The connectors, C100, C200 etc etc etc, are not labled on the vehicle. They are only numbered in the wiring schematic. The legend in the GM shop manual tells the approximate location of each connector, describes what type of connector it is, and the pinout for each connector. The "<<" symbols on the schematic denote a connection. Next to the symbol it gives the connector number and pin location in the connector.

The oil pressure sending unit provides a path for current to flow to the fuel pump only when the oil pressure is above ~7psi. Disconnecting the oil pressure sending unit will not affect the fuel pump while cranking, or while the engine is running. The original intent was to keep the fuel pump activated for a few seconds after the ignition was shut off, to prime the fuel system for the next start up. When the ignition was shut off, the fuel pump relay was deactivated, and the fuel pump remained activated through the oil pressure switch until residual oil pressure dropped below ~7 psi. At which point the fuel pump shut off. The circuit is in no way a "safety feature" intended to deactivate the fuel pump if oil pressure is below ~7psi, (GM hasn't used that circuitry since the mid 1980's). GM realized they could program the PCM to leave the fuel pump relay activated for ~2 seconds after the ignition was turned off. This made the oil pressure switch circuit totally unnecessary, and the last year it was used on S & T vehicles was 1999.

The fuel pump relay is the primary activation means. It activates the fuel pump for ~ 2 seconds when the ignition is initially turned to the RUN position. If the PCM does not receive a signal from the crankshaft position sensor during that time frame, (which it won't) the PCM deactivates the fuel pump relay. If the engine isn't cranking or running, there's no sense in the fuel pump being activated When the ignition is in the START/CRANK position, the fuel pump relay is activated continuously.

The seals around the wires in the connectors is a type of silicone rubber designed to keep water out. They will slide up the wire for testing, and slide back down and seal again.

 

That really clears things up a lot. I was wondering what the theory of operation was on these cars.

Thanks again Captain Hook.

.

 

Captain Hook, I second Snuffy's thanks for your explanation of the function of the oil pressure sensor in the fuel pump circuit. I disconnected that sensor and found the KOEO voltage drop under load from battery(+) to fuel pump(+) unchanged at 1.52V, confirming what you described.

I seriously need to know if the 1.52V voltage drop reported in post #17 from battery(+) to fuel pump connector(+) under load with pump running indicates a definite problem. Is that the likely cause of my erratic fuel pressure tests? How much voltage drop is normal?

To isolate the cause of the 1.52V drop, I am continuing to test segments of the power side of the fuel pump circuit under load with these results so far: Battery(+) to fuse 9: .42V drop; Fuse 9 to fuel pump relay: .234V drop; Swapping the other 2 relays with the fuel pump relay resulted in no significant change in the overall drop from battery to pump: 1.46V vs. 1.52V. I am not finding any single dramatic drop in one segment or device.

Snuffy, you are certainly right that with the engine on I would have 14.2 V, instead of 12.6 V, for the fuel pump. The motive for this thread is to find why I could not meet the KOEO GM fuel pressure specs, and I am still trying to solve that.

 

Rule of thumb is less than .5 volts. In a perfect world, .1 or less.

You can use the procedure listed under "checking the starter circuit" and apply it to the fuel pump circuit: Voltage Drop Testing

 

Now that I know I have a 1.52V voltage drop from battery(+) to fuel pump(+), what can I do to fix that? The drops are spread fairly evenly throughout the circuit: battery to fuse .41; fuse to relay .23; relay to pump .73; pump to ground .29. It is not a very complicated circuit, just a few wires, splices and connectors. Most of the connectors are under the dash and never got wet or corroded. I do not intend to replace the wiring harness, so what can I do to fix the voltage drop. The only thing I can think of is put silicone grease in each connector, but I doubt that will change anything.

Second and more serious problem is: In spite of the voltage drop, the fuel pump does create over 60 psi deadhead. The fuel just flows into the feed line of the fuel injector and right out the return line without even going through the fuel pressure regulator, so zero pressure is measured at the Schrader test port. It sounds strange, but I am very sure that is what is happening. Does anyone have any idea how that could be happening inside the injector? This is the standard CSFI spider injector.

 

You can decrease voltage drop by eliminating connections, soldering the connections that are necessary with 60/40 lead/tin rosin core solder, and using a larger gauge wire. The original wire is 18ga, and by increasing to 14ga, voltage drop will decrease. Same principal as "the big 3" upgrade. Silicone dielectric is a good idea for weatherproofing, but it will do nothing to improve already corroded connections.

The diaphragm inside the regulator is the last component in the fuel delivery system to be pressurized, (see post #13). Spring pressure on the back side of the diaphragm keeps the valve plate closed until fuel pressure reaches regulated pressure, (fuel pressure overcomes spring pressure on the diaphragm). If regulated pressure is above pump maximum output pressure, the valve plate can not open. Provided maximum output pressure is high enough, the engine will run on what the pump can provide. The fuel that the engine consumes will cause the pressure to change. For example: At idle, when fuel consumption is low, the pressure will be higher. At wide open throttle, when consumption is high, the pressure will drop like a rock. With low maximum output pressure from the pump, it might idle just fine, but when it's -20F outside, and you hop in and hit the starter, it probably won't start.

Here's how it's supposed to be
With the engine off, (at start up) or at wide open throttle, (low manifold vacuum) fuel demand is high, so the available pressure must be high. With the engine at idle or decellerating, (high manifold vacuum) fuel demand is low, and the pressure is low.

 

Thanks for the techniques for reducing voltage drop.

Regarding the injector, I am asking what happens when the fuel pressure regulator is circumvented by a leak from the feed to return sides of the injector that goes around the fuel pressure regulator, not through it. In that case the fuel pressure regulator's function is irrelevant. The fuel pressure regulator just stays closed because the leak to the return line prevents enough pressure from building to open the regulator. There are o-rings inside the injector that separate the feed and return, but mine look fine. This is exactly my situation. Fuel goes right down the return line without generating any pressure (zero) in the injector or at the Schrader test port. Does anyone know how that might happen and what could be going on inside the injector? I am hoping so much to get an answer to the question I am asking.

 

Impossible with this system. The ONLY passage for fuel to return to the tank is past the fuel pressure regulator valve plate.

In the link below, the entire area between the two O rings is pressurized with fuel in the fuel metering block. Fuel either goes out the pintle valve and into the spider line, or it goes to the regulator and back to the tank. http://chevythunder.com/CPI%204.3L%2...20assembly.jpg

If that's the case, the regulator is stuck open, (not allowing pressure to build).

 

Attached is a photo of the black plastic pipe from inside my CSFI spider injector. The 3 parts in the photo are in the order they would be assembled: On the right is the injector body; next the black pipe is inserted into the hole where the fuel pressure regulator is installed; lastly the fuel pressure regulator is installed over the grate on the left end of the pipe. The only thing holding this pipe in the injector body is the o-ring on its right end. The pipe can be pulled out of the injector through the fuel pressure regulator hole.

The purpose of this pipe is to conduct fuel from the low pressure side of the fuel pressure regulator through the whole length of the injector to the bottom of the vertical return line inside the injector. During normal operation this pipe is surrounded everywhere by high pressure fuel. What prevents that high pressure fuel from getting into the pipe and thus the return line without going through the fuel pressure regulator is the tiny o-ring on the nose of the fuel pressure regulator that seats in the left end of the pipe and another o-ring visible in the photo on the right end of the pipe that seats in the vertical return line deep inside the injector body on the opposite end from the fuel pressure regulator. If these o-rings are defective or not seated properly, fuel can go directly from the high pressure side of the injector directly to the return line without going through the fuel pressure regulator.

I suspect that is what is happening to mine. My basis for concluding that is that I have installed two new fuel pressure regulators, one Delphi and one BWD, and in both cases fuel leaked down the return line into a jar that I could see without creating any pressure at the Schrader valve. I tried to blow air through each regulator both before and after installation to assure that it was not stuck open. No air would go through.

I hope to be able to seat the o-rings properly this time.

 

It took reading this twice but I understand exactly what your saying.

Cal, your conclusions sound logical to me if that helps. Very informative thread thanks for sharing, and thanks to Captain Hook for his input, great info.

Is it possible that the injector body is cracked or defective and is allowing high pressure fuel right into the return port?

Do you get any pressure build at all then it leaks down slowly? Or you get zero pressure and it all pours into the return jar? I would think a small leak could be a small crack or leaking O-rings. No pressure at all has to be a large leak that should be obvious.


Keep us posted.

.