Smoke signals

Motor,  Jul 2003  by Marinucci, Dan

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Other techs do the basic engine smoke check when there is misfiring, rough idling or any land of lean-condition symptom such as the classic P0172 code. Sure, this code confirms that the computer is driving the mixture richer in response to some kind of lean condition-but which lean condition? Restricted fuel filter? Unmetered or "false" air? Dirty injectors? Here again, experienced driveability techs using smoke machines agree that the basic engine smoke check described earlier is so fast and effective it doesn't make sense noi to do it nrstl That way, they eliminate vacuum leaks from the leancondition search right away.

At Stocks Underhood Specialists in Belleville, Illinois, driveability veteran Paul Stock succinctly summed up this approach: "No matter how sophisticated these vehicles become, our job is still a big, logical process of elimination-separating the knowns from the unknowns under that hood. I've watched too many guys run themselves in a circle because they don't qualify the known things first," he explained. "When the smoke machine quickly establishes that there are no air leaks, that's an extremely valuable 'known' to anyone diagnosing a lean condition."

Meanwhile, let's return to this basic test. You can usually fill an engine with enough smoke for an adequate vacuum-leak check in 60 to 90 seconds.

Then grab your halogen spotlight and patiently inspect the engine and its vacuum plumbing for smoke signals. Among techs who use smoke machines extensively, the consensus is that the more you use the machine, the quicker you'll spot leaks-including the smaller, more elusive ones-and the easier it will be to make discretionary calls on certain leaks.

This basic engine check reveals two kinds of air leaks-those that have already caused a symptom and those that will eventually cause a symptom. And smoke repeatedly pinpoints air leaks that would be extremely difficult to find via other means.

For example, fohn Bradley at Sawchuks Garage in Pottstown, Pennsylvania, showed me an intake gasket leak under the intake manifold of an LH Chrysler V6. This car idled rough and flunked the emissions test due to excessive HC, etc. Within minutes, Bradley proved it was gasket time because smoke was pouring out of the valley of the engine.

"Some guys still leak-check with propane or spray carb cleaner," Bradley observed. "But how would you pinpoint the exact location of that Chrysler s leak with those old methods?"

Bradley explained how helpful it is to smoke-check the individual vacuum circuit related to the vehicle's symptom. Recently, this approach flagged a rusted-out vacuum reservoir on a Ford truck that might have taken a long time to pinpoint with other methods. These coffee can-shaped vacuum reservoirs are often hidden far out of sight on Ford products.

Time and again, smoke signals have proven to be an ideal "flag" for problems such as a disconnected or cracked vacuum hose, cracked plastic vacuum fitting or vacuum switch, leaking injector O-ring, loose or cracked intake air duct, leaking EGR valve or EGR gasket-the list is endless!