Fuel System Delivery Issue Mystery HELP

No lake test today since we had off and on thunderstorms. My next chance won't be until Monday.

OH, please be careful. Working around a running engine with the cowl off is dangerous on its own. Working on one running across the water even more so. We try to avoid this as much as possible. My suggestions, in order of safety.
1. in a test tank
2. on a dyno
3. secured to a trailer and backed down the ramp
4. secured in a slip
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86. running across the lake

I understand you trust your hubby, but do you trust the other boaters on the lake? What about a log floating 6" below the surface? How about a malfunction with the engine during testing?
If you feel you must do this at least wear a good PFD.

Here are a couple of what I call "quick and dirty" tests. They aren't 100% but if the fuel system fails these tests, you know there is a problem.

With engine off, remove vent line from vapor separator (this will be a rubber hose that comes pretty much straight up from the aft top) give the primer bulb a few good hard pumps (bulb should get hard), this tests the needle, seat and float.
With engine running and high pressure gauge installed, make a loop in the clear hose on the gauge, run the open end to a fuel tank or can. Now slowly trigger the relief valve on the gauge just enough to allow some fuel into the hose, but not enough to kill the engine. We are looking for air bubbles to see if there is a vacuum leak in the system. You want to run this test for at least 20-30 seconds. It can be hard to do with the push button valve your gauge has.

I forgot to ask him but I'm curious if there's a way to bench test a fuel pump, like check its electrical condition to see if it has been damaged by the harness plug issue or is it just one of those things that you "test it by using it"?

No real bench test. Mercury doesn't publish amp draw rating or volume for this pump that I'm aware of. Harness plug is a visual inspection or using a non contact thermometer.

Thanks for the advice regarding on-the-water testing. I thought to be truly "under load", it had to be moving through the water as that adds some resistance and would be more of a taxing test on an engine rather than just using a test tank. It's currently tied up at a slip at the lake, and forgive me if this is a really stupid question but how can I bring up the rpms yet stay in neutral? My shifter is a forward/center neutral/backwards is reverse. So once I've moved the shift forward to increase rpms, that would also move the boat forward. Even secured with the three lines I have on it, I don't think I'd trust them to rev it up to past midway and not have it straining at the lines. Again, I'm so sorry if that's a stupid question.
I've already done the "hanging over the back seat" method so far with no worries but I understand your reasons to be safe (I also caught that my go-to method is #86...just a wee bit down the list hahahaha).
The weather is supposed to be nice tomorrow and the hubby is off work so we are planning on testing it. I have a couple questions for clarification in relation to your "quick and dirty" hints. When you refer to (with the engine off) removing the vent line from the VST, is that the black rubber DRAIN line that goes from the external bottom of the tank or are you referring to the black rubber hose INSIDE the tank that is zip-tied to the high pressure pump and I believe it goes to the fuel regulator? The first option is an easy removal but the second would require me to open the VST to access that hose. I wanted to check with you before I try that test.

There are no stupid questions. One of my biggest problems posting on forums is skipping things that I see as common sense, What I forget is that they aren't common sense, they were taught to me soo long ago that I have forgotten that learning experience.

Yes it needs to be under load, that means in gear. But the boat doesn't need to be moving thru the water, just the prop needs to be moving water. If that makes sense?

A test tank is something very few shops have anymore, I only included it because it really is best. Depending on the size of the tank and the HP of the motor we would replace the prop with a modified prop or use something called a "test wheel".

Secured in a slip requires extra lines tied to the correct places and that the dock is also well secured to the lake bottom and/or shore.

On a trailer we will add a couple of ratchet straps to be sure the boat can't move. I'm guessing this method would be best for you.

I'm only bringing this up because I've had some close calls hanging off the back and one of my past co-workers had lost one of his past co-workers to serious injury doing this (he told me the story when he refused to drive the boat when I wanted to hang off the back). Many of our safety precautions were written in blood!

I have never worked on your exact engine so I'm going off of pictures in the manual and service bulletins that I can't post here.
The hose I'm talking about comes out of the top of the vapor separator tank at the top near the back of the tank. It will be next to the fuel line that comes from the low pressure pump. Should be pointing straight up and be secured with a tie strap. It's possible this vent hose will go back to the low pressure pump --- has to go somewhere and that would be the logical place to vent a possible fuel leak in case of a VST malfunction.

As a side note; your throttle/shifter will have a button that allows you to rev the engine in neutral. But there are VERY few tests we do in neutral.

I appreciate your understanding and saying that there are no stupid questions. I LOVE learning new things and the only hindrance I've had is people who took one look at a 5'3" female and said "nah, I'm not wasting my time with her". In addition to working on this outboard issue, I just rebuilt the hydraulics on my mini skid steer and will be replacing all four idler wheels today after replacing the bearings and seals. So with a knowledgeable and patient teacher, I can learn anything.
Without the proper means of securing the boat to the dock or trailer and not having a test tank any better than a 55 gallon trash can, I went ahead and did the lake test today. BUT I didn't hang over the seat. I removed the spare gas tank and sat back by the motor and strapped myself to the boat railing for good measure. Here's what I got with the fuel testing at load: upon start up, the gauge locked in at around 42/43ish and did great at low to mid speeds. When he pushed it up to high and then full throttle, it held the pressure for about thirty seconds and then the needle would quiver and drop suddenly to 30 (at that time though, there was no noticeable bogging-down sensation or sound of the motor slowing, the pressure dropped BEFORE we felt the reaction to that happening. It then dropped to 20 and then 10 and my husband had to back down the throttle to keep it from dying. When he backed off, it would recover (I didn't not pump the primer bulb since I already had too much going on holding the gauge but I didn't need to because him throttling down kept it from dying). We did that two more times to see if it would repeat and it did each time. Absolutely smooth as silk at low speeds and up to about half throttle. Then at 3/4 to full throttle, the pressure would drop from 42 to 30 and on down.

The marine tech had told me to call him from the boat when I was testing it and tell him what it did so I called. His thoughts are that it seems like the VST tank is filling up but when pushed to give more fuel, it's not STAYING full and keeping up with demand. The issue, to him, is definitely a fuel one and not the computer or harness at this point. He said it's possible it could be the low pressure pump but he's not leaning towards that because it is fill the tank regularly at lower speeds. He is leaning towards it being a float and needle issue that is preventing the tank from filling and staying filled when there is a higher demand for fuel. It could also be something going on with the new high pressure pump but he's pretty convinced the issue is inside that VST. It was getting ready to storm so we headed back to the dock. I will go back another day and pull the VST off and take just it to his shop, as per his request. He wants to look inside it and check everything himself.

So that's where I'm at today; what is your opinion of what my testing showed and what the tech thinks it's pointing to?

Great! You're getting closer to fixing this.

Thank you for trying to stay safe.

I think your tech is proposing a visual inspection of the VST. They probably have more experience with this style of motor, so I won't go against their recommendation. I don't think this will do any harm because you should have replaced the cover o ring when you did the high pressure pump (it comes with the new pump) so there is a good chance it can be reused.

Is this how I would handle this? No. But as I said earlier maybe your tech knows something I don't. And I never worked at a dealer that wasn't on the water and/or didn't have a test tank and/or dyno. Sending a boat to the lake for testing is expensive because it usually requires two techs plus travel time.

You have a problem with the fuel system. You might have more problems, but until this problem is corrected you can't know. Diagnostics is finding a problem, correcting it and retesting.
The VST is just a small part of a much larger system that includes the fuel tank to the VST and everything in between. When a problem shows up in the system, I'm a fan of testing the complete system not just a small part of it.

So, like I said. Go ahead and give your tech another chance and see what happens. If they don't get it fixed then you can start troubleshooting.

muc said:

Great! You're getting closer to fixing this.

Thank you for trying to stay safe.

I think your tech is proposing a visual inspection of the VST. They probably have more experience with this style of motor, so I won't go against their recommendation. I don't think this will do any harm because you should have replaced the cover o ring when you did the high pressure pump (it comes with the new pump) so there is a good chance it can be reused.

Is this how I would handle this? No. But as I said earlier maybe your tech knows something I don't. And I never worked at a dealer that wasn't on the water and/or didn't have a test tank and/or dyno. Sending a boat to the lake for testing is expensive because it usually requires two techs plus travel time.

You have a problem with the fuel system. You might have more problems, but until this problem is corrected you can't know. Diagnostics is finding a problem, correcting it and retesting.
The VST is just a small part of a much larger system that includes the fuel tank to the VST and everything in between. When a problem shows up in the system, I'm a fan of testing the complete system not just a small part of it.

So, like I said. Go ahead and give your tech another chance and see what happens. If they don't get it fixed then you can start troubleshooting.

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I think we may have the answer! I took the VST to the tech this morning and he called back right before 5:00. He said he opened the tank and saw that everything looked ok, that I'd put the new high pressure pump in correctly and cleaned the screens and the needle and float seemed to be functioning ok. He decided to blow through the brass 90 degree coupler from the low pressure pump into the VST tank barely let any air through. He cleaned it out and it allowed a bit more air but was still occluded. He cleaned it out further until he could easily blow air through it. He said all kinds of "gunk" came flying out of it but it's clear as can be now. He thinks that the low pressure pump was doing its job filling the tank for low fuel needs at low speed but when we cranked it up to higher speeds or full throttle, the VST couldn't fill fast enough to keep up, due to the clogged brass fitting. He said he'd seen that before on other equipment that still had a carburetor but he feels certain that was the issue. I'll pick it up tomorrow and get it back together in the afternoon. Hoping to do a test ride on the lake that will end up FINALLY being a "pleasure cruise" instead of another "haul it back home" day. I will let you know!!

Sounds like great news!
Got my fingers crossed for you.

But of course the big question is where did the crud plugging the fitting come from? Did the tech have any ideas?

I asked him that too. I was concerned that perhaps the inside of the gas line coming from the low pressure pump could be degrading or something but he didn't seem to be concerned about that. If it runs ok the rest of the season and next, I'll be very happy. If it starts with the same symptoms again any time in the future, I will pull that part first thing and see if it's gunked up again. If so, I'll work backwards from there and remove that low pressure fuel line and check it/replace if necessary. We have that lousy ethanol fuel here and I do buy the additive now to temper it but I didn't always because I didn't know to do that before. I'm hoping for a day on the lake tomorrow but it might not be until Sunday; going to be super hot here tomorrow.
PS I finally got my mini skid steer back together and working like a champ; I've been out in the heat the last couple of days moving a giant pile of dirt and grading that in my yard. I'll be thrilled to be done with my large fix-it projects. I just have to replace a carburetor in my husbands motorcycle and I'll be done with all projects (knock on wood!)

muc said:

But of course the big question is where did the crud plugging the fitting come from? Did the tech have any ideas?

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SUCCESS!!!!!!!!!!!!! When I went to put the VST back on, I carefully inspected each of the fuel lines and on the one going from the low pressure fuel pump to the VST, I could see that a small part of the inner lining had degraded and was hanging down like a little flap. I pulled that out of there with a dental tool but I just couldn't put that back on there, not knowing if the rest of the fuel line was degrading somewhere along the length. So I took it off and bought a new piece of fuel line. I later cut the old one from one end to the other to see if there were any other areas where it was degrading and there were none but I don't regret changing it out. My motto is "Do it right and you'll only do it once".
We took the boat out this afternoon and I was praying the whole time that this was the answer. It seems to have done the trick!

Thank you for taking the time and interest in my little mystery to offer from your years of experience. I really and truly appreciate it.