Getting an accurate vehicle history and description of symptoms is always vital to a successful diagnosis.
But it's particularly important when diagnosing and repairing evaporative emission systems.
Some bosses and service writers roll their eyes-or their eyes glaze-at the mention of anything that sounds too technical for them. Understanding the evaporative (``evap'') system is only as difficult as you choose to make it. Furthermore, your own technicians will attest to the number of diagnostic trouble codes (DTCs) they encounter on evap-related problems.
Here's a very simplified description of an evaporative emission system. In bygone days, gasoline fumes from the vehicle's fuel tank were simply vented to the atmosphere. Then scientists discovered that noxious stuff called photochemical smog occurred when sunlight hit these gasoline vapors. Evap systems were developed to contain and store fuel vapors until the engine could burn them.
The charcoal canister, which first appeared circa 1970-71, is the heart of the evap system. The vapors from the fuel tank are vented into the canister. There, tiny pellets of activated charcoal absorb and store the vapors. (Activated charcoal is used for everything from cleansing the air the astronauts on the space shuttle breathe to filtering your favorite sippin' whiskey.)
Predictably, this canister can only store a certain amount of gasoline vapor. What's more, it's only designed to store vapor-NOT liquid gasoline! Gasoline can damage the activated charcoal and shorten the life of the canister. As a general statement, overfilling the fuel tank increases the risk of liquid fuel entering the evap system. Consequently, auto makers have gone to great lengths to reduce or eliminate the risk of liquid fuel entering the canister.
A long hose connects the charcoal canister to the engine's intake area; periodically, the engine sucks or purges gasoline vapor form the canister and burns those vapors. Note that this cleansing or purging process has to be closely monitored and controlled. Otherwise the ``richness'' of that gasoline vapor would overwhelm the engine, causing it to stall, stumble and/or run roughly.
En route to the engine, this hose goes through a critical component that's aptly named the purge solenoid valve. The engine control computer regulates the purging process-the flow of gas vapors to the engine-by carefully controlling the purge valve.
Remember that the engine computer only opens this purge valve when it knows the engine is ready and able to burn those extra gasoline fumes from the charcoal canister.
The process is so sensitive that when it does open the purge valve, it never opens the valve fully. Instead, it pulses the valve open and closed very rapidly and precisely.
The purge valve should be fully closed during routine vehicle operation so that none of the gasoline vapors from the evap system pass through it until the engine computer thinks so. But sometimes these purge valves leak.
Experience shows that distinctive, important symptoms of this problem may appear long before the engine control computer decides to set a relevant trouble code. For example, the engine may not start or may be difficult to restart immediately after the motorist refuels the vehicle. Or the engine may stumble and run roughly immediately after refueling. The driver may not specify that these occur after he fills the tank.
Many otherwise capable technicians have run themselves in circles over this condition because they didn't know this important symptom as well as an accurate vehicle history. This is another instance where service writers really can earn their pay by gathering meaningful information from the motorist. I think this detail is so important that it ought to be on every customer questionnaire a service shop or tire dealership uses.
For instance, this simple question would yield big diagnostic dividends: ``Does the vehicle's problem(s) occur only after you refuel?''
This is another example where the smartest service personnel don't always successfully diagnose problems. Rather, the best-informed people diagnose and repair the vehicle most profitably. You can never have too much information to diagnose a vehicle.