Use compressed air to find vacuum leak?

Question

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:

1. Some leaks could be unidirectional in nature (a crack that expands under vacuum might actually get squeezed shut by positive pressure)
2. I might possibly break something that is not designed for positive pressure
3. Might not actually be possible (for instance, maybe at least 1 exhaust valve is always guaranteed to be open, at any given time.)

Answer 1

I think you have a misconception of how the soapy water should help you discover the leak. It's not used to create bubbles. It's used to temporarily seal (or partially seal) a vacuum leak. Most leaks which are small enough you cannot detect through normal means (sight/hearing), can and do change how the engine runs when sealed. You are just trying to find where the soapy water makes the engine change how it's running so you can pinpoint where the leak may be at. Any vacuum leak in an EFI setup is going to introduce unmetered air, which will throw the air/fuel mixture off as well as change how it runs.

1. Some leaks could be unidirectional in nature (a crack that expands under vacuum might actually get squeezed shut by positive pressure)

By its nature, pressure would expand a crack and a vacuum would bring it together to seal it. This would be true in any situation where the material around the leak is flexible enough to move, which in most vacuum leak is where cracks and leaks occur (though not always the case).

2. I might possibly break something that is not designed for positive pressure

I don't think pressurizing the system would cause any issues, as long as the pressure was kept to a minimum. I'd suggest nothing greater than 5psi continuous.

3. Might not actually be possible (for instance, maybe at least 1 exhaust valve is always guaranteed to be open, at any given time.)

As far as your idea for pressurizing the system, I think you'll find it will fail without a LOT of air put continually into the system. The reason for this is, in any multi-cylinder 4-stroke engine, there will usually be one cylinder which has both the intake and exhaust valves open (between the exhaust and intake strokes). It would be more likely in a 4+ cylinder engine than below this number (like a V-twin motorcycle engine). This will allow the pressure to escape quickly. Finding your bubbles will not be easy in this scenario.