Having problems with your 1996 Pontiac Sunfire ?

Low gas pressure in 96 sunfire. Replaced the oxygen sensor, vacuum hose, fuel pump and fuel filter. Fixed the ignition regulator. When I shut off the car it sometimes fires right up right away. It used to fire up right away when shut off for less than 10 minutes. This happens even when it is cold out.

Posted / edited by AnonymousUser on : 19-11-2018

Answers :

You have mentioned that you replaced the O2 sensor. The oxygen sensor is completely ignored by the engine computer while the car is starting, and doesn't come into play at all until the engine warms up completely. Are you sure you have a fuel pressure problem? If so, replacing the O2 sensor would not have any affect.

To be sure you have a fuel pressure problem, you need to measure it. This car does not have a fuel pressure test port, so you need an inline fuel pressure test gauge. To test the pressure, connect an in-line gauge and switch\015\012your ignition switch to the "run" position. (Do not start the engine.) The fuel pump should run\015\012for two seconds. During this time when the engine is not running, you\015\012should have between 41 and 47 PSI of fuel pressure. If you start the engine, the fuel pressure should drop somewhat at idle (maybe somewhere closer to 28-35 PSI). Fuel pressure is regulated according to intake manifold vacuum, so at idle, it should be fairly low, while at higher throttle settings, it should be higher.

From what I gather, your car cranks for a long time before starting. If you suspect low fuel pressure, a defective fuel pump relay could cause that condition. The relay is located in a relay/fuse box under the hood on the driver's side of the car. If the relay fails, the fuel pump can still be energized by the engine oil pressure switch. This switch closes when engine oil pressure reaches ~4PSI. If your engine is not producing 4PSI of oil pressure while cranking, or if it takes a while to reach 4PSI, that would explain your problem.
Repair Help & Product Troubleshooting for 1996 Pontiac Sunfire

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