My blend door actuator on my 97 grand cherokee 4.0 works sometimes then other times it doesn't. When I first start the jeep the actuator works fine and heat blows good. After 5-10 minutes the actuator goes back to its neutral or cold position and starts blowing cold air. I think the doors are fine. Can't figure out why the actuator is only working a few minutes. Please help!!!!

 

Try by checking all vacuum lines, it sounds like it´s leaking vacuum,good luck!

 

Rev the engine to ~3000. If the heat gets hot after that, it's a vacuum leak. Well, I say "leak" but high mileage engines can have less vacuum due to wear.

 

Don't forget to check the vacuum reservoir. I think it's underneath the battery.