Smoke signals

Motor,  Jul 2003  by Marinucci, Dan

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Smoke-generating machines have continually evolved and improved. Almost unknown just a few years ago, this technology is now OE-approved and, in many cases, mandatory. Here's an overview of this leak-detection method today.

This story starts out simply enough. I first used a smoke machine in the spring of 1995 and loved it, so I included it in a feature story on leak detection in our june 1995 issue. I also began recommending the technique to anyone and everyone who'd listen. Those who took my advice always responded, "Gee, I wish I'd gotten a smoke machine sooner!"

Since then, smoke technology has come a long way both inside and outside the automotive industry. For example, it's been used to diagnose fuel tank and fuselage leaks on jet aircraft. It's been used to quality-check new tanks for over-the-road tanker trucks and to diagnose high-pressure leaks in nuclear power plants.

In the auto repair business, the cost of labor, increasing complexity of vehicles and limited access to many components have all made smoke technology more important and cost-effective than ever.

Longtime smoke machine users emphasize that the equipment lends itself to demonstration to skeptical car owners. Simply put, smoke signals have proven to be a very powerful, convincing service sales tool.

Even if you have used one, stay tuned anyway. Enhanced evaporative emissions systems have forced major upgrades in the smoke technology that you need to understand. We'll try to update you on the types of smoke machines available and how they're used.

General-Purpose Smoke Machines

As the name suggests, general-purpose smoke machines are used to diagnose leaks all around a vehicle. The lone exception is evaporative emissions systems, which I'll discuss later. A typical general-purpose machine creates smoke, pumping it at approximately 1.00 to 1.50 psi. If the smoke pressure exceeds this for any reason, a regulator inside the machine vents off the excess pressure. In the case of the original Vacutec unit, you knew this was happening when you saw smoke streaming from an overpressure port on the back of the machine.

For many smoke machine users, this overpressure vent has been a simple but very effective diagnostic tool. For example, I pump smoke into a vacuum brake booster when I suspect it's leaking. Obviously, that smoke has to go somewhere. If the machine continues pumping smoke, there's a leak in that booster somewhere. But if the booster fills up and then the smoke blows out the machine's relief vent, you know the booster is tight-otherwise, smoke pressure wouldn't have gone high enough to pop open the relief valve.

This pressure-venting system makes it extremely unlikely that a smoke machine would actually create a leak or damage any component due to excessive pressure. Nonetheless, be sure to read about sensitive evap system pressures later in this article.

Modern machines have user-refillable smoke chambers or reservoirs that last anywhere from 200 to 300 tests. A good rule of thumb is that the newer the model, the better the quality and quantity of smoke it produces. Better-quality smoke is denser, thicker and whiter, which means it's much easier to see during a leak test. Higher-quantity smoke speeds up the test appreciably when you're filling something relatively large, such as an exhaust system, air intake or big vacuum reservoir.

A few general-purpose machines produce smoke with ultraviolet (UV) dye in it. So if somehow you overlook the smoke streaming out of the leak, the smoke will also leave a telltale stain you can see with a UV-type spotlight. Speaking of spotlights, some smoke machines come with a powerful halogen spotlight that's invaluable for pinpointing "smoke signals" during a leak test. This 12-volt spotlight has an extra-long cord that allows you to walk the length of the vehicle with it, if necessary.

Leak-Testing

Finding vacuum leaks in the engine and related vacuum systems is the single most popular application for general-purpose smoke machines. The most-common technique is to leak-check the engine with the engine off and the throttle closed. Disconnect the hose from the PCV valve and pump smoke into the PCV hose. If it's easier to reach, disconnect the vacuum hose from the power brake booster and pump smoke into that hose. If you do this, remember to also smoke-check the power brake booster itself.

Some techs routinely do this on every engine as a quick state-of-health check because it takes so little time. They're repeatedly amazed, they tell me, at the leaks they find on supposedly tight engines that have no symptoms or trouble codes...yet. The tech carefully notes the condition on the work order, explaining that the leak won't heal itself. Then, at the veiy least, he or she tries to schedule another appointment for the actual leak repairs. That additional work is found money!

Obviously, we deal with ECMs today that work overtime compensating for rich and lean conditions. Unless they're monitoring their fuel consumption very closely, many motorists don't notice anything's wrong until the MIL or Check Engine light comes on. A smoke machine will catch vacuum/air leaks long before they cause a trouble code.