95 Mercruiser 350 Magnum Tournament Ski: Got to WOT!

This is a fuel flow problem. Nothing else. There are some folks on the forum who know this engine and my problem, so they’ll know what I’m talking about. Before anyone else chimes in about anything other than fuel flow, please this ENTIRE thread: http://www.marineengine.com/boat-forum/showthread.php?438706-Inboard-95-Mercruiser-GM-350-EFI-Magnum-Skier-limited-top-speed
Then you’ll know what I’m talking about too. Thank you.


After doing a tuneup that included plugs, plug wires, cap and rotor, fuel filter and cleaning the flame arrestor, I removed the mechanical fuel pump and plumbed a direct line from the fuel filter to the electric pump. No mechanical pump in line at all. On a test run, she took off like a scalded cat and ran right up to 4400rpm! First time ever since I got the boat 4 years ago. But it only lasted for about 90 seconds. The it bogged and spit and limped back to the dock.


So at least now we know she will run to WOT, though I’m pretty sure I have to get the mechanical pump back online to make it happen permanently.


Before the Vapor Separator Tank (VST) was tossed by the previous owner, the TBI fuel return ran to it, as did a supply line from the mechanical fuel pump. They entered separate chambers in the VST, which then metered the fuel to the electric pump for the TBI.


Now, the fuel return is hooked up to an inlet on the fuel filter, and when I first got the boat, it ran to a brass tee in the tank fuel line before the mechanical pump. Is it possible/probable that, without the VST, these two low-pressure fuel flows are fighting each other, causing the electric pump not to be able to supply enough fuel to reach WOT?


I’m installing a new mechanical pump and hope to lake-test mechanical/electric setup tmrw nite. If it doesn’t get me to WOT again, I have a couple other workarounds to try. Interested to hear from the folks who are up on this story from the beginning. Many thanks for all your help thus far.

""
Before the Vapor Separator Tank (VST) was tossed by the previous owner, the TBI fuel return ran to it, as did a supply line from the mechanical fuel pump. They entered separate chambers in the VST, which then metered the fuel to the electric pump for the TBI.""

Did the two chambers connect in any way? It would appear the return fuel would eventually over fill the VSP? or did it have an additional return line to the gs tank?

How does the VSP meter the fuel?


I would think,

Based on the image from prior post you reference, The vapor separator is just that, the return line and tank feed should combine within the VST.
An electric pump for some reason cannot do the job alone therefore a mechanical pump is needed?

I am not sure why a quality electric pump couldn't handle the whole job.............maybe someone else can answer this.

Hi Jack. Thanks for your reply. Can't answer your questions about the VST. Never seen one in the flesh. There are none available new or used. I've searched the internet for months. I probably wouldn't revert to it even it I could. That fuel setup was so cursed, that Mercruiser stopped building them after three years. Crashing on work right now. Will try to get back to you over the weekend.

I think the VST provides some buffer capacity between the two pumps, which I think is probably missing in your current set up. The vst appeared to have a float bowl not unlike a carb that the high pressure pump sat in. I can only think this is a revision for the high pressure pump to draw from and allow vapor separation. Did you measure fuel pressure at WOT?

i would suspect that if you got one electric pump Sufficient capacity pressure and flow wise mounted Close to fuel tank and put the pressure relief return back to the tank that would work. Also can’t recall if the fuel pressure regulator was checked or replaced.

interesting posts for sure

Did you start a new thread because you weren't getting the answers you wanted on the old one? I firmly believe that you were given all the information required to solve the problem. Now you are starting all over? I'm sorry to be a dick, but this new thread only adds to your confusion. Go back and read the old thread again, especially the part about the fuel pressure gauge and the pump.
You also never mentioned whether you did anything with the fuel pressure regulator in the TBI unit itself.

o2batsea said:

Did you start a new thread because you weren't getting the answers you wanted on the old one? I firmly believe that you were given all the information required to solve the problem. Now you are starting all over? I'm sorry to be a dick, but this new thread only adds to your confusion. Go back and read the old thread again, especially the part about the fuel pressure gauge and the pump.
You also never mentioned whether you did anything with the fuel pressure regulator in the TBI unit itself.

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You sound as though you are disgruntled.
There is a simple solution.... don't post to a thread that you find to be redundant.

Also, most of us find it to be much more effective when we are kind and polite. You should give it a try!


.

You're only posting here so you can take a swipe at me. Is the entire state of Oregon filled with snarks like you? Practice your own medicine there, Doc.

Sorry, o2bat. If you read the old thread you'd find that I tested fuel pressure at the TBI both at idle and at speed and it tested OK. Regardless of what you think, I tried everything everybody suggested and got NO RESULTS. I started a new thread because this was a completely new problem: I wouldn't run AT ALL for more than a few minutes.

The problem now is the main fuel tank. I hooked up an outboard tank and it runs fine all the way to WOT. The solution included installing new mechanical and electric fuel pumps, and plumbing the system so that the mechanical pump and the TBI fuel return both send fuel to the water separator/fuel filter, and the electric pump takes fuel from that. Now, fuel filter acts kind of like the missing Vapor Separator Tank. (Not exactly your solution, Dieter, but it works! To protect the mechanical pump, which now draws from the main tank ahead of the water separator/fuel filter, I installed a metal 40-micron fuel filter in the main fuel line. The main fuel tank has also been part of this problem for all the time I've owned the boat. I suspect the pickup tube is damage or clogged. I'll have the tank inspected to see if it can be cleaned/repaired. If not, I'll replace it. Thanks to all who have pitched in with good advice and positivity. I'll try not to post anything for awhile. Otherwise, o2batsea will have to increase the dose on his BP meds...

Likely there is an anti-syphon valve that you may want to check...

Otherwise, o2batsea will have to increase the dose on his BP meds...

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They don't make enough bourbon for that. Glad that you are making progress. This is why old boats are so charming; they are full of surprises!