VST flow in a injected 4stroke engine

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VST flow in a injected 4stroke engine

Postby chris oak » July 16th, 2016, 9:53 pm

I'll move this later but wanted to see if anyone here can jump in, I've tried on several other outboard sites. I've been doing a google search for days and cannot find a clear answer. If anyone has knowledge in this area jump in, otherwise when I solve it I'll post it up.

I am fairly certain the problems I am having are related to running out of gas, we ran Gigas low because my dumbass was trying to see how far I can get on a full tank. When the boat stalled at idle I threw in 5 gallons of gas and could not hear the high pressure fuel pump priming. I think I need to know the general flow of fuel through a outboard engine. I am guessing it goes this way:

Fuel from tank goes through racor filter to primer bulb into engine. Travels through another fuel/water separator into low pressure fuel pump which is mechanical in nature. Then goes upwards to the VST. The high pressure fuel pump is located in the VST. The high pressure pump is electrical and transfers gas into the injectors.

Here is what I know: Primer bulb can be pumped until it is hard. It is not the fuses because I checked them. It is not the fuel relay as I have continuity verified via multimeter. It probably is not the low pressure pump because pumping the primer bulb lets fuel flow thru the low pressure pump into the vst. It is not the high pressure fuel pump as I took apart the vst and ordered a new pump and I can hear it priming on the first key click before ignition. The VST filter screen was clean. I have spark as sometimes the engine will turn over.

Gas is going through the low pressure pump, but when I open up the high pressure bleeder valve (and also depress the pressure valve) no gas is coming from the VST. I read online about a stuck float valve and needle, if that is the case I need to know how to clean the needle so gas will flow out.

I guess I need to know 2 questions. 1. Is my flow of fuel statement correct, ie does it flow from the tank into the injectors as described? 2. If the vst is not filling what could be the probably causes that I need to check? I am going to trace each hose one by one to see where the fuel is going but I'm fairly certain it will stop at the VST it could be an upstream clog or a downstream one.
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Re: VST flow in a injected 4stroke engine

Postby chris oak » July 17th, 2016, 9:48 pm

Okay I talked to some guys online at iboats.com and marineengine.net. The flow of the fuel goes like this:
fuel bulb to under cowling fuel filter to low pressure pump to VST to high pressure pump (inside vst) to fuel rail. My clog could have been a stuck float valve but it turned out I was lame and pinched a fuel hose, I saw it when I took off the intake silencer again (it took me 2 hours the first time and now I can do it in 15 minutes). Once I moved it the VST is now getting gas. I confirmed this by opening up the drain valve on the vst and a ton of gas poured out.

The new aftermarket fuel pump however is not moving the gas to the fuel rail. I can hear the pump fire up (I couldn't hear the old pump fire at all before), but the engine would not start. When I took off the fuel line from the VST going to the fuel rail and turned the key, no gas was moving to the fuel rail. I tried the first click several times and never saw a drop of gas going towards the fuel rail.

My guess is the aftermarket fuel pump is a POS. At least I hope so. On the good news iboats had a ten percent of sale and two day shipping so I ordered the oem yamaha fuel pump for 180, about double what the aftermarket one cost. I'll send that one back.
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Re: VST flow in a injected 4stroke engine

Postby chris oak » July 20th, 2016, 8:01 pm

Confirmed and fixed. The aftermarket pump was a pos and I sent it back. Got the oem one from iboats with a ten percent off coupon so it ended up about 170 shipped. It would have ran me at least 800 to have it done at the dealership.

Here's what I know now. The flow of fuel is pretty much what I stated above. When you click the first click on the key of a four stroke you should be able to hear the fuel pump whine if you aren't making too much noise. If it's not then either the fuel relay is dead or the fuel pump. On the yamaha f90 there are seven bolts you will have to undo, one of them is a sob that is located on the bottom of the intake manifold and you have to use a special tool to break it free. Then you will have to disconnect about 4 plugs and several gas/air/vent lines. It's not a big deal, it took me 2 hours the first time because I had to keep running to the manual or computer to figure things out. I can do it in 10 minutes flat now. After removing the intake manifold and carefully setting it on the side you have access to the VST. The high pressure pump is located in there. Remove it, if you order the oem yamaha pump then you won't have to mess with the wiring, I'd highly recommend that. The aftermarket one I had to remove the clips and that was a bitch as they are under the float. I ended up having to put back the regular oem one and then installing the oem fuel pump.

You have to prime the vst, to be sure I kept the lid off and filled it using the fuel primer bulb about three quarters full. Then I screwed on the vst lid and made sure that fuel primer bulb was hard. Then I clicked the firsdt key and kept the fuel line coming from the vst into a small container and sure enough the gas poured out meaning the fuel pump was running.

I buttoned it all up, put the mufffs on it and ran it several times, it fired up every single time. I'm not sure what killed that pump as I only had 500 hours on it, I have a feeling it was running the gas too low. Even though there are 3 filters before the vst somehow something must have gotten in there.
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Re: VST flow in a injected 4stroke engine

Postby no bananas » August 2nd, 2016, 2:46 pm

Good job Chris. I will make a mental note of this thread in the event my F225 craps out one day. Although, it is difference displacement engine, I would imagine flow path of the fuel is same.

I have noticed a nagging oil leak on my F225. I could see the oil stain, but when I open up the cowling, the leak is not visible. The research continues for me.......
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Re: VST flow in a injected 4stroke engine

Postby chris oak » August 2nd, 2016, 7:17 pm

Brian where is the oil showing up? You can ask the guys on marineengine.com, iboats, thehulltruth.com and bloodydecks. I hit them all and got enough answers to start searching, some of the guys on those sites are incredibly cool.
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Re: VST flow in a injected 4stroke engine

Postby no bananas » August 2nd, 2016, 8:34 pm

Hi Chris, I should have taken a picture before I put the boat away. But here is a picture of someone else's boat with my "markings" with arrows.



After a few hours of running the engine at higher RPM, an oil stain is visible below the engine but above the lower unit. I ruled out the marine grease that we use on the pivoting point for the steering as the viscosity of the stain is not that of the grease. I also ruled out the lower unit leaking as the stain is above the lower unit. I am reading that it might be a leaky crankshaft seal or a oil pump leak. Both of which sound like a massive project. Fortunately, the leak is small and I don't see a change in oil dip stick. I might continue to run it until the off season.
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Re: VST flow in a injected 4stroke engine

Postby alexanderwhite » December 3rd, 2016, 10:14 pm

Yes You may call them or mail them and they will visit you at your place within 30 minutes.They are experts and are affordable.
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Re: VST flow in a injected 4stroke engine

Postby Qressor » February 15th, 2017, 11:32 pm

Hi Chris, I should have taken a picture before I put the boat away. But here is a picture of someone else's boat with my "markings" with arrows.
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