if you have a vacuum leak you can try using a small hose or vacuum hose even and use it to listen for air leaks sround the air intake manifold. I had a leak in my gasket and wished i checked it first. i ended up replacing 2 o2 sensors, egr valve, and a bunch of other things i can remember before i found the leak. dont use propane. i don't know if do any harm but sounds dangerous. good luck