Q: Our 1993 Lumina minivan has the 3800 V-6 engine. Fuel pressure is good when the key is turned on and while it warms up. As soon as it warms up, and I’m assuming goes into “closed loop" operation for fuel-air mixture calculations, the fuel pressure smoothly drops to zero over about 3 seconds. I’m assuming the computer is turning the fuel pump off. The factory manual doesn’t describe what inputs to the computer would cause it to shut down the in-tank fuel pump. My suspicions would point toward either the fuel pressure regulator or the oxygen sensor. Can you enlighten me?
A: There’s one more scenario that could shut the fuel pump off with the engine running — low oil pressure. The fuel pump is controlled two ways. The ECM — electronic control module — grounds the fuel pump relay when the key is turned on for 2 seconds to bring fuel pressure up quickly for startup. When the ECM sees a “start" signal indicating the engine is cranking, it grounds the fuel pump relay to allow the engine to start. Once running, the relay is continuously grounded by the oil pressure sender — as long as oil pressure is adequate. If oil pressure drops below a safe level, the oil pressure sender will open the ground to the relay to stop the pump and protect the engine from mechanical damage. Low oil pressure or a faulty oil pressure sender could conceivably be the problem. It is possible to bypass the sender to operate the relay out of circuit as a test.
Another possibility is a clogged sock filter on the fuel pickup in the tank. As the fuel pump picks up fuel to pump forward to the engine, debris in the tank may be progressively restricting fuel flow through this fine-screen filter until fuel pressure falls to the point of stalling the engine.
Q: My 2005 Nissan Altima 2.5-liter automatic with 72,000 miles has a shift shock when shifting from first to second gear and from second gear to third gear. We have already done transmission oil flushing but no change. An OBDII scan by our local dealer showed no codes. The auto transmission light and service engine light are not on. What could be the solution to this?
A: That’s a tough one, but here are some possibilities. Harsh shifts are often related to high hydraulic line pressure in the transmission, sometimes related to high operating temperatures. Other potential issues include solenoids or accumulators that buffer pressure during shifts or a problem with the control valve assembly. But don’t overlook the possibility of a misadjusted throttle position sensor, inaccurate speed sensor or transmission fluid temperature sensor.