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Evap Test Leak Tester Smoke Machine Tips P0440 P0442 P0456 P0455

Updated on August 22, 2014

Evap Leak Tester

What Leak?

This article will discuss what they forgot to tell you in evap class and you won't find in the booklet that came with your smoke machine.

A smoke machine uses pressure to help you find leaks while the car uses vacuum to run the evap monitor.

Not a very big distinction right? A leak is a leak and it will behave the same under pressure or vacuum. Not exactly true. Fuel caps can leak under vacuum but not pressure. Take it from someone who uses their smoke machine every day, it can get tricky.

On some vehicles when you hook your smoke machine to the evap test port under the hood you wait a few minutes and there is no flow at all. It seems there are no leaks. Take the extra step to remove the fuel cap before the end of the leak check to verify you now have a large leak flowing with the cap off.

Some times a rollover valve in the gas tank can seal off the filler neck completely so your pressure testing everything except the filler neck and fuel cap. Removing the gas cap is a quick and easy way to verify this condition.

If your filler neck is not being pressure tested you can then test it separately from the bottom by removing the hoses or make up your own gas cap with a hose through it for testing from the filler neck down.

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