I've seen 'the smoke test' referenced several times in answers to OP's.
I would like to ask;
What is a smoke test?
What kind of tool(s) do I need to do a smoke test?
What is the proper procedure to do a smoke test?
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A smoke test is used to find leaks in any hollow body like the exhaust system, evaporative emission system or intake manifold. The smoke test works like finding a leak in a bucket. Fill the bucket with water and see where it drips out. Similar with the smoke test you fill the hollow body with smoke and see where the smoke comes out. A bright lamp can assist you in finding the source of the smoke. Use the light and scan along the body being tested looking for smoke.
For smoke testing you need a smoke machine. The machine pictured is a fairly inexpensive one that is commonly used in garages.
The important features of any smoke machine are
Inducing smoke into a system is the very basic test that can be run. This is a common test for intake manifolds and exhaust systems. When working with an evaporative emissions system the flow meter and pressure gauge become your friend because they are more sensitive to very very small leaks.
To test an evaporative emissions system find a place to induce the smoke. The line connected to the vent solenoid or into the filler neck are the most common. The best practice is to Tee into the vent line or purge line and leave the whole system as intact as possible.
If going into the vent solenoid line then there is no need to command the solenoid. If Teed into a line or any where else you will need to command the vent solenoid closed with either a scan tool or by jumping power and ground to the solenoid.
Now turn on the machine, adjust the flow until the ball is at the top of the flow meter and watch the flow meter. If the meter never drops then there is a large leak. Watch for large quantities of smoke. As the gas tank and all the lines fill with pressure the ball should drop down. If the ball never reaches zero then there is a medium to small leak. The lower the flow meter ball drops the smaller the leak. Look for smoke but much more closely. The leak will be very small. A spray bottle filled with soapy water is your friend. Spry down anything that looks questionable and watch for bubbles. If the ball drops to zero there is one more test to run. Seal the system and let it sit watching the pressure gauge. The gauge should not drop at all. If it does there is a very very small leak. Let the machine induce smoke into the system for a while to maximize how much smoke comes out. The smoke with such a leak may not help. Again, use the soapy water. Also a set of needle nose vise grips or hose pinch off pliers may help. Pinch off the hoses to the vent solenoid, purge solenoid or the filler neck one at a time and see if the leak goes away.
Some evaporative emissions systems like on select Chryslers and Toyotas is too complex to explain in a simple tutorial. Check the manufacturer service information.
P.S. inches of water or in of H2O is a very sensitive way of measuring pressure. One psi is about 27.7 inches of water. Also there is no need to use a nitrogen based system. Use a good dryer on the shop air line to your machine. The gas tank will not explode if filled with air. Everyone who says that is just trying to sell you nitrogen. The gas tank already has air in it and there was no explosion.