Niagara Gazette

October 20, 2013

TOM'S CORNER: Taking a look at EVAP system problems

By Tom Torbjornsen
Niagara Gazette

Niagara Gazette — SANDY FROM BOSTON: I own a ’06 Ford Taurus and the engine light went on. The last time this happened I replaced the gas cap and the light went out. However, this time when I replaced the gas cap the light stayed on. I had a mechanic look at it and he said something about the evaporation system? Is this just mumbo-jumbo?

TOM: The MIL (Malfunction Indicator Light) went on because the engine management computer detected a malfunction in the computer system and, in your case, the EVAP system. No mumbo jumbo here; your car has a genuine problem and it must be repaired in order to pass state inspection emissions laws.

I get a lot of questions relating to this topic. “Tom, I was told to replace my gas cap. I did and the light went out. Why?” “Tom, what is an EVAP code?” “Tom, can a bad gas cap cause my check engine light to come on?” Let’s explore answers to some of these questions.

What is an EVAP system?

The EVAP system is an emission control system in your car designed to trap and re-circulate fuel vapors back into the engine to be burned. It also ensures that harmful fuel vapors do not escape into the atmosphere. It is a closed and sealed system comprised of a series of steel and rubber lines, valves, and a canister. When the system is compromised, the engine management system senses it and trips a check engine light.

Can a bad gas cap cause the check engine light to come on?

Yes, absolutely it can. The gas cap has a large rubber O-Ring that seals the mouth of the gas tank inlet. When the rubber O-Ring goes bad, the sealed system is compromised and thus trips a MIL (Malfunction Indicator Light) or check engine light. Also, it’s a good idea to shut your car off during fueling. If you leave the car running, the system will sense a leak when the gas cap is off, and it can trip a check engine light as well. If when the tech checks the gas cap; notes that the sealing surface where the gas cap seals is marred or worn out, this will warrant replacing the fuel filler neck. This is the hose/metal tubing that goes into the gas tank, providing a means by which to fill the tank.

What’s it take to fix the problem?

Sometimes, as in Sandy’s case, simply replacing the gas cap will fix the problem. But when an EVAP code comes up in the diagnostics, the job becomes more complicated. In order to find a way to identify where a leak is in the EVAP system, tool designers came up with a device called a “Smoke Machine.” Much like a fog machine in a stage production, the smoke machine generates vapor. An EVAP line is disconnected and hooked up to the smoke machine, which feeds vapor into the EVAP system. The tech then scans the lines, hoses, valves, and canister for any escaping vapor, pinpointing the source of the leak. The repair the leak, reset the computer, perform a drive cycle and you’re off to the races again!


LINDA FROM LITTLE ROCK: Is there a product on the market that can stop oil from leaking in an engine? My Lexus is leaking oil and I was quoted $1200 to fix it. I don’t have the money and want to drive the car. Please help.

TOM: No product stops oil leaks. The only way to stop an oil leak is to restore the sealing capacity of either the mating surface or the seal. Mating surfaces must be clean and have enough material to fit securely to a gasket or seal. Seals and gaskets must be flexible and pliable so that they can form a tight seal against the mating surface. Linda, you’re going to have to find a way to get it fixed. Sorry.

Linda brings up a hot topic. This one falls under the category I call “Mechanic in a Can.” Some companies take advantage of the ignorance of consumers, and promise them instant cures for mechanical problems by simply adding their wonder elixirs. Be wary of products that claim to “Stops Engine Oil, Transmission Fluid, and Coolant Leaks.” What are the hard facts surrounding these snake oil compounds?

Engine oil and transmission fluid sealers: This category of chemicals, when added to engine oil or trans oil, restores aged-hardened rubber seals that are leaking so that they are once again soft and supple. As the seals swell, they tighten against the mating surface and the leak stops. However, in short order the sealer chemical wears out, the seals return to their hardened state, and the leak is back. It is for this reason I do not recommend using these products. Have the component re-sealed or rebuilt. You can’t restore rubber to seals, nor can you restore metal to sealing surfaces. Don't waste your money on such foolishness. Fix the problem or risk causing more damage (and that means more money out of your pocket in the long run).

Radiator stop leak: These chemicals are designed to course through the cooling system to the location of a leak. When the stop leak product finds an exit from the cooling system, it starts to close the leak by “building on itself” as it exits the system. However, problems arise when system pressure is restored. For example, in the case of aged and brittle radiators, wear is relative and another leak will spring up. In addition, “stop leak” tends to stop up other small orifices in the system such as heater cores. A plugged heater core causes the vehicle’s heater to malfunction. Finally, I have seen so much stop leak used in an engine that it literally clogged up an entire bank of water jackets. These are passageways lining the cylinder walls through which engine coolant flows, transferring heat away from the engine. This stuff harms more cars than it fixes.

‘Til next time ... Keep Rollin’

"America's Car Show" with Tom Torbjornsen airs 7:30 p.m. Wednesday, 9 a.m. Thursday and 11 a.m. Saturday on WBBZ-TV.