Irritrol hasn't been around that long and I don't know what a 2909 valve is. Maybe you read the date code, 2606. If the valve is totally plastic and grey (2700 series), make sure the solenoid seat is tight. You might have to take the solenoid off the valve to tighten it up. Since you have the solenoid off. Take that white thing off holding the plunger inside the solenoid, if there is one. Throw away that white thing. Look for debris. If that doesn't work I'd unscrew the bonnet (top half of the valve) and check the rubber diaphragm and seat. Look for pits or a small hole. Look for debris.
If the valve is black plastic (311) series. Do the same as above with the solenoid. If that doesn't work I'd unscrew the valve adapter and check the seat washer and seat. Look for debris. On one occasion I saw a cracked seat on the 311 valve. Hairline and very hard to see. Good luck!
If I can't fix it, it's broken!