Pressure Regulator Problems on the 1998 Grand Marquis

The 1998 Mercury Grand Marquis engine is available in one size, the 4.6-liter V-8. The fuel system is the sequential port style with the injectors fed through the use of a metal fuel rail atop the injectors. Fuel enters one end of the rail from the fuel pump in the fuel tank, and the excess fuel exits through the opposite end after passing through the fuel pressure regulator.

Operation

The Marquiss fuel pressure regulator is preset to maintain a maximum pressure of 41 psi on the injectors. It does so with the use of a calibrated spring pressure on a fixed flexible diaphragm. Incoming fuel from the fuel pump is allowed to build to the specified pressure. Any pressure in excess of 41 psi will overcome the spring pressure on the diaphragm and bypass it on its return to the fuel tank. Less fuel pressure is needed at an idle or the mixture would be too rich and foul the spark plugs in short order. To drop the pressure, the regulator uses the increased vacuum from the engine at an idle. The vacuum is directed to a vacuum port situated above the diaphragm on the regulator. The vacuum lessens the spring pressure on the diaphragm with its negative pressure lifting the diaphragm and allowing the excess fuel to return to the tank.

Symptoms of a Faulty Fuel Pressure Regulator

A faulty regulator will cause various driveability problems with corresponding symptoms. There are several ways the regulator can fail, so any number of these symptoms may be present but not necessarily simultaneously. They include: black smoke from the exhaust indicating a rich mixture, check engine light with a code for the oxygen sensor indicating a rich mixture, rough running engine with fouled spark plugs, smell of fuel, hesitation when accelerating because of a slow reacting regulator, lack of power, a big drop in fuel economy and hard starting.

Quick Visual Check

A fast way to check the fuel pressure regulator for the most common obvious problems is to open the hood and look at the rear of the fuel rail on the drivers side. You can plainly see the round regulator on the top of the fuel rail. With the engine off, pull the vacuum hose off and shake it to see if fuel has entered the hose. If the diaphragm broke, vacuum draws fuel into the hose. The fuel will travel through the hose into the intake manifold, where it will enter the cylinders and burn. This will richen the mixture and foul the plugs, not to mention kill the fuel economy.

Look at the regulator itself where its bolted to the rail and see if the rail is discolored because of fuel leaking past its O-ring seal. If you see either of these indicators, the regulator is bad and must be replaced.

If the last inspection did not turn up any indicators, then a fuel pressure gauge must be used. Look on the fuel rail for a Schrader valve that looks like an air valve on a bicycle tire. It is usually on the drivers side of the rail, but could be on the opposite side. Install the gauge and start the engine. The fuel pressure should be around 8 to 12 psi at an idle and should rise to 39 to 41 psi above idle. Replace it if it differs. It also should rise swiftly when the engine rpm is raised.

Replacement

Replacing a fuel pressure regulator is a simple proposition. The fuel pressure, if any remains, must be dropped in the system before removal. Pull the fuel pump fuse out of the fuse relay box on the drivers fender well and start the engine. It will only run for a second, but that is all that is needed. Reinstall the fuse, then replace the pressure regulator.