Blazer Forum - Chevy Blazer Forums - Engine vacuum loss/related 4wd issue

Hi folks,

I am new to this forum and have a 2000 blazer 4x4 with the 4.3 engine and 194,000 miles. It has been a lemon since I bought it last fall and I am trying to get it decent enough to sell it so I can move on. I recently stuck $800 into it replacing the front differential, actuator and cable. The problem I am having I believe is vacuum related. When I get out on the highway, the actuator is losing vacuum enough that the 4wd is trying to engage, making a whining/whirring noise. It does this until I put it in 4hi then I can put it back to 2hi. Won't do it again until I get to hiway speeds. I just had it at the shop again and the mechanic is telling me the engine is losing vacuum. He said the valves could be bad, or possibly an intake issue. Have any of you experienced a similar problem? I already have $2600 into a $1200 vehicle with a salvage title so don't want to spend alot of money. It is also getting poor gas mileage, like 8 to 10 mpg and up til now I figured the injectors were probably bad. Hoping someone on here has some ideas on what this could be. Thanks!