Exactly how did you determine this?

I determined this by first testing the new Delphi fuel pump at the fuel filter location with the adapter I built with a ball valve shutoff and pressure gauge connector. The fuel pump was isolated from the engine during this test. I reported the results of the deadhead test in this thread and you stated the pump was within spec, so I accepted that and eliminated the pump as a possible cause for my failed regulated pressure/leakdown test even though it is hard for me to believe that a pump which drops 12 psi in 10 minutes in the range of 80-68 psi during the deadhead test will stop dropping in the range 40-60 psi during the regulated pressure/leakdown test at the Schrader port. I acknowledge that is possible though.

Then I proceeded to the regulated pressure/leakdown test at the Schrader port. I opened the ball valve on the adapter I had built at the fuel filter location to reconnect the fuel pump to the engine. I attached an Autozone loaner pressure gauge to the Schrader fuel test port and operated the fuel pump using the key-on-engine-off. I turned it on several times and bled the gauge of air until I reached a starting test pressure with pump on that was over 60 psi. During the test the plenum was open. I could see the fuel pressure regulator, the injector body and all 6 poppets. There was not a drop of fuel or any wetness whatsoever anywhere in the locations associated with the injector that you mentioned as potential leakers in this thread. I performed this test a second time in exactly the same way with a loaner gauge from O'Reilly to eliminate the inaccuracy or failure of these loaner gauges. They are sometimes in bad shape. I reported the results of these 2 tests in this thread and you responded that the leakdown was "excessive" and technically out of spec. I had also called the Delphi technical support rep and he said that a drop to 40 psi was OK. Since they manufacture the Delphi fuel pressure regulator I am testing, it is hard not to take him seriously. He stated that the purpose of the fuel pressure regulator is not to maintain the pressure but to maintain the fuel column without letting air get into it so the next time you start you have a solid column of fuel from the fuel pump to the engine and the fuel pump can pressurize it easily. You say drops into the 40s is out of spec and it is hard for someone learning like me to know who to believe. When you say it is out of spec I take that to mean it represents a real leak that I need to find and eliminate.

Searching for the leak, I removed the fuel pressure regulator that provided the out of spec results and found that I had installed it for the test without removing the tiny o-ring from the old pressure regulator. So there were two tiny o-rings on the end of the fuel pressure regulator during the test. I felt sure that I had found the leak and the cause of the out of spec results, but when I removed the extra o-ring, re-installed the new fuel pressure regulator and performed the regulated pressure/leakdown test the same as before, I could not get any pressure at the Schrader valve whatsoever. Then I opened up the fuel return line right above the fuel filter location and put a plastic drain pipe on the return line coming from the engine so I could see if the fuel was being discharged down the return line during the tests. It was. Since I had had a moderately successful test with the two o-rings, I reinstalled the second one, but I still got zero pressure at the Schrader valve and fuel running out through the fuel return line. Thinking that I may have damaged (distorted or cracked) the injector body internals by having two o-rings on the regulator, I took the injector out, took it apart and inspected the body for leaks. I could not find any problem. I filled the injector feed port with gas to see if it would appear in the return port. It did not leak either through or around the pressure regulator (although there was no pressure during this test) and appear in the return port. But fuel is definitely coming down the return line as soon as I turn the pump on.

I had an out of spec result. Now I have no pressure whatsoever -- even more mystifying. I am using a new Delphi FP10075 fuel pressure regulator, which Rockauto says is correct for my Blazer. I'm about one-half of the way to wits end and any help will be appreciated.

Good job testing;) When the pump is activated, it pressurizes the line from the pump to the injector housing, (including the schrader valve), the injector housing, all of the injectors up to their pintle valve, and the regulator up to the valve plate, in that order. The regulator is last in line so the injectors have unrestricted access to max pump output pressure on acceleration. The image below shows fuel entering the regulator on the left, (after the injectors are pressurized), and exiting from the top, going back to the tank:

Attachment 31181

Spring pressure on the diaphragm keeps the valve plate closed until fuel pressure overcomes the spring pressure. At that point, the valve opens, pressure drops, and sends the excess fuel back to the tank. When spring pressure overcomes fuel pressure, the valve closes, pressure rises, and the cycle repeats. The vacuum connection is exposed to manifold vacuum at all times. When manifold vacuum is low, (engine off and WOT) spring pressure is high, causing fuel pressure to be high, (regulated pressure). When manifold vacuum is high, (deceleration & idle) spring pressure on the diaphragm is low, causing lower fuel pressure. If you're not getting pressurized fuel at the schrader, it's not the fault of the regulator, the pump is not delivering pressurized fuel. Need to look into that;)

The shut off valve that you installed in the pressure line, isolates the tank from the plenum. If the tank leaks down to 40psi, regulated pressure at the schrader will also leak down to 40psi, (or lower if there's a leak in the plenum). Max pump output and leakdown must both pass before you can diagnose any other leaks. To check the regulator for internal leaks, there is another adapter that goes in the return line, (with a ball valve), that isolates the regulator from the plenum. The fact that you visually checked each poppet, and no leaks were present, don't bother with the return line adapter;) If your leakdown went lower than the results you got at the pump, (and there are no external leaks anywhere) the valve plate in the regulator is leaking.

Not sure why the Delphi rep told you leakdown to 40psi is ok. It clearly states no lower than 55psi after 10 minutes in the GM dealer issue shop manual.

Hope all this helps. If you have questions, just ask;)

I have run many fuel pressure tests. The reason is that I don't get consistent results. This makes it hard to draw any dependable conclusion. For example, today I ran a dead head fuel pressure test using my adapter at the fuel filter location. The pump came on and the pressure generated was 4 psi. I have previously reported in this thread that this same test generated a pressure of 80 psi.

I have been paying attention to the voltage of my battery each day and have had suspicions that running these fuel pump tests drains the battery after about 4-5 pump-on tests. My battery voltage right now is 12.67 and I am getting a 4 psi deadhead fuel pressure. I'm charging the battery overnight and will perform the same test in the morning at 13 volts.

In the meantime, how do professional mechanics power the electric tests that they apply to auto components so they always get consistent results and are not depending a battery which may give completely different results if the voltage decreases by .4 volts? Do they have a 12 volt power supply that's plugged into an A/C outlet? Do they have something they connect to the battery temporarily so they can draw as many amps as they need for any component on the vehicle without discharging the battery? What is a good solution to this for the DIY person? And if I did have that device, how would I power my fuel pump with it in the tank? Is there a "fuel pump prime" electrical terminal on the 1996 Blazers like I've seen on other posts for the 1998 and up?

The results of today's deadhead fuel pump pressure tests using my adapter at the fuel filter locator are:
Starting battery voltage: 13.24V. The fuel pump is isolated from the engine.
Turned KOEO 6 times to reach 72 psi.
Ending battery voltage: 12.80V

Second test:
Starting pressure 26 psi
after 1 KOEO 50 psi
after 2 KOEO 61 psi
after 3 KOEO 64 psi
after 4 KOEO 67.5 psi
after 5 KOEO 68 psi
ending battery voltage: 12.65V
The pressure then dropped from 68 psi to 57.5 psi in 10 minutes.

I called Delphi tech support for the fuel pump and he said that these results indicate inadequate voltage for the pump and said that if the voltage from power to ground in the fuel pump electrical connector while the pump is operating is more than .2V lower than the voltage across the battery terminals, there is a problem with the power provided to the pump.

The voltage measured in the pump electrical connector while the pump was not plugged into it was .26V lower than the voltage across the battery terminals. While the pump was running the voltage was substantially lower at 10.41V, 2.03V less than across the battery terminals (12.44V).

Can anyone confirm the Delphi support tech's view that these voltage measurements indicate a problem? If so, can you tell if it is a problem with the power provided by the vehicle or a problem in the pump?

Tech support is correct, and may be what's causing your erratic readings. The test he suggested is called "voltage drop". This link explains: Voltage Drop Testing

The article talks about voltage drop at the starter and alternator, but it can be done on any electrical component the same way.

Most of the time you'll find a poor ground. Each time I replace a fuel pump, I solder the level sender ground wire, and the fuel pump ground wires together. Then add a section of 14 gauge wire and run it to a good, clean chassis ground close to the tank. If your 96 is a 2 door, the original pump ground is on the top left rear corner of the frame. Can't remember where the 4 door grounds;)

While the engine is at idle, the pump should only draw 5-8 amps.

Momentarily activating the pump 5 or 6 times shouldn't noticeably affect your voltage on the battery.

You can use a battery charger, or a jump pack, to supply more voltage/amperage while testing pressure, but you really shouldn't have to.

Thanks, Captain, for your valued support.

Today I did a voltage drop test on the fuel pump with these results:
Voltage drop from battery(-) to fuel pump connector(-) under load with pump running: .288 V
Voltage drop from battery(+) to fuel pump connector(+) under load with pump running: 1.52 V (same value on 2 tests)
Voltage drop from fuel pump connector(+) to fuel pump connector(-) under load with pump running: 10.42 V
These 3 voltages add up to 12.23 V, which was the measured voltage from battery(+) to battery(-) under load with the pump running.

I am guessing the 1.52V drop on the positive side of the circuit is too high. Could this cause my erratic fuel pump pressure measurements? If so, I have the fuel pump circuit wiring diagram for fuse 9 to diagnose the pump electrical, but I don't know the locations of these electrical connectors:
S222, P100, C200, C100 are the main ones, but there are others. Can you help with their locations? Are splices on the wiring diagram physically accessible for testing? 1996 4.3L V6 Vortec 2WD 4DR CSFI Vin W

Do you have any tips on how to do this electrical test? Is there any history of the oil pressure sensor in this circuit causing a problem?

Electrical Diagnosis, Connectors and Wiring Diagrams
 

Attempting to diagnose the 1.52V voltage drop on the power side of my fuel pump, these questions arose:
1) I believe I found connector C100 under a plastic flap immediately above the brake booster. It is 15 pin and black, as my wiring diagram said, but it did not have any mark C100 on it. I wanted to disconnect pin M to cut the engine oil pressure sensor out of the fuel pump circuit for a test. I could not find any markings for the individual contacts (such as "M" or "E"). It seems incredible to me that they have these detailed wiring diagrams, but when you go find the connector or the contact in the connector of interest, it is not marked! Am I missing something?
2) If I knew which contact was "M", how would I disconnect just that contact temporarily and put the connector back together for a test? Can I pull the male blade out for "M" or will that break the connector permanently?
3) I think I can see the C200 connector under the dash way up high to the right of the glove box after removing a small lower dash panel, but I can barely touch C200 much less see if it has contact markings. What is the most effective way of getting to it. I took off the door of the glove box and the mesh vent on top of the dash above this location, but neither provided a path to C200.
4) GM wiring diagrams have arrows for connectors like this "<<". What does the direction of these arrows indicate?

If you have "How to read GM wiring Diagrams", that would be very helpful on this website.

Regarding item 1 in the prior post: To find the individual wire connector markings on C100, "E", "M", etc., remove the gray plastic clip where all the colored wires enter the connector on the female half of the connector. Look at the clip to determine how to remove it. Once the clip is removed you can see the wire identifiers "E", "M", etc. embossed in the black plastic body where each wire enters it.

Regarding item 2 in the prior post: Each wire has brightly colored plastic/rubber material in the hole where it enters the connector to keep water out. Is it meant to be pulled out temporarily for a test? Will it ever reseal? Will it come out at all or break off?

Cal,

Does your wire diagram show a fuel pump relay? I am looking at a Haynes manual and it does show a fuel pump relay. However the way I read it the relay is only needed during koeo or "prime". Once the oil pressure is at normal operation the oil pressure switch is closed and provides power to the pump.

Your tests are koeo so you are testing with this relay in the circuit.

Also something to consider, the voltage with the engine running is almost 14 volts.

Sorry I can't answer all of your questions, just thought I'd point this out.

.