I have a 99 Grand Cherokee 4.0L that has been running really weird lately. When you first start it's fine after 30sec it will run rough and backfire through the throttle body. And then all of the sudden its fine. And it will be fine until you shut it off. It will usually last between 15-60sec. I've changed the fuel filter and also the tps and cleaned the iac and tb. And it still does it. There is no codes and no check engine light. Before it starts to run rough I can hear it sucking air like crazy through the throttle body. I'm not sure if I just have a vacuum leak or a sensor freaking out. And my air vents will switch on their own from time to time. This is why I'm thinking it's a vacuum leak. I tried tonight after work to find a leak with propane and no luck. Any thoughts? I'm also thinking buying a bluetooth OBD II scanner so that might help me figure it out.