Truck Trend Garage: 1991 Chevy Suburban -- Fuel Cutoff
Question: I have a 1991 Chevy Suburban with a 350 V-8, 4x4, and 216,000 miles. I call it my work truck and don't drive it much on long trips, and sure enough the problem accrued after a long trip. The truck cut off on me at a stoplight but started right up. I was low on gas (the gas gauge doesn't really work), and it began to drive rough. It was sputtering and hesitant -- I thought it may have happened because it was low on gas, so I went straight to a gas station and filled it up. That didn't fix problem. No matter how hard I pressed the gas pedal, the Suburban wouldn't go anywhere. Replacing the fuel filter didn't help either. It also revs up and down at idle. It drops to 400 rpm and then back to 1000 rpm. I replaced the throttle body gasket, spark plugs, took the cat converter off, and replaced the oxygen sensor. I was told it could be the idle control sensor, so I took it off and cleaned it. It appears to be working, and the spring moves in and out. Then I was told maybe the fuel pump doesn't have proper pressure in drive and is causing it not to run right.
| Z Truck Trend Garage Fuel Cutoff
Answer: You're getting deep with a lot of parts swapping as opposed to good-old diagnostics. If it runs rough and there's a lack of power, that eliminates the Idle Air Control valve as the culprit. The IAC valve only controls idle speed. I know you also have an idle fluctuation issue, but that's probably just an after effect from the rough running and power loss problem. You have to do the basics, which require some special tools. First off, use a scan tool to check the powertrain control system for diagnostic trouble codes and blatantly incorrect engine data -- sensor inputs and PCM output commands. Second, confirm if it's an overall lack of power or a misfire at one or two cylinders. You can do that by shorting out each ignition wire one at a time to see which cylinders drop in rpm and which don't. Cylinders that don't affect engine speed with spark taken away, are not firing. The other basics are to check that the ignition is producing a strong enough spark all the way around--there's a tester for that too -- and checking for adequate fuel pressure with an accurate fuel pressure gauge. Your problem can have a root cause in a number of areas, including mechanical engine issues. My hunch is that it's a fuel pressure problem. You may have a fuel pump on its way out.
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