floods just a lot of rain and surface water. The problem is when the car is first started - it's ok after a minute or two. After starting the accelerator is almost completely unresponsive. He presses the pedal down and nothing happens for several seconds and then after a while the revs build up and then it's ok. Any ideas?

from the last news http://ift.tt/1AJ92oQ

via IFTTT