1. Chilton's or Haynes Repair Manual: These manuals are your best bet. They often include detailed vacuum diagrams specific to the year, make, and model. You can find these at auto parts stores or online retailers. Make sure you get the one specifically for a 1987 Chevy G20 with a 350 engine.
2. Online Forums: Websites dedicated to Chevy vans or classic car restoration might have members who have scanned their repair manuals and posted the vacuum diagrams. Search forums such as the Chevrolet Forum or others dedicated to vans or the 350 engine. Be prepared to sift through posts.
3. Local Mechanic: A mechanic specializing in classic vehicles or GM vehicles is likely to have access to these diagrams either through their own resources or online subscription services.
4. Dealership (unlikely): While a long shot, some dealerships might still have access to old service manuals, but this is less likely for a vehicle this old.
Important Considerations:
* Engine specifics: A TBI (Throttle Body Injection) system will have a vastly different vacuum routing than a carburetor. Knowing your exact engine setup (carburetor type or TBI specifics) will significantly improve your search.
* Diagram variations: Even within the same year and model, minor variations in options can affect vacuum routing. The diagram you find might not be 100% identical to your van's system, but it should be close enough to be helpful.
Start with the repair manual; it's the most reliable and detailed source. Good luck!
