FUEL PUMP OPERATION
Michael, I’ve been giving this thread and your descriptions a lot of thought and offer the following comments on some of the issues and your potential conclusions discussed earlier.
I believe a diode is unnecessary.
There is a wiring diagram discrepancy between the two drawings that you should resolve first.
You can’t kill electrical gremlins with luck.

Old metal fuel pumps have grounded cases. See Napa electronic pumps FLIX post, here:
http://c34.org/bbs/index.php?topic=2515.0 The ground wire is shown in the picture and discussed in the text, reply #3.
Four family simplicity: It occurs to me that your goal is to get things working, and that nothing would change in terms of operation IF your fuel pump was operated all the time from the just the ignition switch in the on position. Wiring changes at and to the cockpit panel are not required. You could either figure it out with what you have, or change it. Your choice.
Universal Manual Starting Instructions: Tonywright’s post in reply # 38 about the Universal manual requiring holding the glow plugs via the keyswitch spring position ON while hitting the start button MEANS that there is a continuous draw on the battery bank from BOTH the glow plugs AND the starter motor. Our C34-group experience is that this INHERENTLY leads to tough & rough starting. The Mark I “trick” has always been to RELEASE the glow plug (button in our Mark I case) BEFORE the start button, so that both heavy amperage draws are NOT on at the same time. There is no reason to need to do what Universal suggests, regardless of how your starting sequence is wired. This is one of the reasons that I cautioned earlier that new designs that complicate originally simple operations are not necessarily “improvements.” What you can try if you choose to is to release the glow plug spring setting on the key switch, wait a bit, and then hit the start button. All this means is that, as “possibly” wired, the fuel pump will only be off for that short amount of time between releasing the spring glow plug and hitting the start button. It should make NO difference in starting operation, since the fuel will “be there” from the pump operating during the period you are holding the glow plugs, and once started the fuel pump will begin operation once the oil pressure switch closes upon engine start. I believe this “duplex” newer wiring setup to the pump is an unnecessary complication to what used to be a simple startup procedure. Interrupting the fuel pump between the glow plugs off and oil pressure switch close is unnecessary, since the fuel pump and oil pressure have nothing to do with each other. On the Mark Is, the pump runs when the ignition switch is in the ON position, runs all the time. Why stop it? If the oil pressure is low, it doesn’t matter if the fuel pump is running or not.
Power to Fuel Pump: Michael, in your reply # 37 you wrote: “…(I say energized rather than operating because there will be cases when the pump is receiving an electrical current but is not operating because the fuel line pressure is such that no demand for fuel is being made - that is, I guess, above about 4 pounds per square inch gauge pressure on these Posi-Flo pumps which are rated for 1.5 to 4 PSI.)” That doesn’t seem to be the case. IF the pump is receiving “…an electrical current…” it will be on. I read the referenced sources on the bio-diesel referenced website, but do not believe that EITHER the original metal Facet pumps OR the new plastic pumps are “on demand” pumps. They’re either on or off. When pumping, the unused fuel is returned to the tank via the fuel return line, whether the engine is on or off.
Grounding & Wiring Diagrams: Continuing with comments on your # 37: Grounding should have nothing to do with your “issue.” It appears that it is one of significant differences between the two quoted Catalina wiring diagrams, and a POSITIVE-side power issue, unless, of course, your grounds aren’t solid. You wrote: “Now the problem as it is theorized to be is that because the electrical circuit ("preheat solenoid activating circuit") that energizes the preheat solenoid (which acts as a switch to close the glow plugs circuit) runs through the electric lift fuel pump (a clever way to make sure that when the glow plugs are energizing fuel is also being delivered by the lift pump to the fuel injection pump for onward delivery to the cylinders when the engine cranks), if the lift pump electronics are making and breaking they make and break the preheat solenoid activating circuit with the result that the preheat solenoid does not close - or closes intermittently…” What’s so “clever?“ If the pump was ON from the ignition switch, it wouldn’t need to be connected to the solenoid at all, and there wouldn’t have to be two wires to the pump. And, it is entirely useless to have the pump running before the engine starts, so why have it "linked" to the glow plugs at all? Although "our" design does have it run then anyway, right? Following your logic would mean that the lift pump is OFF just when you need it: when the engine starts!! Think about it. And, based on the wiring diagrams noted in your later post # 47, that simply does NOT seem to be the case. The power to the solenoid simply is NOT shown on either diagram to “run through” the fuel pump: the circuit goes FROM the keyswitch TO the solenoid and then TO the fuel pump. The fuel pump has two feeds on the page 25 diagram: one from the solenoid when the keyswitch is being held to energize the glow plugs via the “S“ terminal of the keyswitch, and second, from the “I” terminal of the keyswitch when the engine starts and the oil switch closes. On the page 26 diagram, the glow plugs and the fuel pump are in PARALLEL, on both pages 25 & 26 the fuel pump and oil pressure switch are in SERIES. BECAUSE the fuel pump is in SERIES with the oil pressure switch, the pump will NOT run until the engine starts unless it is wired with the double inputs on page 25. On that page 25, the pump WILL run when the glow plugs (solenoid) is held ON with the “S” spring on the keyswitch. You are correct in your summary, but my point is that the wiring just doesn’t go THROUGH the fuel pump to the solenoid, it runs FROM the solenoid TO the pump when the keyswitch is held on. It’s a subtle, but important difference, because I see no way that your theory that the pump is affecting the glow plugs should apply. You can easily TEST my understanding of the circuits: either disconnect the fuel pump or close the circuit breaker, if there is one, from the solenoid to the pump.
Wiring Diagram Discrepancies: These are TWO serious discrepancies between the two wiring diagrams. (1) Circuit breakers: Page 25 of the wiring diagram shows the circuit breaker downstream of the solenoid TO the pump. Page 26 shows a circuit breaker between the oil pressure switch and the resistor to ground. (2) Connections to the pump from the Positive (+) Wiring Side: Page 25 shows the wiring from the downstream side of the solenoid to the + side of the fuel pump, while page 26 shows the wiring from the end run of the glow plugs to the (-) side of the pump. They are numbered the same (#200360) but they are very different re: the pump operation. On the page 26 drawing the pump simply should not start from anything BUT the oil pressure switch. Follow both the positive wiring TO the pump and the grounds from and around it. You need to find out what you have on YOUR boat before you go adding stuff. I would find that out first if it was my boat.
Another thought: maybe the circuit breaker, if there is one, is faulty. This has happened before.
Lift pumps: the reason they call them that is because on the C30s and C36s, the fuel tank is well below the engine. As you may already know, our tanks, when at least half full, have been reported to siphon to provide fuel even if the pump is broken or off. If your fuel tank is full, you can perform my suggested test.
Sequence of Operations: I agree with your presentation of the operations based on the page 25 ONLY wiring diagrams and your field experience: keyswitch held to energize glow plugs means the fuel pump runs, release spring loaded keyswitch, pump stops, start engine, pump runs when the oil pressure switch closes. However, IF you follow Tony’s post, item 4 above, then the pump would continue to run since you are “supposed to” hold the keyswitch (and keep energizing the glow plugs) until you start the engine. What I suggested too indirectly in my # 39 was that the Mark I sequence is simpler: the fuel pump always runs when the keyswitch is on; the Mark I keyswitch is ONLY for on or off, since we have separate glow plug and start buttons; turn keyswitch on - pump runs - hold glow plug button in - pumps continues to run - release glow plug button (we have a solenoid we installed ourselves since the Mark Is with M25s don’t come with one) - pump runs - hit start button. It’s simpler, and is what I was suggesting you consider. The short period of time that the pump is OFF in the page 25 ONLY wiring diagram, and its dependency on the oil pressure switch, I personally believe, is meaningless. Page 26 operationally is NOT what you describe.
Suggestions: If you agree with that, then you really don’t have to do anything with the wiring to the cockpit control panel, assuming you‘ve resolved the wiring diagram discrepancies noted in my item 6 above: do two things all “underneath“ and not in the stanchion to the cockpit panel -- disconnect the wire from the solenoid output to the fuel pump [
correction](
per page 25 wiring diagram which shows two wires into the pump vs. page 26 which shows only one) and jumper the oil pressure switch so that the pump always runs when the keyswitch is ON (the oil pressure switch will still provide an alarm).
Options: OTOH, if everything works with my suggestions, then you‘d be changing the engine wiring “design“ retroactively to the M25 - simpler is sometimes better. If it works for you, either way, quit while you‘re ahead, but at least you‘ll KNOW what you have.
Last: If all THAT doesn’t work, you DO have a FUEL problem.
This has been fascinating, but we still have to remember that the whole purpose of your part of this thread was hard engine starting. I do not believe the fuel pump has anything ELECTRICALLY to do with your starting problems (other than swapping out the fuel pump again, or disconnecting it as a test as I‘ve suggested to get to the heart of your issue). Perhaps it would be helpful to go back up to Mike Stimmler’s original issues on this thread and what he did to resolve HIS problem. Or, wait a few seconds between draining the battery bank with the glow plugs before you start. Also suggest advancing the throttle a bit as previously suggested.