454 XL Bypassing knock sensor/oil pressure switch

I know full-well that these sensors exist for a reason, and I know full-well that the OP sending unit is different than the OP switch...

That said, I'm trouble shooting an issue where the engine runs great up to 1900, revs past 1900 fine at idle (no pops backfires, etc.) but will not: 1) sustain an RPM above 1900 at idle (not 2000, not 2100, nothing, 1900 is the magic number, perfect to that point, **** after 1900), and 2) will not go above 1900 while under power.

I have a hard time believing that it's fuel system related, but acknowledge that it could be. I have changed all fuel filters and checked the anti siphon valve - all fine. The engine runs the exact same under a load (which requires more fuel) as it does at idle - perfect to 1900, falls on its face after 1900. Has good fuel pressure through 1900, which may indicate that it's not sensor related (if memory serves, the "safety" cuts power/pressure to the fuel pump), but the pressure is ultimately controlled by a mechanical/spring loaded diaphragm on the TBI.

To the best of my knowledge, there are only two safety switches on the boat - the knock sensor and OP switch. To rule those out, I would like to bypass each, temporarily, one at a time.

Does disconnecting the switch bypass the switch for each, or do they need to be grounded, or otherwise lied to?

Had the exact presentation on a carbed XL. Plumbed in a fuel pressure gauge between the fuel pump and carb. Engine ran great when first started with fuel pressure at 7 psi. After about five minutes rpm dropped off as the fuel pressure dropped off to 4 psi. Then connected a volt meter to the fuel pump. Engine ran great until fuel pressure again dropped with a reduction in voltage to the pump. Problem was the pressure switch (behind the alternator). Problem corrected with new switch. These switches are problematic and also pricey. Some folks replace them by wiring in aftermarket switches (by Holley I think). Recommend trouble shooting before buying a new switch. Good luck.

if you are referring to the oil pressure switch controlling the fuel pump, yes, it can be bypassed for troubleshooting purposes...most use a fused 'hot lead' direct to the pump....

for the knock sensor, I'd just disconnect it...

The EST pickup coil seems to have a finite lifetime...and can limit RPM...

Thanks, are you talking about the regular ignition coil, next to the distributor? It was my understanding that they either worked or didn't. I'll try swapping it out and see if I get any improvement. I believe I have a spare.

Can the oil pressure switch be bypassed by either disconnecting it, or grounding out the lead that plugs into the end?

no, the pickup coil inside the distributor...the one you have to disassemble the distributor to change.

the oil pressure switch controls the +12VDC power going to the fuel pump...for testing purposes, I'd suggest taking a fused lead and applying the current to the pump's lead.

The switch itself is rather expensive from Crusader...I've recommended more than once to redo the wiring to use one of the more economical options. I know of one instance where a different pigtail was spliced in so one could use the factory or an aftermarket switch...if you go that route, I'd suggest getting a 'dummy plug' to fill the unused socket to minimize any corrosion...

Should just be a module in the EST dist., but mine is new, so I don't think that's my issue.

Is the crusader oil pressure switch much different than the auto switch? Sure looks the same. I have not switched it out. I was hoping I could either jump it out or disconnect it, to circumvent the switch. It was my understanding that the switch is normally open, and if the engine lacks the requisite oil pressure, it closes and grounds out the switch, therefore, disconnecting it and leaving it open would accomplish the same affect, is that not the case?

Do you have any diagrams to bypass the switch? I suppose I can run a 12v lead right from the circuit breaker box down the side of the engine.

The pressure switch is a safety measure to shut the fuel pump off (by creating an open circuit) if the engine quits. The idea is to protect you from explosion if you have a broken fuel line which would continue to gush fuel in an engine stopped but ignition on situation. I don't see how anything that may be wrong with the switch could cause your symptoms (rpm limited to 1900). The switch and therefore the fuel pump is either off or on. If the engine keeps running at limited rpms that means the fuel pump is still on. You may have a fuel delivery problem but it does not seem possible the problem is the pressure switch. The fuel problems (that cause limited fuel flow) could be a failing pump or some kind of clog.

But I would want to measure the fuel pressure and actually see the drop before I believed that was the problem. To me, if the problem is as described (hard rpm limit), it sounds more like an ignition system issue or perhaps a rev limiter due to a code fault (if your system has such a feature). You can wire across the switch if you want to check it, but I don't believe that will change anything.

Good luck, let us know how it goes.

Your understanding of the oil pressure switch isn't accurate.

Its a SPDT switch, with the common side going to the pump. the normally closed contact connects to the R terminal of the starter's solenoid and engages the pump during cranking. the normally open side is usually fed from the ignition terminal (hot in RUN) and provides the current for the pump once the engine has built enough oil pressure to close the switch.

The switches I have changed have nothing uniquely 'marine' about them...the OEM ones usually have a decent plating on them to mitigate corrosion.

I'll take a pic of my switch, but I'm pretty sure there's only one wire that goes to the switch.