Welcome to Modern Agriculture!
You had hoped to get a late-season fungicide application on your corn crop, but the sprayer blew a hose. You jump into your gasoline-powered pickup and head to town to get the parts. About 5 miles into the trip, the dreaded amber Service Engine Soon light pops up, but the truck is running fine. While in town, you have the code read, and it is for the EVAP system. The code is erased, and the SES light doesn’t come back on for a week. What’s going on?
Welcome to the world of evaporative emission control systems!
In the early 1970s, when the auto industry faced scrutiny and pressure to reduce air pollution, it was recognized that vehicles were polluting even when they were not running.
The fumes from the fuel tank and carburetor bowl vent were emitting hydrocarbons into the atmosphere. In addition, the plastics used in the interior were gassing when heated, and even the engine paint was releasing hydrocarbons.
The discovery prompted a new test procedure called the SHED test (Sealed Housing for Evaporative Emission Determination) that every vehicle had to pass.
The SHED looks like an auto body paint booth. The vehicle is placed inside, and the hydrocarbon emissions over a specified period are measured with the engine off. There are ambient temperature and heat-soak SHED protocols.
As years went by and EPA standards tightened, it resulted in the elimination of glossy engine paints and the use of a flat finish. (Most consumers incorrectly thought this was a cost-cutting measure.) Interior plastic components were made from new formulations and resulted in a large reduction in that new car smell we all loved.
The largest offenders were the fuel tank vapors and carburetor. The introduction of fuel injection eliminated the float bowl.
This spawned the early charcoal canister evaporative emission system that would store the fuel vapors. At a predetermined time when the engine was running, the vapors would be purged from the canister by applying engine vacuum via a purge valve.
The hydrocarbons would be burned as part of the normal combustion event. This worked extremely well and required little to no maintenance by the consumer other than a charcoal canister filter change, which no one did anyway.
For the 1996 model year, a second generation of onboard diagnostics was introduced (OBD-II). It mandated that the system would self-check for leaks and evoke the SES light if one was found.
The protocol for a maximum leak (1996 to 1999) was a hydrocarbon escape equivalent to what would be realized with a 0.040-inch hole. For model year 2000 and beyond, the standard was tightened to a 0.020-inch hole.
What was at first a simple design instantly became very complicated since a series of solenoids was required to have a self-test function. It works by systematically applying engine vacuum to the EVAP system lines, while a solenoid trapped the low-pressure air in that part of the system. It then determines if the vacuum held and, if not, what the leak rate was.
A differential that would have the system lose integrity that is currently equivalent to a 0.020-inch hole would cause the test to fail and illuminate the SES light. If the area where you live has emissions inspection, the vehicle will fail if there is an EVAP system code.
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A modern OBD-II EVAP system that would be found on a gas-powered truck has a series of vapor and vacuum lines running from the fuel tank to the engine along with the charcoal canister. Often, the lines may pass through sections of the frame, but, at some point, they are exposed.
On a farm or ranch where the truck is used off-road, there is a greater possibility for one of the lines to be damaged or kinked. In most instances, they are made of a hard plastic material that can easily crack.
In like fashion, the connections under the hood are exposed to extreme heat cycles that, over time, will cause them to become brittle and leak.
The solenoids that control the testing and purge cycle are also exposed to the elements.
What needs to be understood is that an EVAP leak test is not constantly occurring. There are generic and application-specific protocols for when the system does a self-check.
If a leak is detected, the SES light will evoke and a code stored. The only method to shut the light off is to use a scan tool and erase the trouble code. In the imaginary scenario I referenced in the beginning of this article, you’d have had to drive this truck a week before the test ran or it failed again.
In most applications, an EVAP test will not run if the fuel tank level is above three quarters full or below one quarter of a tank. The system also monitors the ambient temperature and will not test above 95°F. or below 30°F. If these protocols are met, an additional criterion is set for an engine that has not run for at least eight hours or needs to be driven between 45 to 65 mph for 10 minutes.
With these criteria, you can see how an EVAP leak is not easily found. If the use of the truck, the weather, or the fuel tank level is in a certain range, you can be tricked into thinking that it’s fixed.
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If any of your vehicles triggers one of several EVAP codes, the first place to look for a problem is at the fuel cap. Make sure it is tight, the cap’s gasket is sound, and the cap is not damaged.
On vehicles that have a capless fuel fill, the fuel fill door will have a gasket on it that needs to seal tight against the vehicle’s body. Check the gasket and make sure it is free of mud, dust, and, in the winter, salt and slush.
Take the emergency fuel fill funnel provided with the vehicle and push it fully in and out about 25 times. Most systems have two flaps in the fill tube that, if they are not sealing, will cause an EVAP failure. Next, do your best to perform a visual and, if possible, a tactile inspection of the lines that you can access. Look for loose fit, cracks, compressed areas, etc. Though this step is important, it will often leave you with inconclusive results.
The only efficient way to fix an EVAP problem is with a smoke test machine.
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In my experience, unless a line was damaged by contact with something, the leak failure is the cumulative effect of several small leaks that all add up to the 0.020-inch threshold.
A smoke machine creates nontoxic and non-flammable smoke that is pushed into a vacuum line via a few pounds of shop air pressure. The entire EVAP system will fill with smoke and, due to the low pressure, will come out at the leak site(s).
If one of your farm vehicles fails a leak test and you take it to a shop in town or to the dealer, ask if they use a smoke machine. If not, find someone who does, or you will spend a good deal of your hard-earned money on a failed repair.