Crank no fuel - oil pressure safety switch ?

[ill r]

Hi all,

What was originally an AQ271c has been modified over the years, most recently with a vortec engine (still carbed) which involved going from mechanical fuel pump to electric. Boat was running fine, albeit not many hours since the new engine, and recently started then immediately stopped and wouldn’t restart, no other symptoms. No smell of gas after cranking and a shot of starter fluid or gas in the carb will try to start but then quickly shut down. 12v jump to fuel pump runs it and when pulled supply line off carb and did same test, good fuel flow. Tracing the circuit leads to an oil pressure switch near the oil filter that supplies the fuel pump with power and in turn is supplied by either the starter or the ignition coil. Power to coil, starter, relay between them and everywhere else I tested. I couldnt find any other fuses in these circuits, though from what I’ve found online seems like there should be… That oil switch is super hard to get at. Before I replace it, anything else I could have overlooked? Am having visions of mangling the darn thing trying to get it out or failing to get a good seal putting it back in - any tips or tricks?

Also bothers me this is less than a year old and has failed. Found online an automotive switch that will run the pump for a few seconds when the switch is turned to ”on” and then will run when the tach is getting signal. Seems to accomplish the same thing, no fuel running when engine isn’t, and maybe a better way to go about it - any thoughts on this?

 

[ESGWheel]

You did not mention it so have to ask: do you have oil pressure? If suspect that is shutting down due to an oil pressure switch, have to ask.

When you jumped the fuel pump, did it start and run like normal?

Also, it does not make sense to me that an oil pressure safety switch – which as its name implies is intended to shutdown the engine on a loss of oil pressure (or not let it start) – would use the fuel pump on a carbureted engine. Here is why I think that > a carb as a reservoir of fuel (the float bowl) that will keep feeding the engine for some time (a minute or more, depending on RPM, etc.) Better to kill the coil.

I may be wrong and others will know better but I suspect that with the engine swap the wiring is not as it should be.

Assuming you do have good oil pressure you can do a workaround as you are thinking until fully troubleshoot the issue. But does not have to be anything like the automotive switch you mention. Simply jump the fuel pump to a +12VDC that is hot whenever the key is on. The positive side of the coil should work. In other words, its ok the fuel pump is running when the key is on. The carb has a float to shut off the flow of fuel into it > do not need to “prime the carb” for a few seconds and then wait for a positive run signal to turn it back on.

Also, what is the engine in it now? And do you have a part number for that suspect oil switch?

 

[ill r]

Hi, thanks for the response. Oil pressure is around 40 while cranking. Haven’t tried running the engine with the pump jumped yet, just did that to ensure it worked at all and had no obstructions to fuel supply, etc. No part number for the switch, won’t have that until I remove it, though looks like there are 2 basic types, differing by the size of the threaded connector, either 1/8 or 1/4. It’s still a carbed 350, just not Volvos branding. DP-A outdrive, volvo exhaust, controls, etc.

 

[ill r]

Also the wiring is pretty basic and lines up with diagrams I found online so I doubt that is the issue, engine was built by a reputable shop, I’m just not local to them anymore.

 

[ill r]

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[alldodge]

Wire the switch as such

 

[ESGWheel]

One of my mottos is to learn something new every day. Today is no exception > was driving me crazy why they would design the system to cut off fuel pump on low oil pressure on a carbureted engine. Clearly not intended to preclude start or cause immediate shutdown based on oil pressure. In high performance and racing applications they do have systems that preclude engine start until proper oil pressure is obtained, but that is not the case here.

What I found is the answer seems to be twofold. (1) a carryover from the automotive side where the safety concern of the fuel pump running after an accident and (2) concern that the carb needle and seat is suspectable to failure due to debris and sticking open > if the engine is not running this can lead to bad things.

I also learned that I should think twice before posting past my bedtime

 

[ESGWheel]

Am having visions of mangling the darn thing trying to get it out or failing to get a good seal putting it back in - any tips or tricks?

It seems your question was not addressed regarding replacing the switch, so here are my thoughts. For getting it back in and not leaking suggest using a thread sealant like Locktite 545 (it’s a liquid) and avoid the first couple of threads. Tighten it up by hand and then another abt ½ turn should do it but if still feels loose keep going. The key, as I am sure you already know, is to be careful to not cross thread it. If need to be a contortionist to get it in, one of the tricks I do is a “trial run” > I figure out the best position to get into to fit the part (with the part in hand and trying to actually fit it), then back out, apply sealant and go. Good luck and please post how it goes.

 

[ill r]

Thanks all, work obligations will interfere with progress on this for a few days, but am looking forward to solving it. I don’t like throwing parts at this type of problem but also picked up a starter relay, fuse/breaker and coil pack as they are all accessible, relatively cheap and probably not bad things to have onboard anyway… will let you know the final outcome…

 

[ill r]

Can’t even get much of a clear path to tap on the thing to see if something internal was stuck as I read elsewhere can happen. This sits 90 degrees from as shown just above the oil filter. If I remove some hoses I may at least be able to see it better while working. iPhones are at least handy for getting a visual on hidden stuff….

 

[ESGWheel]

That’s tight! Be sure to record the wire color vs. positions on the switch! Your photo did not appear to capture them. Good luck!

 

[Horigan]

Your connections don't look too bad, but I ran into a similar limp mode issue due to corrosion at the connection to the oil pressure switch. Maybe just remove and clean the connections as a first attempt.

 

[Bondo]

Ayuh,..... As in the diagram from AD, the fuel pump is fed power from 2 points,....
It primes when the starter is engaged from the outside tiny terminal on the starter's solenoid,....
When the motor starts, the power is fed by the key switch, in yer case, off the coil's terminal, through the oil pressure switch,....

 

[ill r]

Well, I am admitting defeat and calling the shop! Back in town this AM and continued looking at this. Jumped across the oil pressure switch first from starter straight to the fuel pump - I don’t think the fuel pump came on when I cranked but didn’t have a helper to validate. No start or even attempt to catch. Jumped from ignition coil to pump and with key at “on” I heard the pump come on. It didn’t sound great to me though. Let it run for a bit to hopefully ensure the carb was full and then cranked, no joy, no hint of trying to catch. Also when the key was switched back to on, pump kept running until I switched to off. Would have thought in pretty short order it would have switched itself off? There is no overflow tube going back to the tank from the carb or anything so I’d assume at a certain back pressure the fuel pump stops or does it just cycle the fuel internally or something.

Terminals/connections at the suspect oil switch all looked fine. Now, this is hard to gauge but I really dont think the carb smelled very strongly of gas for all the cranking that was going on. My old Rochester carb had a fuel inlet filter that once caused me a hard start problem but this Holley does not appear to. I tried a new starter relay and ignition coil even though the test light indicated they weren’t the issue (went through the starting circuit per instructions on this forum, which are great!) Almost brand new fuel tank, what little fuel has gone through it has been ethanol free, recent fuel filter change, so don’t see it being that kind of thing… so, all kinds of fuel is pumped when 12v applied directly from battery to pump and when supply hose is running to open container, but I feel like it’s still fuel starved in every other configuration I’ve tried… Engine starts or tries to fire at least with starting fluid or gas right down the carb but immediately shuts off. without bench testing the various components not sure what else to do and I have no time for all that…

 

[alldodge]

Also when the key was switched back to on, pump kept running until I switched to off. Would have thought in pretty short order it would have switched itself off? There is no overflow tube going back to the tank from the carb or anything so I’d assume at a certain back pressure the fuel pump stops or does it just cycle the fuel internally or something

The fuel pump does not have a regulator switch of some sort that would turn it OFF once pressure is reached. It has only an internal regulator which keeps pressure from exceeding a set point. Once pressure set point is reached the just stays running at that pressure

With key OFF you should be able to pump the throttle and see gas squirt down the throat of the carb. Would need to disconnect throttle cable to do this by your self

If no gas comes out, then the carb float bowl needle valve might be stuck in the needle seat. The needle and seat can get stuck and it will take a lot of pressure to dislodge.

Can remove fuel line and with a air compressor blow maybe 20 psi into car fuel line, or remove the top of carb and inspect needle and seat

 

[ill r]

No gas via accelerator pump but if there is no fuel in the bowl that doesn’t work anyway does it? I tried some press. air and gentle tapping but that didn’t free anything up if it is the problem. Was a little surprised no gas at all dribbled out when popped the fuel line off, its bone dry for sure…

 

[alldodge]

No gas via accelerator pump but if there is no fuel in the bowl that doesn’t work anyway does it? I tried some press. air and gentle tapping but that didn’t free anything up if it is the problem. Was a little surprised no gas at all dribbled out when popped the fuel line off, its bone dry for sure…

That's the issue, still not getting fuel to the carb