1997 Jimmy Low Fuel Pressure

[Jake524Rocks]

I bought a 97 Jimmy and I'm working on getting it running. It doesn't want to start but if I put a cup of fuel in the throttle body and step on the gas a little it starts right up but then stalls at low rpms under a load. I hooked up a fuel pressure tester on the shrader valve on the fuel rail. When I turn the key on it reads between 43 and 45 psi even when I cycle it a few times. If I let it sit for a minute or so the line loses pressure. If I put the cup of gas in and it gets running the tester reads around 50 psi. I want to know if the problem is in the fuel pressure regulator or if the problem is the fuel pump. I don't want to buy either unless I know I need them cause this car still has brake and interior issues that I need to spend money on. Any insight would be greatly appreciated. Someone on one of the other threads said to pinch off the return line to see if that increases the pressue and if so the problem is in the regulator. The problem I had doing that was that there were three fuel lines with two of them resembling a return line. The other problem is that these fuel lines are either metal or plastic so pinching them off would ruin them. Please tell me what my options are.

One last question: If I do have to replace the fuel pump what are the good brands to buy because I know some of the aftermarket pumps are garbage.

Peace,

Jake

[old_master]

Fuel pump operation:
When the ignition switch is turned to the RUN position, the Powertrain Control Module (PCM) activates the fuel pump relay for approximately 2 to 3 seconds, and then deactivates the relay. This is commonly referred to as “fuel pump prime”. When the ignition switch is turned to the START position, the PCM activates the fuel pump relay continuously. When the ignition key is turned back to the RUN position, the PCM will keep the fuel pump relay activated provided the PCM is receiving a signal from the crankshaft position sensor indicating the engine is running. If the engine stalls, or is not running for any reason, the PCM will deactivate the fuel pump relay after approximately 2 to 3 seconds.

Fuel pressure test:
This determines if the fuel pump is capable of building adequate pressure to satisfy the fuel pressure regulator. It will also determine if the system is able to hold the fuel pressure. If the pressure is insufficient, or the pressure does not hold, this test does not determine what the problem is. It only confirms that a problem exists. Fuel pressure must be tested only when the engine is cold. This avoids increases in pressure by heat radiating from the engine causing erroneous readings. There are no specifications for fuel pressure while the engine is running.
Here is a link to an adequate, inexpensive, fuel pressure tester: http://www.harborfreight.com/cpi/cta...emnumber=92699 Install the fuel pressure gauge to the test port near the distributor. Turn the key to the RUN position but do not start the engine. While the fuel pump is running, fuel pressure must be 60-66psi. If the pressure is not within range, there is a problem. After 2 to 3 seconds, the fuel pump will shut off. Fuel pressure must remain between 55-60psi for 3 to 5 minutes. A drop in pressure below 55psi indicates excessive leak down. If a problem exists, leakdown must be checked and repaired before testing fuel pump output.

Diagnosing excessive leak down:
External fuel leaks can occur from the pressure connection at the fuel pump, up to the connection going to the injector assembly. Internal fuel leaks can occur at the check valve in the fuel pump assembly, the fuel pressure regulator, the fuel lines under the upper intake manifold, or a fuel injector. Accurate diagnosis for pinpointing the cause of an internal fuel leak requires the temporary installation of a shut off valve in the fuel pressure line. Here is an image of the tool: http://img139.imageshack.us/my.php?i...tool002vu8.jpg Install the 3/8” shut off tool inline at the fuel filter outlet fitting. Open the valve, turn the ignition switch to the RUN position and allow the pump to pressurize the system. When the pump shuts off, immediately shut the valve off. If fuel pressure remains above 55psi the fuel pump check valve is leaking. If the pressure drops below 55psi the leak is under the intake plenum.

Testing fuel pump output:
Install the 5/16” shut off tool in the fuel return line. Open the valve. Install a fused jumper wire to activate the fuel pump continuously. Slowly close the valve while watching the fuel pressure. Do not allow fuel pressure to rise above 75psi or the fuel pressure regulator will be damaged. If fuel pressure will achieve 75psi, the pump is OK. If pressure will not reach 75psi, the pump is weak and needs replacement.

[Jake524Rocks]

Old Master, you are wise!

I will perform these tests today. Where do I get the parts to make that shut off valve? The only parts I'm wondering about are the quick connects on the ends. The rest I'm sure I can get at the hardware or autoparts store. I did disconnect the return line at the filter yesterday and crank the engine for a while. No fuel came out though. Is that an indication that the fuel pump is not even pressurizing enough to get past the pressure regulator? I will still perform the above tests. Thank you so much for your help.

Peace.

[old_master]

Thanks for the kind words, been doing this for a "few" years. It depends what ends are on your fuel filter. If you have quick connects, they are the easiest to deal with. The male and female ends are available at most auto parts stores. They're made by Dorman and are intended as a fuel line repair. Use high pressure fuel injection line ONLY, and use good hose clamps. The valves you can find at any good hardware store, Home Depot, Lowes etc. The valves must have threads in each end to accomodate the adapters, (NPT thread). Use a compression adapter in one end of the valve and a barbed adapter in the other. The valves are rated to 600psi, and the compression adapter is rated to 170psi. The fuel pressure line on the vehicle is 3/8" and the return line is 5/16" so you'll need to make up two testers to be able to fully diagnose the system. The vapor line and the fuel return line are both 5/16", don't confuse the two when installing the test valve.