SPONTANEOUS SHUTDOWN
| 2002 Chevy Suburban Shut Down Z
Question: My wife and I own a 2002 Chevy Suburban with 112,000 miles on it. Last week, on our way to Dallas, after almost 300 miles of driving on the Interstate with the cruise control set at 70 mph, and with no warning, the vehicle shut down. I shifted to neutral and tried to crank the engine but it would just turn over. Stopped on the side of the Interstate, I put the vehicle in Park, turned everything off, and tried to restart the vehicle. It started back up after a few minutes and ran fine. We drove another 50 miles before it did the same thing. Once I restarted it, I went to a local AutoZone to have the Suburban scanned for any trouble codes, and none was found. We drove it for the weekend and back home, with no other problems. The same thing happened about three years ago on the way to San Antonio. The vehicle died a few blocks away from our hotel when I braked at a stop sign. I then pushed the vehicle across the intersection and was able to start it after a few tries. I brought it to a Chevy dealership, and they could not find anything wrong with it. Last summer it did the same thing driving to Houston. Could this be the summer heat, vaporlock, or the fuel pump going out? The Suburban always runs fine and never stutters or runs rough.
Answer: It sounds like the fuel pressure has gone awry. Most General Motors fuel pumps have a life expectancy of only about 100,000 miles. As always, you want to test and confirm; it's worth another trip to a repair shop to have the fuel pressure accurately tested. You may not be aware, but going by the VIN, you're driving a FlexFuel Chevy (5.3-liter VIN Z engine). That means the fuel system is specifically designed to run on E85, an 85/15 mixture of ethanol and gasoline, or typical unleaded. Accommodation for E85 requires different injectors and a sensor to calibrate the air/fuel ratio, along with a fuel pump, tank, and lines that can withstand the highly corrosive properties of alcohol (methanol). Here's a little trick I've learned to help diagnose an intermittent low-fuel-pressure condition. The service manual fuel pressure test gives you the basics: Hook up a fuel-pressure gauge and power up the pump to see if it's supplying the specified amount of pressure. While running the engine, disconnect the vacuum hose from the fuel-pressure regulator to be sure pressure climbs accordingly, and then shut off the pump (engine) and check to see if residual pressure is bleeding off excessively. That's fine, but the problem is that a fuel pump on its last legs may pass all tests with flying colors. In your case, specified fuel pressure is 48 to 54 psi (55 to 62 for the non-flex 5.3-liter). While viewing correct pressures on the fuel pressure gauge with the engine running, pinch off the fuel return line (the excess fuel pressure is fed back to the tank by the regulator). On a good pump, pressure will shoot up to about 80 psi. That's the maximum pressure the pump has to offer. On a bad pump, it may rise just slightly above the specified values. While driving, this borderline available pressure may dip down low enough to cause drivability issues, or even shut down the engine. Typical fuel-injected engines require about 40 or 50 psi of fuel pressure to run as planned. The high pressure is needed behind the injectors so when the onboard computer rapidly pulses them open and closed, they shoot out a uniform fine mist of fuel, which produces optimum combustion. Carburetors are another story. They don't require much "pressure" from the pump at all, but just as much if not a higher "volume" of fuel. The gas merely has to be fed to the carburetor, past the needle and seat, and into the float bowl. There the fuel is sucked into the engine by vacuum through jets, with a little mechanical squirt from the accelerator pump when needed. Carbureted motorcycles have an advantage over cars or trucks because the gas tank is situated above the level of the carburetor(s). So gravity is more than enough persuasion to flow the fuel down to the carbs and run the engine-no fuel pump needed.
If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 831 S. Douglas Street, El Segundo, CA 90245, or e-mail us at trucktrend@sourceinterlink.com. Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.
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