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

[PW]

The electric fuel pump that was used to replace the Vapor Separator Tank (VST) is a Walbro GSL395 (description below). It?s an automotive part and I don?t think it can replicate the function of the VST fuel pump. Here?s why: I did the high-pressure fuel pump test per the service manual. Key on, the electric pump runs for two seconds and the test gauge indicates 30psi, which is right on spec. The pump shuts off, and the pressure is supposed to hold, but in my boat, the gauge immediately drops to zero. Then I did the fuel pressure regulator leak down test, plumbing a fuel shut off into the fuel return line and applying 12V to the pump. The gauge shot up to 60psi, which is right on spec, and held, which is exactly what it?s supposed to do. So, does the VST have some kind of check valve to hold the initial pressure from the 2-sec pump operation? Is that functionality missing from the Walbro pump? If so, I?m screwed, because there is no part number other than the VST, which, while I said I found one earlier, is actually no longer available. The pump that goes in the VST is available for $725, but you gotta have the rest of the assembly for it to work. Lastly, I took the boat out today, and with my buddy driving, saw that the test gauge on the high-pressure pump held 30psi steady at all throttle settings, and the gauge on the mechanical pump rattled between 4-5psi at anything above 2000rpm. Took the test gauges off, and ran the boat. The surging is still there, but not as pronounced, and the boat still only tops out around 36mph, and the tach goes wonky when I try WOT, so I?m not even sure how high the boat is revving. None of my buddies or I ski faster than 30, and I drove it around the lake a few times at 30 and it was OK. Not perfect, but OK. I guess I could replace the electric fuel pump with a new one and see if that improves anything. That would only be $135. Or convert the TBI to carburetor. That would cost probably $1k. That said, there?s a new anti-siphon valve on the fuel tank, a new Tygon sight tube running from the mechanical fuel pump to the flame arrestor, USCG Type 1A fuel line throughout and new stainless-steel hose clamps at all unions.

 

[alldodge]

The electric fuel pump that was used to replace the Vapor Separator Tank (VST) is a Walbro GSL395 (description below). It?s an automotive part and I don?t think it can replicate the function of the VST fuel pump. Here?s why: I did the high-pressure fuel pump test per the service manual. Key on, the electric pump runs for two seconds and the test gauge indicates 30psi, which is right on spec. The pump shuts off, and the pressure is supposed to hold, but in my boat, the gauge immediately drops to zero.

Then I did the fuel pressure regulator leak down test, plumbing a fuel shut off into the fuel return line and applying 12V to the pump. The gauge shot up to 60psi, which is right on spec, and held, which is exactly what it?s supposed to do. So, does the VST have some kind of check valve to hold the initial pressure from the 2-sec pump operation?

Is that functionality missing from the Walbro pump? If so, I?m screwed, because there is no part number other than the VST, which, while I said I found one earlier, is actually no longer available. The pump that goes in the VST is available for $725, but you gotta have the rest of the assembly for it to work.
Lastly, I took the boat out today, and with my buddy driving, saw that the test gauge on the high-pressure pump held 30psi steady at all throttle settings, and the gauge on the mechanical pump rattled between 4-5psi at anything above 2000rpm. Took the test gauges off, and ran the boat. The surging is still there, but not as pronounced, and the boat still only tops out around 36mph, and the tach goes wonky when I try WOT, so I?m not even sure how high the boat is revving.

None of my buddies or I ski faster than 30, and I drove it around the lake a few times at 30 and it was OK. Not perfect, but OK. I guess I could replace the electric fuel pump with a new one and see if that improves anything. That would only be $135. Or convert the TBI to carburetor. That would cost probably $1k. That said, there?s a new anti-siphon valve on the fuel tank, a new Tygon sight tube running from the mechanical fuel pump to the flame arrestor, USCG Type 1A fuel line throughout and new stainless-steel hose clamps at all unions.

Had to divide up your post to make it more readable.

With the tach going wacky, my first thought is there is something causing a losses in spark

 

[PW]

@ AllDodge. Sorry for the long post. "With the tach going wacky, my first thought is there is something causing a losses in spark" The stainless fuel tube into the throttle body is within microns of touching the wire from the coil to the distributor cap. Possible short?

 

[PW]

AllDodge Found this from u on another thread about the same engine as mine:
I blocked the return line from the throttle body to the VST as per the Service Manual and cycled the pump on via the ignition. The Service Manual says that if the pressure climbs above 30 psi replace the pressure regulator. If the pressure does not climb above 30 psi replace the fuel pump. Mine climbed to 50 psi (still dropped quickly to zero after turning off the key).

With this I would say one or more of your injectors are your issue. If the return line is blocked off the pressure will rise above the regulator 30 psi setting, Reason being is the reg is set at 30, the pressure goes above it so the reg is full open. The fuel pressure falls right off, so if the pump in the VST is not leaking back (could but not likely) its your injectors leaking.

To test the theory, disconnect output line and block off (remove injectors from the mix). Attach pressure gauge and see how high it goes and does it hold

AllDodge: I did the same test blocking the fuel return line to test the pressure regulator and got 60psi and holding. How do I "remove injectors from the mix" as u suggest? I don't understand what I'm supposed to disconnect and where to attach the pressure gauge.

Thanks for yr help, PW

 

[alldodge]

If your testing the pump only connect direct from the output of the high pressure pump. The regulator or injectors are not in the mix and have no effect. If pressure drops back off then the HP pump is leaking internally.

If block off is measured after the TBI then the regulator and the injectors are in the mix. If pressure gets to or exceeds 60 psi the regulator can be damaged. If injectors are leaking the pressure will drop when pump is turned OFF

 

[PW]

AllDodge. Got it. Thanks. More news: A poster on another forum turned me on to this site: http://www.boat-fuel-economy.com/mercury-mercruiser-350-5.7-fuel-consumption-us-gallons. It shows my engine requires 21gph @ 30 psi at WOT of 4800 rpm. Then a different poster linked me a to flow chart that shows my current pump (Walbro GSL395) can't possibly pump that much fuel: http://walbrofuelpumps.com/walbro-gsl395-fuel-pump.html. Scroll down the page and you'll see the chart. I think this solves the mystery of the surging across the rev range and problems at WOT. The next pump up looks like the answer: http://walbrofuelpumps.com/walbro-gsl393-fuel-pump.html. Please let me know if you agree.

 

[alldodge]

If you have the 395 then I agree

 

[PW]

If you have the 395 then I agree

The Carter P61171 is a marine rated pump, preferable for use over the Walbro GSL392 automotive part. Here are its specs:

- Inlet: 3/8 24 UNF
- Outlet: 5/8-18 UNF
? spade
- Flow: 35gph@40psi
- Draw: 7.8amp@40psi
- 60-120 psi Static

I think "spade" is the type of connector. Do you know what "60-120 Static" means? Do you think this is the right pump for the job? As far as marine pumps go, it looks like my only choice. Thanks for all your help.

 

[alldodge]

Do you know what "60-120 Static" means?

Static means nothing is moving, so pump can take 60-120 psi if output is block off

As far as marine pumps go, it looks like my only choice

There are others, but it should be fine
Aeromotive A1000
Weldon
http://www.hardin-marine.com/c-489-electric-fuel-pumps.aspx

Also found where Indmar uses GSL392 so it is marine rated
http://www.ebay.com/itm/INDMAR-FUEL-PUMP-GSL392-/282576691036?_trksid=p2141725.m3641.l6368

 

[PW]

Today I installed a Walbro GSL393, which has a flow rate of 38gph at 30psi, as opposed to the old GSL395, with a flow rate of 0gph at 30psi. Also put in a 10-micron fuel filter/separator with a flow rate of 90gph, which is required as a prefilter. Took the boat out and it ran to 4400 WOT, but gradually dropped off RPM to below 4000. After cruising at 3000 for awhile, engine began coughing and stalled. I sat in the lake for 5 minutes, and got it started, took if up to 2100, and it stalled again. After getting it started one more time, I idled back to the dock. Here is a quote from post #30 by Dieter:

Looks to me that the low pressure pump is inadequate, would expect a constant output pressure. This maybe a result of the fact this pump is discharging directly into the suction of the high pressure pump with no accumulation. The set up originally on there had in effect a reservoir the low pressure pump pushed into, the drop to zero you see now may just be the cycling of the diaphragm in the pump. I would think with no reservoir between the two pumps this would lead to a fuel starvation and surging.

The only "reservoir" in the current set up is the filter/water separator, which is supplied by the mechanical pump on the engine. Is the new high-pressure pump sucking all the fuel out of the filter/separator and the mechanical pump can't keep up?

Ski season is over and I'm at a loss. Will any remedy work without the damned unobtainable Vapor Separator Tank? Should I remove the mechanical fuel pump and try using the electric pump alone?

Remove the mechanical pump and install the low-pressure electric pump I just removed in its place? (This would put 20psi on the intake side of the fuel filter/separator. Will the filter/separator handle that pressure as opposed to the <4-5psi the mechanical pump supplies?)

Should I buy another high-pressure pump and substitute it for the mechanical pump?

Should I put a carburetor on the damned thing? Your guess is as good as mine.

 

[PW]

NOTE: The fuel return from the TBI is dumping into the filter/separator. That is sending a huge amount of fuel back to that "reservoir." How can the system be starving?

 

[alldodge]

Did a quick read back thru every post, not in depth read.

I still want to know is what is the fuel pressure at the TB while running from idle thru WOT?

The TB has an inlet fitting and a plug fitting. In many cases a fuel pressure sensor is installed in this area, such as pic below


Note there should be two pieces to this fitting. Remove the top portion and install pressure gauge on the 1/8 NPT fitting

 

[PW]

Hi AllDodge. In post #41, 9/10/17: I ran idle to WOT with a Schrader valve adapter and high-pressure test gauge fitted to the fuel inlet of the TBI, and fed by the GSL395 (the pump that came to me with the boat). The gauge never varied from 30psi, which is right on spec according to the service manual. The Walbro GSL395 spec sheet says that while the pump holds the right psi, its flow rate at 30psi is about 0gph instead of the 21gph required at WOT, Are you saying I should mount the Schrader valve adapter to a different spot on the TBI? I'll go out to the boat tonite. and see if I can spot what yr talking about. Thanks again.

 

[alldodge]

No further testing needed.

You ran with the 395 better then the higher output 393, Your within spec so I'm out of ideas.

 

[Scott06]

If you're maintaining 30 psi it's within spec, no matter how non stock it looks - it appears to be doing its job. My thoughts would be you're fuel system is ok. Coil was mentioned in early posts, what is the condition of cap rotor wires etc. assuming you already checked would look at the pick up coil and ignition module 811639t and 811637001. In my mind if the fuel pressure is correct and you are confident the injectors are flowing correctly, it can only be ignition related