Fuel Pump Issues

[jdav360]

Hello everyone. New to the forum. I know that everyone "new" needs help...lol, no difference here!

I just replaced my fuel pump. When I went to wire up the new harness, I discovered that the wiring was kind of "rigged up". There were the standard wires coming from the factory wiring (gray, purple, black, black/white), but there was also a wire running from a kill switch under the dash to the gray wire (the kill switch is wired from the fuel pump fuse in the engine compartment fuse and relay box).

When I first bought the tahoe, I figured that the kill switch was a theft deterrent since i bought it from a bad neighborhood. After driving for a while, I figured out that wasnt the case. Sometimes when I shut the vehicle off, the engine keeps running and I have to use the kill switch to shut it off. So something in the electrical system is messed up causing the engine to stay on intermittently.

So, when wiring the new fuel pump, I bypassed the gray wire all together since apparently the wire from the kill switch controls power to the pump. But now, the pump wont turn on at all when I turn the key forward.

I have the purple wires (sender power) connected, the black and black/white wires (pump ground, sender ground) soldered together and bolted directly to the frame by the pump. and I have the power wire from the kill switch connected to the gray wire (pump power).

so...why is the fuel pump not turning on? any thoughts?

Thanks!

 

[MO Viet Vet]

Problem with ignition switch feeding power to the fuel pump circuit or a relay?

 

[jdav360]

Thats what I originally thought too. When the truck stopped running, I knew it was a fuel pump failure because there was a loud grinding noise coming from the pump, and it would cut off after 2-3 seconds. before this, the system worked fine. Horrible gas mileage and sending unit was bad causing gauge to bounce around and be highly inaccurate, but it still fired every day. I checked the fuse and relay, and both are fine. Maybe I should just wire it back up the way it was and try that.

 

[MO Viet Vet]

I would for now and then get a wiring schematic for that circuit and see what is going on.

 

[jdav360]

Thats what I am going to do...but it still doesn't make any sense. The fuel pump relay provides power to the pump until proper pressure is reached, then the oil pressure switch provides power. Running a wire directly from the fuse box should make the pump kick on regardless of a bad oil pressure switch...one would think anyway. I hate wiring issues:whymewhyme:

 

[MO Viet Vet]

Everybody does. All those problems are always the ones most hidden.

 

[jdav360]

Well, I got the fuel pump working. Turns out the ground from the fuel pump has to be attached to the fuel relay and can't just be grounded to the frame.

But...the darn thing still wont start. acts like it wants to on first crank, but just wont fire. Thought it was flooded, so I choked it manually....didn't work. So fuel and air are fine...guess im gonna check spark next...although all of that is 9 months old...cap, rotor, plugs and wires.

 

[lesterl]

Unplug the MAF sensor electrical plug, crank. If that is NG, spray some carb cleaner down the intake, crank. If still NG check spark. If cap/rotor has been replaced recently and it wasnt an AC DELCO/DELPHI, suspect it is bad, check for spark at coil, coil wire, plug wire.

 

[jdav360]

Found out that the fuel pressure regulator was bad. And it had been for a while because the intake below the regulator was crystal clean. Replaced that last night, put the whole thing back together. This morning I checked spark on every spark plug...all firing just fine. Checked fuel pressure, 62psi. Still wont start. Keeps backfiring and trying to start, but just wont. I checked to ensure the timing chain wasn't broken by turning the motor by hand with the valve cover off and watching the valves open and close. I did timing chain, cap, rotor, plugs, wires all at the same time in February.

Is it possible that the engine is just totally flooded by the leaking fuel pressure regulator? Because I cranked and cranked and cranked after I got the fuel pump working, and there was a lot of gas in intake, and the oil smelled like gas. But man....i've been cranking, and even choked it, and it still wont start. I tried starter fluid, and that actually shot flames out of the air filter housing!

 

[jdav360]

Does the spider injectors have certain cylinders that they go into? I thought they fired when pressure in the cylinder dropped, so there was no need to plug into certain cylinders...maybe I have this wrong and switched an injector around?

 

[jdav360]

scratch my previous post...this is a second generation throttle body injection. I did have 4 of the 8 injectors wrong. Problem solved....well, kinda..

Starts fine now, but when I hit about 2500 rpm, serious backfiring...like throwing flames out the tail pipe kinda backfiring. Any ideas?? I know it needs plugs...but that wont cause flames...

 

[MO Viet Vet]

Intake leak.

 

[jdav360]

fuel or air? because the seal that went around the spider assembly did break...and I didn't put it back on. can it really cause backfiring that bad?

 

[MO Viet Vet]

Air leak. Let off and backfire is usually an exhaust leak. Accel and backfire is usually a vacuum leak or timing or fuel mixture problem. Best guess is the vacuum leak.

 

[jdav360]

guess its off to the parts store tomorrow and buy the whole plenum set. They don't sell just the injector seal. timing cant be it...ran fine before pump / regulator went out, and I already did timing this year...thanks, I will try the seal tomorrow and see if that does it.

 

[MO Viet Vet]

Any codes?

 

[jdav360]

no...i suspect the ecu doesn't work correctly anymore...or the cel bulb is burnt out. i did notice the MAP sensor was broken off...that could cause rich mixture too. but seeing as it didnt run this rough before, I doubt that is it. I'm going with the seal and new plugs for now, then MAP sensor...then scrap yard...lol

 

[MO Viet Vet]

Here is a very long winded way to check the MAP sensor. If it is broke you should replace it.


Your engine’s Manifold Absolute Pressure (MAP) sensor could be the culprit if you're still getting lousy gas mileage after a tune up. A MAP sensor continually compares atmospheric absolute pressure to the intake manifold vacuum and sends the appropriate voltage signal to the vehicle’s computer. The computer then decides whether to increase or decrease fuel supply to the engine and adjust spark advancement accordingly. A bad MAP sensor sending the wrong signal to the computer can disrupt fuel consumption and overall engine performance. But with these simple steps, we will be able to determine if the MAP sensor in your vehicle is giving out.

Things You'll Need

Voltmeter or tachometer

Hand vacuum pump

Instructions

Locate the MAP sensor along the firewall or to the passenger side of the engine compartment.

Check the vacuum hose attached to the sensor and make sure it is in good condition and free of obstructions. Make sure there are no loose wires and the sensor’s electrical connector is in good shape.

Determine what type of MAP sensor is installed in your vehicle--voltage or frequency type. If you have a voltage type, you will use a voltmeter for this test; if it is a frequency type, you will use a tachometer. Consult your vehicle service manual if you're not sure what type you have.

Unplug the electrical connector from the MAP sensor and turn the ignition key on, but do not start the engine. With a voltmeter, measure the reference voltage by touching the reference wire with the meter’s positive lead and the negative lead to ground-black wire, which is at the connector’s end coming from the computer. If the reference voltage--about 5 volts--is out of specification, you have found the source of the problem. Reconnect the MAP sensor and turn off the ignition key. Take your vehicle to a professional auto electrical shop for further testing.

Attach the voltmeter or tachometer negative probe to the ground (black) wire and the positive probe to the signal wire. The sensor's connector should be plugged in. The third wire--the reference wire--produces a steady 5-volt signal only. Consult the wiring diagram in your vehicle's service manual if necessary. You might want to insert pins into the wires to attach the probes to for this test.

Disconnect the vacuum hose from the MAP sensor and attach a hand vacuum pump.

Turn the ignition key on, but do not start the engine.

Read the voltage or rpm. With zero inches of mercury (in-Hg) vacuum, you should get between 4.5 and 5 volts, or between 300 and 320 rpm.

Apply 5 in-Hg of vacuum. You should be reading now about 3.75 volts, or between 275 and 295 rpm.

Apply 20 in-Hg of vacuum now. This time you should be reading about 1.1 volts, or between 200 and 215 rpm. Compare your readings with those specified in your vehicle service manual. If the values are out of specification, replace the MAP sensor.

 

[jdav360]

i am certainly going to save that for future reference!

I'm pretty darn certain its bad though...lol. It is broken off where it plugs into the intake cover.

 

[lesterl]

Yes, injectors are cylinder specific on CSFI, CPI is non specific..... Each CSFI injector fires in sequence so make shure it is right..... Holding the accelerator pedal to the floor is flood clear mode. If MAF sensor is bad unplugging the electrical connection will bypass it.