[AT] propane to detect vacuum leaks.

[James Peck]

Here is a post outlining the procedure for vacuum leak testing with propane.

https://www.hunker.com/13414896/how-to-find-a-vacuum-leak-with-propane

48 years ago a man taught me how to use a vacuum gauge in diagnosing a blown head gasket. 

I will have to remember Stephens tip for using propane on hard starting gasoline engines.

[Jack] If a bare plug wire were sparking to ground it could light the propane torch if you moved the torch nozzle so propane flowed over the spark. A bare plug wire itself would not heat propane to its ignition point.  The torch itself restricts the propane flow.

However, the only engines I have encountered with hard to find engine vacuum leaks are V6 and V8s where the intake manifold seals against both the block and heads. You can’t usually see the rear manifold seals and they do get out of place. Tractor spark ignited engines are generally straight 4s and 6s.  I suspect that the use of propane to find vacuum leaks dates to the introduction of the OHV (overhead valve) V8. 

We did have a flathead Ford V8 on a piece of equipment when I was a kid.

I once had a pinhole leak in an anti-dieseling pneumatic damper diaphragm that I found using a handheld vacuum device. This was on a vehicle without ECM (engine control module).

[Jim Becker] I’ve wondered how often that method accidentally finds a plug wire with bad insulation.
 
[Jack] In the automotive courses I took, we would use an unlit propane torch and direct the gas flow over spark ignited engine areas of possible vacuum leaks. If the engine speed increased, you had found the leak.
 
[Stephen Offiler] Anybody use propane/MAPP?  Unlit torch pointing into the air box.  I use it on reluctant gasoline engines with a fair amount of success, but not so sure about diesel.  Since we're talking about gas-soaked rags or gas-soaked air filters on diesel engines, seems like a similar approach.  
 
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