Mechanics

CHEVY FUEL PRESSURE TEST

1990 Chevrolet Lumina

Engine Mechanical problem
1990 Chevy Lumina 6 cyl Front Wheel Drive Automatic 220000 miles

I submitted a question several days ago. 1990 lumina hard to start cold or hot. Your response was to test the fuel pressure first, if that checks out, then change the ECT sensor, TPS, and the ingnition switch.
I tested the fuel pressure and not sure what to make of it. First I tested the fuel pump by applying 12 volts to the test terminal. All is well, I hear the whinning sound. Now connecting the gauge to the schrader valve, key in the on position, the pressure is low, 10 psi. I started the car, pressure jumped to specs, 32 psi. From there all checks out OK. I tapped the throttle, pressure jumps about 5psi. Removed the pressure regulator vacuum, pressure jumps about 10 psi. I did change the ECT, no effect on starting, however I believe the idle is improved and the exhaust smells better. I believe it was a little rich before.
I did notice when tapping the throttle, there is a very slight hesitation. Very slight! What should I do at this point? Why is the fuel pressure low when the key is in the ON position?
Avatar
Golfnguitars
July 29, 2010.



Pressure is tested with key on engine off and should read 41-47 psi for 3.1 and 40-47 3.8 So replace the fuel pump and you are good to go. You can check the filter, but I haven't come across one that clogged and still run! Here it is from the book: Fuel Pressure Check (PFI) 1. Relieve fuel pressure as previously described in FUEL PRESSURE RELIEF (PFI). Connect Fuel Pressure Gauge (J-34730-1) to fuel pressure fitting on fuel rail. 2. With gauge installed at fuel rail connector, turn ignition on. With ignition on and engine off, pressure should read within specification. See SPECIFICATIONS - 4-CYL article (for 4- cylinder models), See SPECIFICATIONS - V6 article (for V6 models), See SPECIFICATIONS - V8 article (for V8 models). Start engine. Pressure should drop 3-10 psi (.2-7 kg/cm 2 ). For more details on fuel system pressure testing, see SYSTEM/COMPONENT TESTS article.

Merlin2021
Jul 29, 2010.
New Info!
I performed another fuel pressure test, this time when the key is in the on position, I am getting a reading of 42psi. However, the pressure drops to ZERO after a few seconds. So, I turned the key off, and on again, 42psi, so I started the car immediately, and the car started right up, with 34 psi while ideling. I perfomed this operation 6 times with the same result. Is the fuel pump weak, or perhaps an intermittent connection somewhere? The car runs and accelerates quite well!

Tiny
Golfnguitars
Jul 29, 2010.
No it's probably OK, this system is designed to bleed the pressure off quicker than most, as long as it's OK with key on engine off and drops 3-10 psi on start up, it's OK.

Merlin2021
Jul 29, 2010.
This seems to be an intermittant problem. Sometimes when I turn the key to the ON position, I have 42psi, sometimes I have zero. What should I do from here? Could it be the TPS or MAP sensors? Could it be the wireing at the fuel pump relay? Or connecting harness near the fuel tank? I guess we have ruled out a faulty fuel pump? I was repaced 4 years ago. Thankyou for all your help.

Tiny
Golfnguitars
Jul 29, 2010.
When it reads zero check for power at the pump, use a volt meter. The pump is designed to run only 2-3 seconds so hook up the meter and get a friend to turn the key on.

Merlin2021
Jul 29, 2010.
OK, you're going to have to instruct me on how to test for power at the pump. I have a multimeter, but where do I test?

Tiny
Golfnguitars
Jul 29, 2010.
The pump is inside the gas tank, jack up and place on stands, then unplug the connector at the pump, test gray wire and ground, meaning black lead to ground red lead to gray wire, this gray wire comes from the pump relay. At least 10 volts minimum is what we want. And you should here the pump run for the 2-3 seconds.

Merlin2021
Jul 29, 2010.
I just realized, when the pressure reads zero, I can hear the pump whining for a few seconds, so I assume it is getting power. This is what I have observed so far, when the car sits for 15 minutes or more, I turn the key on, the pressure reads zero, and I can hear the pump. After several attempts turning the key to the on position, then off, wait 10 seconds, turn on and off, wait, then maybe the 3rd or 4th try, the pressure is 42psi, and all is well from there. This is the best way I can describe what is happening here. What would cause this?

Tiny
Golfnguitars
Jul 29, 2010.
- Check the following items.  Check fuel pump relay. To do this, connect test light between fuel pump test terminal (terminal " G" of ALDL on most vehicles) and ground. Turn ignition on. Light should illuminate for 2 seconds. If not, see FUEL PUMP RELAY in SYSTEM/COMPONENT TESTS article. For location of fuel pump test connector, see COMPONENT LOCATIONS in the article SYSTEM/COMPONENT TESTS in this section.  Check for poor quality or water contaminated fuel.  Check that TPS is not sticking or binding.  Check EGR operation.  Check for a leaking injector. To do this on TBI systems, disconnect injector electrical connector at injector. Crank engine and watch for fuel leakage.  Check that resistance of coolant sensor circuit or coolant sensor is not too high. See CODE 15 chart in appropriate TESTS W/CODES article in this section, or the SENSOR RANGE CHARTS article in this section.  Check ignition system for a worn distributor shaft, bare or shorted wires, incorrect pick-up coil resistance, loose ignition coil ground or moisture in distributor cap. Check for adequate spark using Spark Tester (ST-125).  Check for shorts by spraying plug wires with fine mist of water.  Remove spark plugs and check for wet plugs, cracks, improper gap, burned electrodes or heavy carbon deposits.  Check for correct fuel pressure in all speed ranges.  A faulty in-tank fuel pump check valve will allow fuel in lines to drain back to tank after engine is stopped. To check this condition, turn ignition off, disconnect fuel pressure line at fuel rail, remove filler cap, and connect a radiator test pump and apply 15 psi (1.0 kg/cm 2 ) pressure. If pressure will hold for 60 seconds, check valve is okay.  Check that PROM/MEM-CAL in vehicle is correct for that vehicle. Check with dealer for latest application information.  Check for restricted exhaust system.
See you in the am, early riser, so early to bed, 8: 30 in my location 1 1/2 hour past bedtime!

Merlin2021
Jul 29, 2010.
WOW! That's a long list of stuff to check. One thing caught my attention, you said check for restricted exhaust flow. This problem started the same day I got a new pipe and muffler installed. Do you think the muffler is bad, or worse, someone at the muffler shop tampered with my vehicle to cause this problem? Would that be possible?

Tiny
Golfnguitars
Jul 29, 2010.
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