Question: My 2001 Chevrolet Suburban with the 5.3-liter V-8 has 115,155 miles on it and is mostly used around town. It started to idle roughly at random about a year ago -- not every day, but it happens in spells. I changed the fuel filter and have run fuel system cleaner through it a few times. This condition seems to go away for a while after trips over an hour, then returns anywhere from a few days to a few weeks later. This past week, a more troubling problem arose: The vehicle died three times while driving on the same trip. Fortunately, all three occurred in a parking lot, and it hasn't happened since. One similarity was that both issues occurred at low rpm. Any thoughts? Do you know if there any related recalls?
Answer: If the "service engine soon" indicator came on at any point, pull the diagnostic trouble code(s) with a scan tool and start there. If not, the best way of diagnosing any intermittent problem is by duplicating the condition with tools attached, in this case a fuel-pressure gauge, a spark tester, and a scan tool to view all powertrain control module data. Unfortunately, intermittent conditions don't always cooperate, and sometimes the events occur randomly with lots of time and mileage in between. That's when you want to analyze the common weak links.
When dealing with a 2001 Chevy, that means you go straight to the fuel pump. Low fuel pressure can cause a number of driveability symptoms. One is that the engine shuts down and won't start. Another can be a rough idle, sometimes because of the fuel boiling in the fuel rail just before it reaches the injectors. The higher the pressure, the higher the boiling point, and vice versa. If you don't have the equipment, get your truck to a quality shop with a fuel pressure gauge. Ask them to test the fuel pressure. It should be at 55-62 psi with the engine running, and the vacuum line removed from the fuel-pressure regulator. It should decrease 3-10 psi with the vacuum line reattached. There are two items you want to stress at the shop. One is to be sure the fuel pressure regulator holds vacuum and that there is no sign of fuel in the attached vacuum line. Both are indications of a cracked rubber vacuum diaphragm inside the regulator.
The other is kinking the fuel return hose and observing the increase in fuel pressure. While shutting off the fuel return path, you'll see the pump's maximum pressure. In this case, it should shoot up to about 80 psi or higher. If it doesn't increase significantly -- even if it's within service manual specifications -- you've got a weak pump that needs to be replaced.
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