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Inboard '95 Mercruiser GM 350 EFI Magnum Skier limited top speed

Try running the return to the tank, not the filter. With the fuel returning ahead of the non return valve you end up pressurizing the return line. TBI no like back pressure on return line.
 
@o2batsea: The engine is in a direct-drive ski boat, not a stern drive, which means the fuel tank is not in proximity to the engine. If I ran the return line down through the bilge and back to the tank, does that line have enough pressure to pump back up from the bilge and into the top of the tank? How would I plumb it in? There are only three holes in the tank: 1) The fuel filler inlet; 2) the fuel main outlet with anti-siphon valve; 3) vent hose through transom. Here is a diagram of the fuel system in my boat that was installed by the previous owners when they yanked out the OEM Vapor Separator tank. In it, you can see that the fuel return line empties into a brass tee in the main fuel line before the fuel filter which is before the low-pressure lift pump. The only change I made was to move the return to the filter, which is still before the high pressure pump. Are you saying that by putting the return after the low-pressure pump, I've created back-pressure in the return circuit? I could always move it back to the brass tee.

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@makomark: In automotive use, the GSL393 is either an in-tank or frame-mounted single-pump application. I could pull the mechanical pump out entirely and install a block off plate; then move the new pump from the top of the engine down to the bilge, which is a considerably cooler location. I'd locate it before the filter/separator, putting a prefilter in the main line between the fuel tank and the pump. I'd also have to extend the power leads from the connector on top of the engine down to the bilge. Lastly, o2batsea suggests in post 61 that moving the fuel return line to the filter/separator creates back pressure on that line, which "TBI no like." I could cure that by plumbing the fuel return to the brass tee located in the main fuel line before the GSL393. The one question that this hypothetical install raises is "Can the fuel filter/water separator canister hold 30psi?" This install would create a 30psi pressurized circuit from the pump, through the filter and to the TBI. What do you think?
 
@makomark: Also, with a new, single-pump install, the boat's 42-gallon fuel tank becomes the "reservoir" for the entire fuel system. Pretty sure there's no way for the pump to overtax that!
 
How far away from the engine is the fuel filler? Just for grins could you run a temporary return line from the TBI to the filler? Is the T something you can cap off temporarily? Also, 30 Psi seems awfully high. TBI runs at 14-17 as I recall.
 
again, without some pressure readings, its hard to know what is going on....

if there's any substantial pressure in the return line, you can always return the excess to the tank....and if you are careful, you could use a temporary setup to testing purposes only.
 
Right now, the tee is gone. I pulled it when I put in a new main fuel line from the tank to the filter/separator. Running a temp return line to the filler is doable. If I can find time this week. I'll try to run that test. However, I'm beginning to think that the lack of sufficient "reservoir" capacity due to the missing Vapor Separator Tank, and maybe the mechanical fuel pump are the roadblocks to a real solution. At this point, I've been chasing this for two months, and the boat comes out for the winter in a week or two. I might just put the thing to bde and wait until next year. Per the Mercruiser No. 17 GM 5.0L/5.7L V8 Marine Engines—Throttle Body Injection Fuel System Diagnosis: "Ignition ON, pump pressure should be 28-32 PSI (234-207 kPa). This pressure is controlled by spring pressure within the regulator assembly." I bought the TBI pressure kit and did the test myself. It is spot on spec.
 
It's the same as what came on yours. I looked at the part numbers. They're virtually all the same thing regardless of the engine. It has a low pressure lift pump, not a high pressure pump. Get that one or continue to flog, your choice mate.
a replacement vst fuel pump for your engine is $30.00, not 600.
 
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pwskicat said:
Per the Mercruiser No. 17 GM 5.0L/5.7L V8 Marine Engines—Throttle Body Injection Fuel System Diagnosis: "Ignition ON, pump pressure should be 28-32 PSI (234-207 kPa). This pressure is controlled by spring pressure within the regulator assembly." I bought the TBI pressure kit and did the test myself. It is spot on spec.
Click to expand...

That's necessary but not sufficient.....you need to see what pressure is maintained across the RPM band....WOT is the driver; if the fuel pump maintains adequate pressure @ WOT, usually all will be well....
 
Most gerotor type pumps are capable of delivering waaaayyy more than the engine can use. I doubt there is a fuel delivery problem here. I think he has it plumbed wrong and the return line is getting pressurized. This is based only on what is written here, so I am merely taking a SWAG at it.
 
The VST, engineered with the right size reservoir and correct high-pressure fuel pump, is missing.

The GSL395 that the previous owners installed to replace the VST delivers ZERO GPH at 30psi per the attached chart.

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I tested the fuel regulator per the attached diagram from the service manual. Where does it show the return line going? To the filter.

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How can there not be a fuel delivery problem? Get a copy of the manual, study up on it, and then come back and tell me all the stuff I'm doing wrong.

Thanks for the tips on the Cool Fuel module and the replacement pump. If the pump seller can't show me a chart like the one above that tells me that the pump can deliver 21gph at 30psi, no sale. There are still some pieces of the puzzle missing: The fitting that plumbs into the raw water line and brings cooling water to the module, and the hardware to mount it to the engine.
 
It's not a Cool Fuel module. It's a VST. You may have found the last used one on earth. The seller doesn't warrant that it's complete, or that it's in working order. Nor does he take returns. If you look at the schematic, there are a ton of parts that are unavailable. If the top is cracked, a new one costs $400. I may end up spending hundreds rebuilding it if I can even find the parts. How are you finding the aftermarket parts? I tried to cross reference a few through google and I get nothing.
 
Think we're done here.
Pull the darn TBI off and put a carb on it for crissakes. Or go buy all the parts you need new from our hosts. You have all the info you need to get 'er going.
 
If you're concerned about the return line coming back in at the fuel filter, temporarily take it off the fuel filter and return to an empty 5 gallon can and take the boat out. If this cures the surge you know you have to plumb the return directly to the tank either b y T'ing into the fill or vent line or putting a new bulkhead fitting into the tank top. If this doesn't cure the surge I think this issue wouldn't be fuel supply related (ignition).
 
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