Hi, first post got a 2004 b2500. 74000 miles , recently changed all filters , oil is perfect truck drives superb, recently though after start up acts like air con has been switched on. Slowly getting worse , runs perfect for about 15 seconds , them revs up to a constant 1.5 rpm and does that for about 30 to 46 seconds before settling down to idle , any ideas greatly appreciated

 

I've got a 97 b2600 and as soon as I start it up the revs jump up to 2 grand revs then settles, does the same when stopped while going for a drive but it's not as much jumping only to 1.5 grand, I'm still trying to find out what is doing it.