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Q: Fuel pressure problem

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recently bought a used 2000 Mitsubishi Montero Sport.my husband changed the fuel filter,and then had a friend change the fuel pump because the vehicle would start and die right away.after changing the pump, it seemed fine it would start everytime and run.we drove it for a few days,then one night it would not stay running again and now we are back to square one any other suggestions

My car has 200000 miles.
My car has an automatic transmission.

This may be related to many different possibilities. I would recommend with first starting with checking the fuel pressure to be sure the pump is producing the proper fuel pressure. In addition to the fuel pump, there is also something called a fuel pressure regulator which helps to regulate the fuel pressure in the system based upon the increase and decrease in fuel demanded by the fuel pump. Fuel injectors are fixed in the vehicle and need an exact amount of pressure at the nozzle, during operation. Fuel pressure regulators help in maintaining set pressure for smooth running of the vehicle.

This may also be related to a dirty or faulty idle air control valve. The idle air control valve monitors the air intake as it is mixed with fuel prior to being injected into the engine at low speeds and at idle. This valve is controlled by the vehicle’s computer and will adjust idle speed based upon other measurements such as engine temperature, intake air temperature and electrical system load or voltage. This is also an important function when starting the motor as it allows the motor to run and idle on it’s own once the motor fires. When you accelerate, the engine RPM increases, and as you let off the gas, the RPM slowly returns to the normal idling speed with the help of the idle air control valve making the transition from a higher RPM back down to idle speed while adjusting the air/fuel ratio constantly to allow this to happen smoothly. When the engine RPM drops below the normal range of about ~800 RPM, this often times will cause the engine to stall indicating a dirty or faulty idle air control valve.

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