Fuel-vapor system could block gas tank

| Friday, Feb. 21, 2014, 9:47 p.m.

Question: I have a 2001 Mercury Sable with a 3.0-liter DOHC engine and 100,000 miles. For about the past four months, it will not take gas when filling it up. The fuel nozzle constantly shuts off, no matter how fast or slow you pump it. Within the last month, I had to change the fuel pump, and before I put it back together, I blew air through the lines, hoping this would fix the problem. It did not. I sure need your help.

Answer: This sounds like an ORVR problem. This term refers to the federally mandated Onboard Refueling Vapor Recovery system, which began in 1998 and phased in to 100 percent of the passenger car fleet by model year 2000. Light- and medium-duty trucks began a five-year phase-in in 2001. ORVR is designed to capture and hold fuel vapors released from the tank/filler neck during refueling. The claimed benefit is up to 400,000 tons of volatile organic compounds and toxins not entering the atmosphere each year, an admirable objective.

ORVR systems employ numerous mechanical parts within the tank filler neck and inside or atop the fuel tank, along with the existing evaporative emissions system components to capture and recycle fuel vapors that would be pushed out of the filler neck as liquid fuel enters. It's a tricky balancing act to properly sense vapor pressure and incoming liquid, so fuel can enter the tank quickly and vapors are held. Vapors are sent to the charcoal-filled evaporative canister for storage, and are consumed by the engine while driving, in a process known as purging.

Difficulty refueling is the typical symptom when an ORVR system fault occurs. Issues with the filler neck, valves atop the tank, various hoses and the canister are unfortunately not that rare. Effectively diagnosing such faults requires a professional-grade scan tool, evaporative system leak tester and detailed testing procedures. The scan tool is used to turn on and off EVAP system control valves and to observe tank pressure during a half-dozen test procedures, as the leak tester applies slight pressure to the system. These tools and procedures are work for a dealer or heads-up independent shop. Lesser methods might result in unnecessary parts, replacement and labor.

My research indicated several possible causes for your symptom, making a hunch too risky to recommend. We might assume the fault does not lie in the partnered EVAP system components, which include the fuel tank pressure sensor, canister, vent valve and purge valve, as you haven't mentioned an illuminated “check engine” light.

A nice but dated write-up of ORVR facts can be found at: http://epa.gov/otaq/regs/ld-hwy/onboard/orvrq-a.txt.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net

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