97 SOHC Fuel Pressure

[Scout3]

Just finished working on my son's Explorer. It was missing bad and slowly got worse. Decided to do basic tune up. Changed the plugs and wires. Plugs needed changing and I found one of the wires was open. Runs pretty good now. I also changed the fuel filter. It is a little slow to star so I checked fuel pressure. When you turn the key to run but not starting the pressure only bumps up to about 5psi. Turn the key off and back on and it bumps up higher. When you go ahead and start the engine it goes up to 30psi. If you slowly rev the engine it stays at 30psi. If you romp on it it will go to 40. If you disconnect the vacuum line from the pressure regulator it goes up to 40psi. When you turn the motor off the pressure goes down to zero within about 15 seconds. I cannot crimp the return line closed because it is a braided wire kind of line into a solid metal line.Does it sound like my pump is weak? 30 psi seems kind of low.

 

[drdoom]

Sounds like it is failing to hold pressure, like it leaking down.

 

[2000StreetRod]

rapid pressure loss

According to my Haynes Repair Manual your fuel pressures are at the lower limits for with (30 psi) and without (40 psi) vacuum to the fuel pressure regulator (FPR) but indicate rapid loss of pressure (should hold for 5 minutes). Rapid loss of pressure may be compensated for by cycling the ignition key three times before cranking the starter or prolonged starter cranking. The PCM enables the fuel pump for a couple seconds after switching the ignition on and while it detects the crankshaft rotating via the crankshaft position sensor. I add a bottle of Techron fuel injector cleaner to my fuel tank annually. My fuel system pressure only decreases to half of normal in about 16 hours. Based on my experience I suggest performing the steps below to isolate the source of rapid pressure loss.

1. Check for external leaks by sniffing for gas fumes and looking for drips on the driveway. The most likely external failure is the flexible hose between the fuel tank and the rigid fuel tube. Other good possibilities are the fuel filter connections, the fuel rail supply connection and the fuel rails.

2. Check for a ruptured FPR diaphragm. Fuel may be present when the vacuum hose is disconnected but a hand vacuum pump is needed for a reliable test to see if the FPR holds vacuum.

3. One or more fuel injectors may be leaking which may be detected by the presence of "wet" spark plugs when examined. Adding a bottle of fuel injector cleaner to the fuel tank may decrease leakage after a few hundred miles of driving.

4. The unserviceable check valve in the fuel pump assembly may be leaking. Replacing the fuel pump will not correct a leaking check valve. Adding a bottle of fuel injector cleaner to the fuel tank may decrease the leakage and is worth trying since the alternatives are to replace the entire fuel pump assembly or to install an external inline check valve. (Some Volvo owners install an inline check valve in the flexible fuel hose near the engine rather than replace the expensive fuel pump assembly with a leaking check valve.)

 

[Scout3]

Street Rod,

New fuel filter and connections are good, no leaks. FPR did not not have any fuel in it when I disconnected the vacuum hose. Spark plugs were worn but all looked the same and looked like they had been burning good. No evidence of oil burning or wet with fuel. Turning key on three times before starting does make it start on the forth try quickly. If you don't do that it has to crack a little while before it starts. So it is obviously a fuel pressure issue.

All this time when it was missing badly there has been no CEL and no pending codes. I would have thought I would have gotten a misfire code or a run lean code.

If you slowly rev the engine, shouldn't fuel pressure rise too?

Thanks

Ed

 

[2000StreetRod]

no codes

. . . All this time when it was missing badly there has been no CEL and no pending codes. I would have thought I would have gotten a misfire code or a run lean code.

If you slowly rev the engine, shouldn't fuel pressure rise too? . . .

Since fuel pressure is normal when the engine is running the PCM does not detect any malfunctions. A fuel pressure sensor was not installed on Explorers until the 3rd Generation models.

On your return type fuel system vacuum (an indication of engine load) affects the fuel pressure. Disconnecting the vacuum hose to the FPR simulates maximum engine load (no vacuum). Slowly increasing engine speed when transmission is in park or neutral does not load the engine or change the vacuum. Consequently, there is no increase in fuel pressure.

 

[Scout3]

Thanks for the info.

Ed