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I’ve been having problems with poor gas mileage and lack of power even after i practically rebuilt the upper part of my engine, new injectors, fpr, plugs/wires, front 02 sensor, head/intake/exhaust gaskets, new maf sensor, cleaned up throttle body, new pcv valve, crank position sensor, radiator, thermostat, water pump, fuel filter, air filter and even ran fuel injector cleaner through multiple tanks. the fuel pump reads 40 psi on my cheap harbor freight fuel pressure tester, thought maybe it wasn’t working properly so i borrowed a nice one from a mechanic friend, it reads 38.3 psi at idle, and reaches 47.2 max driving down the highway. with the fuel pressure regulator’s vacuum line disconnected, the gauge reads 48.1. when i reconnect it drops back to 38.3. when the engine is turned off the gauge read 46.4 and dropped to 46.2 after 45 minutes of resting. i’m not sure if its a bad fuel pump or a bad fuel pressure regulator(which now has maybe about 1,000 miles total on it.) any help would be greatly appreciated. almost forgot, 2005 buick rendezvous cxl(i think), 3.4l 2wd 120,000 miles.
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The FPR behavior seems normal, so I doubt that is the problem. Remember that fuel pumps do not create pressure, they create volume. The pump pushing the volume of liquid through the lines and filter and small orifices are what creates the pressure. Fuel injected engines are very sensitive to fuel pressure as well as fuel volume. Low pressure will cause starting and driveability problems. A pump that can deliver adequate pressure but not enough volume may allow the engine to start and idle normally, but it will starve the engine for fuel and cause a loss of power when the engine is under load, accelerating hard or cruising at highway speeds. Therefore it is quite possible to have proper pressure but reduced volume. This could be due to a failing pump or a clogged in-tank strainer. To test pump volume, disconnect the fuel line past the filter and run it into a clean container (attach a hose to the line if necessary). Make sure there are no ignition sources near the testing area. Turn the key on (do not crank), wait 30 seconds, turn key off. In the container, the pump should have expelled approximately one quart of fuel. The fuel can be returned to the tank.
I will do this, but I do have a question, you said for 30 seconds but if I’m correct in my understanding, without the car running does the fuel pump not shut off after a few seconds? I guess I will find out in a few but if that’s the case how do I measure the volume without a decent amount of data to collect.
[quote=”bryanwarden” post=138009]I will do this, but I do have a question, you said for 30 seconds but if I’m correct in my understanding, without the car running does the fuel pump not shut off after a few seconds?[/quote]
Many GM vehicles do not shut off after 2 seconds, but some do. The ones that have a timer also have a fuel pump test connector under the hood that can be shorted to battery voltage for testing and priming the fuel system.I guess I will find out in a few but if that’s the case how do I measure the volume without a decent amount of data to collect.
One way I do it is to take a container, measure 16 oz of water and pour it in the container, then make a mark on the outside of the container with a permanent marker where the water line is. Then I add another 8 oz of water and make another mark, and repeat adding 8 oz and making additional marks 2-3 more times. This gives a graduation to measure the amount of fuel collected. Obiviously you do not want a very large container or it will be hard to discriminate the liquid levels. 1/2-gallon to 1 gallon container is a good size. I use an empty 1/2-gallon glass pickle jar. You do want to make sure to dry the water out of the container before collecting the fuel if you want to return the fuel to the tank.
It shuts off after a few seconds, where would one look for this test connection to short to battery positive
It should be a single gray wire, not connected to anything, behind or near the alternator. You can short it to positive by touching it to the post on the alternator where the battery cable connects.
I’ve tried apply voltage to the correct pin but it doesn’t effect the fuel pump, I don’t know what to try.
[quote=”bryanwarden” post=138037]I’ve tried apply voltage to the correct pin but it doesn’t effect the fuel pump, I don’t know what to try.[/quote]
Here is the wiring diagram specific to your vehicle:There is nothing between the relay and the pump. If you apply voltage to the relay, the pump should work. If the pump runs when you turn the key, the fuses and wiring are good.
If you place a jumper where I drew the red line the pump should run:
Yes, that sounds good. So, your pressure is good, your volume is good, your FPR is good, throttle body is clean, and you’ve got a lot of new parts. Now it is time to hook up a scan tool and get some live data from the data stream to find out what the fuel trims and O2 sensors say.
Well but the pressure is supposed to be 50 psi at idle, it’s 38 at least that’s what my service manual says
The manual I have says 41 to 47, 36 min, 55 max. At 38psi at idle you are at the low end, but you said it reached 47-48 while running, which still places it within good range. However, since you know the flow is good now, the pressure regulator is suspect, and fortunately is easy to replace.
Ok so update, car is worse than ever know. I decided to replace the fpr, it was warranties so free, after replacement no real change, the next day my wife experienced a very serious change in symptoms. A rough idle, serious lack of power and gas mileage is nearly in half. I put my obd scanner on it and hooked it to the computer, now here’s where I’m needing advice.
At idle(800-1100) map reading is approx 6.5-7.5, short term/long term fuel trim is 27.3% and pegged on it, timing advance on #1 cylinder is around 25, maf reading is approx 3 g/s, calculated Load value at idle 28%, coolant temp is around 190-200 back and forth. I’ve replaced front 02 sensor so I know fuel trim is being calculated correctly because of that, tested vacuum and its 16, unplugged maf and it ran slightly smoother with a slow bouncing idle at about 9-1100 over about 2 seconds. Fuel press is slightly better with new for at 46 but the spec for this vehicle is 52-59 psi, the values you pulled are incorrect I’m sure, if you reference oreillys even the for says the same. I have system lean code, multiple random misfire codes, and a p0449(evap vent solenoid valve/circuit). Which I messed up trying an electrical connection fix but the car already had the symptoms I’m trying to correct when I fouled it. Please someone advise anything I should check. Also at 5000 rpms the maf reading is about 160g/s
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