I need to hunt for a possible vacuum leak in my 1999 Toyota 4runner. I've seen all the usual recommendations for this (propane, cigar, smoke machine, carb cleaner, soapy water.) Of those, soapy water seems like the lowest cost, lowest risk option.
The thing I can't get over though, is that I'm looking for a negative pressure leak, not positive pressure. I can't imagine being able to see bubbles on a vacuum line.
So I'm wondering: Is it a bad idea to put the vacuum network (with engine off) under positive pressure using my shop compressor, and then using the soapy water?
I imagine this working by plugging up my PCV valves, and then using one of the vacuum lines to introduce maybe 10psi of pressure.
I have three concerns:
- Some leaks could be unidirectional in nature (a crack that expands under vacuum might actually get squeezed shut by positive pressure)
- I might possibly break something that is not designed for positive pressure
- Might not actually be possible (for instance, maybe at least 1 exhaust valve is always guaranteed to be open, at any given time.)