You know the situation. Your customer pulls up and says that their check engine light is on. You agree to check it, but you believe deep down that it’s just going to be another loose gas cap code. You connect your OBD II scanner and it does pull up an evap system code, but it’s not necessarily a loose gas cap. After clearing the code and removing and reinstalling the cap, the light comes back on. What’s your next step? Unless there’s obvious damage somewhere, you could be stuck. This is where a smoke machine can help.

Why Does It Matter?

An evap/emissions related code in the computer can do a lot of things other than just illuminate the MIL. It can cause the vehicle to fail an emissions inspection for one thing. Even if emissions testing isn’t required in your geographic area yet, it can also compromise engine performance, reduce fuel economy, and even lead to other problems like surging or sputtering. Leaving the car alone isn’t an option, which means that you need to figure out where the leak is. The diagnostic computer has given you a rough starting point, but now you need to pinpoint the problem.

What’s a Smoke Machine All About?

Really, the name says it all – a smoke machine creates, you guessed it, smoke. It then pumps that smoke into the evap system at a low pressure to avoid causing any damage. You can then visually inspect the entire evap system and watch for where the smoke is coming out. This allows you to pinpoint the leak, and make super-accurate repairs, saving yourself time and effort, and ensuring better customer satisfaction.

How Smoke Machines Work

If you’ve never used a smoke machine before, you’re not alone. While they’ve been around for a while, they are only now becoming commonly needed pieces of equipment. In fact, you might have worked in a mechanic shop for years without ever needing one. They’re actually very simple devices that do their job quite well.

First, understand that the machine will need power to create the smoke that will ultimately be pumped through the evap system. Electrical power is provided by the car’s battery – just connect the machine’s clamps to the battery’s positive and negative terminals.

Next, you’ll need to fill the machine with its required smoke-producing material. This is where we start to see some differences. Some machines use mineral oil or baby oil. Others use air and colored dye instead of burning oil to create actual smoke. You will also need a connection to your shop’s air lines, or to a nitrogen machine if you have one.

Finally, you will need to connect the smoke machine to the evap purge solenoid (in most cases – your machine may differ, in which case you’ll need to read the operating instructions). This is the point at which the smoke machine injects vapor or dye into the system. Once activated, the machine will pump smoke/air into the evap system, allowing you to locate the leak.

Note that some smoke machines come with warnings that only nitrogen should be used because it is an inert gas and does not react with other gases. This is good advice, but most real-world shops simply use the shop air system instead.

It’s also important to look for a multi-use smoke machine that can be used with turbochargers, superchargers and to check for vacuum leaks as well as evap system leaks.

When Should You Use a Smoke Machine?

The first step in diagnosing an issue with a vehicle’s evap system has nothing to do with the smoke machine. Rather, you need to connect your OBD II scanner to the car’s computer and download the error codes. If you note a minor or major emissions system leak, you can do one of several things:

  • Visually inspect the system for obvious signs of leaks or damage
  • Clear the code(s) and drive the vehicle to see if it recodes
  • Go ahead and prep the vehicle for the smoke machine

The best option is really just to go ahead and connect the smoke machine if you don’t see anything obvious with the gas cap/gas cap gasket. You can waste a lot of time in a basic visual inspection and still not find anything. Clearing the codes and driving the vehicle likewise does little good if they simply return once more – you’re still back at square one.

To get everything set up, you’ll need to connect the smoke machine to the emissions hose running to the rear of the vehicle, and then you’ll need to block that line using a pair of vice grips to crimp it closed (do this near the gas tank). Go ahead and connect the clamps to the battery, and the line to the purge solenoid. Then activate the smoke machine.

The only thing you’ll need to do now is watch for smoke issuing from a cracked line or failed fitting, and then make the repairs necessary. When you’re done, clear the codes from the computer and test drive the vehicle.

A Few Caveats

With all of that being said, smoke machines are not the end-all-be-all of evaporative/emissions system testing. Common sense and a good understanding of the most common causes for setting evap codes are both vital. For instance, a quick visual inspection of a gas cap gasket could show you that it’s dry rotted and the cap needs to be replaced. Simply checking the gas cap’s tightness when seeing an evap code can be all that’s necessary (drivers constantly forget to tighten their caps all the way).

You’ll also need other tools to augment or supplement your smoke machine. There’s no reason not to have a good multimeter, and you’ll definitely need a hand pump and a decent scanner with diagnostic capabilities.

Check out one of our favorite smoke machines on the market:

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