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Diagnosing Fuel Pressure Problems When Starting

Diagnosing Fuel Pressure Problems When Starting

Dan Fitch


I've been reading your starting saga. From all indications I read into your account, the fuel pressure is dropping off after the start. This may be due to several problems, but easily traced.

Do you have a schematic? What year is your Series III? I have the factory schematic for my 82' XJ6-III which may be helpful. Here are a few ideas that might help get you into the right area. BTW, my first inclination is to suspect either the fuel pump relay, air flow meter, or connections to them.

When starting the car, turning the key to position #3 bypasses a switch in the air flow meter and feeds power to the fuel pump via the diode pack to the fuel pump relay. When the car starts, it sucks air through the air flow meter, closing the switch which keeps the car running. If it doesn't start, the fuel pump is shut down to keep fires from happening.

Here are a few ideas for diagnosis: (remember that this is based on my 82').

There is a row of relays on the firewall, over the center of the engine. They are the following from left to right - diode pack, pump relay, main relay, and starter relay. Battery voltage for the fuel pump to keep running after start comes from the main relay (terminal #87, brown wire with slate tracer) when the key is turned to position #2 (run), or position #3 (start). One of the brown-slate wires from terminal #87 on the main relay goes to terminal #10 on the air flow meter. The air flow switch (the one that closes when the engine starts) is inside the air flow meter and connected internally to the terminal with the pink-blue wire (not numbered on my schematic!). The pink-blue wire is connected to terminal #2 of the diode pack. From there, the diode pack steers the voltage to the fuel pump relay via the white-purple wire from terminal #5 of the diode pack to terminal #86 on the fuel pump relay. This is probably confusing, at best, but you can check it out, one step at a time.

Test for voltage at the brown-slate wire on terminal #87 of the main relay when the switch is in position #2 (run) and #3. Yes = go on. No = possible bad relay, inertia switch, or source from ignition key.

Take off the air cleaner for this next test. With key in position #2 (run), check for voltage at pink-blue wire on terminal #2 of diode pack. It should be zero initially. Now depress the air door at the inlet to the air flow meter. It is usually pulled open slightly by the vacuum from the engine after it starts. The switch should close in the air flow meter, and the test voltage should rise to full battery voltage. You should also be able to hear the fuel pump relay click at the same time. Voltage yes = go on. No = bad switch in air flow meter or connections to it. This is where I suspect you will find the problem. If all of that looks ok;

Again with the key in position #2, test for voltage on the white-purple wire on term #5 of the diode pack while you press the inlet door on the air flow meter. The voltage should be zero, climbing to battery voltage when you depress the door. Voltage yes = go on. No = probable bad diode pack.

Again with the key in position #2, test for voltage at the white-green wire on terminal #87 of the fuel pump relay while again depressing the air flow meter inlet door. Voltage yes = go on. No = bad relay or connections to it. I really doubt the relay is bad, because the engine will attempt to start.

I have assumed from your test with starting fluid; i.e., that the engine can be made to run with external fuel applied and that you have pulsing of the test light on the injector, that the main relay is operating correctly, and that the ECU is attempting to pulse the injectors. About the only things left are directly fuel related: very weak fuel pump, plugged fuel filter, or bad pressure regulator. A quick check of the fuel rail pressure (more than 30#) while cranking will help there. If the pressure is there, then check pressure at the pump itself. If it is good at the pump, but not at the rail, the fuel pressure regulator has failed or a blocked fuel filter is probable.

I hope this helps you some. Please let me know how it goes. This is an interesting problem.

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