Closed raw water intake -- no fuel pressure

[SeaHarlequin]

I recently installed a fuel pressure gauge to monitor my electric fuel pump and it appears to be working fine -- the PSI fluctuated between 2 to 4.5 PSI while idling for about 20 minutes.

I shut off the raw water intake and opened a bypass intake which was immersed in a bucket of fresh water to flush out the salt water. I noticed the fuel pressure gauge stopped registering any pressure even though the engine was still running. I thought maybe it got stuck or the fuel pump turned off (I had the engine running for less than a minute so presumably there was sufficient gas in the carb bowl to continue operation.)

Out of curiosity, I repeated the steps -- opened up the raw intake, observed PSI readout within expected parameters, shut down the raw intake and started fresh water intake and observed the PSI readout drop to 0.

I'm not yet understanding the interplay between intake of two sources of water and the fuel pressure. Is this expected behavior, and if so -- why?

I've not had a fuel pressure gauge on this boat before so maybe jumping between pressure and no pressure based on engine needs is normal and I just happened to time things twice so that it looked like there was interplay? (I'm operating under the assumption that an electric fuel pump would provide consistent pressure and was surprised to see the needle jump between 2 and 4.5 actively, although I didn't see it drop to 0 during those times.)

Thanks and apologies for what is probably a stupid question/mis-timed set of observations but I'm trying to get a better understanding of how the systems work together.

[Surcouf]

I can't think of any link between water pressure and fuel pressure... So pretty curious to say the least. I will not make you the insult of asking if you fuel pressure gage is not a water pressure gage...

Absolutely stupid theory question: is your fuel pressure gage an electrical one with a sender, or just a local gage?

[roadnsky]

Hi Sea-
First... there are NO stupid questions here.
This is a classroom of discussions and we are all learning together.

As an experiment, does the fuel pressure return to normal when you switch the water valve back over to raw (sea) water?

I can not think of a reason the changing the source of cooling water would have any relationship or effect on fuel pressure.

[Dave Neptune]

SeaHarliquin, has to be a coincidence. The pressure fluctuation has more to do with the "pumps" regulator. The pressure should stay steady and only fluctuate a tiny bit. If you are getting 4.5PSI you have a bit more pump than necessary and it should stay close to the 4.5 if it is operating properly.

By the look of the pump it just may be time to take a look at it or replace it.

The 4.5PSI is not a big deal at all just a bit much BUT the pressure should not vary so much so I think the internal regulator of the pump is becoming compromised. The fluctuations were probably just noticed when you changed to valves position as there is no connection between them.

Dave Neptune

[Dave Neptune]

Part II

Oops, also take a look at the OPSS if so equipped or add one if not. A faulty OPSS switch could cause a zero reading and then kick back on. Again a coincidence.

Dave Neptune

[JOHN COOKSON]

Oops, also take a look at the OPSS if so equipped or add one if not. A faulty OPSS switch could cause a zero reading and then kick back on. Again a coincidence.

Dave Neptune

Loose wiring would have the same effect.

Try shaking the wiring while the engine is running and see if the PSI varies. Or run a wire from the battery to the fuel pump, bypassing the boat's wiring and the OPSS, and see what happens. This will tell if the fuel pump has a problem or if it is something before the fuel pump.

When troubleshooting a electrical device it is important to troubleshoot the device and the associated electrical circuit.

ex TRUE GRIT

[SeaHarlequin]

First... there are NO stupid questions here

Give me time, I may surprise you yet.

You've all given a lot of good information to process. I was cleaning up for the day so didn't pursue this further than that one test. Now that I know that's not normal, I'll play with it the next time I'm on the boat.

It's a local gauge. I do have an OPSS. The fuel pump does look a bit rough (I posted in another thread about it with pictures). I'll try replicating the experiment by monitoring the gauge for a longer period of time (given that I know it's supposed to stay relatively steady, it's fluctuations are now a red flag to me).

I'll jiggle wires, hoses, bypass switches, etc until I get more data to process and debug this. Thanks!

[edwardc]

Rather than just jiggling, get a multimeter and monitor the voltage at the fuel pump when the pressure goes to zero.

[SeaHarlequin]

Rather than just jiggling, get a multimeter and monitor the voltage at the fuel pump when the pressure goes to zero.

I do have a multimeter. Where would I connect them on the pump? The positive terminal and anywhere on the case? What would I be looking for?

Thanks!

[edwardc]

Correct. The positive terminal of the pump and the pump case.

You're looking to see if the voltage drops to zero when the fuel pressure goes to zero.

[romantic comedy]

I am curious as to what would happen if you used salt water for the flush.
The fresh water may be a variable here.

Salt water tend to conduct and fresh tends not to conduct electricity.

Just a thought.

[Surcouf]

I am curious as to what would happen if you used salt water for the flush.
The fresh water may be a variable here.

Salt water tend to conduct and fresh tends not to conduct electricity.

Just a thought.

That was my “stupid theory”, would have been the sign of a really compromised engine ground. But it is a local gage, not electrical... so no crazy theory

[roadnsky]

Sea-
WHERE does the ground for the fuel pump come from?
I'd check both of those ground connections.
You may want to consider having a dedicated BUS BAR for your various ground terminations.
Same suggestion for any hot connections hanging on the battery.

[SeaHarlequin]

That's interesting -- thanks for the ideas re: the saltwater.

Sea-
WHERE does the ground for the fuel pump come from?
I'd check both of those ground connections.

I don't understand the question. The fuel pump was previously attached to the engine block itself and had no dedicated ground wire (presumably because of its mount points on the engine). When I relocated it to the bulkhead, I added a dedicated ground from the mounting platform to the engine block.

What do you mean by "both of those ground connections"? Do you mean to check continuity between the pump to ground wire terminating on the engine block or is there another ground pathway I'm neglecting to consider?

You may want to consider having a dedicated BUS BAR for your various ground terminations.
Same suggestion for any hot connections hanging on the battery.

Good call. The PO had hot wires running from the batteries to bus bars -- ditto for the grounds. There were a few exceptions in the engine compartment where ground wires were attached to the block itself (I think the exceptions were the coil and alternator) and since the pump used to be on the block without a ground going back to the ground bus bar, I figured grounding to the engine would be acceptable. I'll check the continuity but please do let me know if I've screwed something up or if any of you have recommendations on improving things other in general. Thanks!

[Surcouf]

what is the ground connected to? A metal plate or a fiber panel? I would recommend to connect directly to the pump mounting bracket to make sure to have a good continuity