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

[Rick Stephens]

Could sure use a engine serial number to verify. Have been unable to verify EFI in a ski boat configuration for 1995. The fuel pump is from a GM truck and should come out of there, also wonder if the rest came from the same truck.
If it was a VST then I wonder where the return line was placed?
Does it have a MEFI 1 computer?

Need to check fuel pressure as another mentioned, otherwise were shooting in the dark

AD, the 350 Mag EFI SKI was specifically made for ski boats from 94-96.

I don't know how to have an opinion on the fuel pump from the truck, seems like it would be the same part in the marine application.

 

[alldodge]

If the electric fuel pump provides specified pressure, it stays unless you have a very good reason why it shouldn't.

The pump is not certified for marine use, if you want to keep it that's your choice. If something happens, and not saying in any fashion it will, then it will be your fault. I would switch to cool fuel, or another marine rated pump.

Never know, the pump may not have come from a truck, might have come from Indmar, they use similar pumps, but not so sure about the wiring studs

 

[PW]

@AllDodge: I've been shopping for a high-pressure pump since I started trying to solve the problem. Online vendors don't want to provide pressure specs, and I'm not going to buy one that isn't spec-ed 30 PSI. If you have a resource for a quality marine-rated pump that's guaranteed to produce 30 PSI from 600 RPM idle all the way through 4400 RPM WOT, please let me know.

Whenever I take the boat out, I open the engine cover and turn on the blower for a few minutes to air out the bilge, close the engine cover and never turn off the blower until we return to the dock. I understand that there may be a higher risk with the wiring studs. I'll try to find a rubber hood similar to ones used on ignition coils to mitigate the risk. I'll take it out it if I can find the correct replacement pump. I appreciate your input. Thank u.

 

[alldodge]

The 30 psi is controlled by the regulator (item 12) on the TBI
http://www.mercruiserparts.com/bam/subassembly/31412/1612/110

Most marine pumps can produce in the area of 60 psi, so the regulator bleeds off excess pressure back to the filter/tank. The VST and Cool fuel are pumps can also produce 60 psi. My guess, is the pump you are using can also produce 60 psi.

 

[PW]

@AllDodge: Thank you. This was info I did not have. This is the first automotive engine I've worked on in 20+ years and the first fuel-injected engine I've ever worked on. I didn't know that the pressure was regulated not by the pump, but by the pressure regulator, which is in the throttle body just upstream of the fuel outlet. Also, the 30PSI figure I found online appears to be incorrect, based on the attached page from the Mercruiser Engine Manual, which says it applies to MPI and TPI engines. Item 5 specifies pressure while cranking engine should be 36 PSI, which doesn't agree with your estimation of 60 PSI. Any idea why the discrepancy?

 

[alldodge]

Keep going thru the pages until you reach 5E-70 testing the fuel pump

 

[PW]

@AllDodge: More good info! Thanks. Not being an experienced mechanic and working full time, it will take me awhile to perform these other operations before I get to testing the electric pump:
1) Disconnect/Clean/Reconnect all threaded unions currently sealed with teflon tape using 592 thread sealer per the engine manual
2) Hook up portable fuel tank with fresh gas to fuel system at fuel filter/separator to rule out any bad gas/fuel tank, vent, anti-siphon valve fuel line issues. The boat came to me from AZ and it had gas in it. Don't know how long it had been sitting or if it was treated with stabilizer or not. I stabilize all fuel that goes in my boat.

I'll probably get to the above work this weekend. Thanks again.

 

[PW]

If anyone is reviewing this topic for the first time, pls read the entire thread so you are up to speed before commenting. Thank u. This weekend's work:

1) Tested Throttle Position Sensor
-- Voltage supply from ECU tested 5.0V, which is correct
-- Throttle position voltage tested .46V at idle, also correct
-- Slow advance of throttle through WOT produced smooth increase in voltage (no sudden jumps) to 4.25V. Anybody know if this is in spec?

2) Half tested Idle Air Control
-- Tested resistance between control terminals A--B and C--D. Both measured 55 ohms, which is in spec.
-- Could not find instructions on how to test female harness plug with multimeter. Effing instructions on the web expect everyone to have a garage full of GM test tools. Is it possible to test the harness with a multimeter or is a test light mandatory? I know this harness is connected to the ECU, so I don't want to don't want to do anything to burn that up.
-- Will remove and inspect IAC soon

3) Hooked up a portable tank of clean gas on the intake side of the fuel filter/water separator. There must have been an error somewhere on my part because this actually made the engine run worse. It acted like it was starved for fuel (bogging) and backfired, which it never did before.

4) Disconnected threaded unions in fuel system, cleaned of teflon tape and reconnected using 592 pipe dope per the shop manual. Reconnected fuel tank.

Everything is the same as it was as in my post on 8/16 @ 2:35pm. I think I'm chasing a vacuum leak or fuel pressure issue. How do I test for vacuum leak(s)?

 

[alldodge]

3) Hooked up a portable tank of clean gas on the intake side of the fuel filter/water separator. There must have been an error somewhere on my part because this actually made the engine run worse. It acted like it was starved for fuel (bogging) and backfired, which it never did before.

Pump may be weak. but your previous posted listed solid 30 psi thru entire rpm range.

Everything is the same as it was as in my post on 8/16 @ 2:35pm.

See the date, don't see the exact time. Suggest using post count numbers

IAC issues is for idle only, will not effect top end

To check for a vacuum leak look for the MAP sensor.. To check fuel vacuum, place a vacuum fuel pump tester between tank and filter

 

[PW]

On an earlier post, I wrote that the high-pressure fuel spec is "30 PSI through entire RPM range," which I later, with your help, found out was wrong. I have yet to test fuel pressure on either the low- or high-pressure pumps, so that and checking the MAP sensor for vacuum are next. Thanks again!

 

[alldodge]

Just to make sure we are talking the same thing. The fuel pump should have the capability of producing up to 60 psi, but the motor needs to see only 30 or so psi. So your running motor should be seeing 30 psi throughout the rpm range. The regulator keeps it in the 30 range

 

[PW]

Got it. Does that mean if I test the pump while the engine is running and it remains at 30 PSI from idle to WOT, that I don't have to perform the 60 PSI test you pointed out in post #26? Or do I have to do both?

A comment I just got on another forum follows. What is your opinion about a possible "closed loop."

Is it common practice for the fuel return line to feed directly into the fuel supply line as you showed with the brass T in your diagram? I thought return lines always dump back into the tank.

What I'm wondering is if the pumps are somehow starving themselves for fuel by drawing from the closed loop directly through the TBI rather than drawing from the fuel tank? That low pressure pump is entirely dependent on a good siphon, and if that return loop is interrupting that vacuum it could cause delivery issues into the filter.

Thanks again for all your help!

 

[PW]

I tried to use colored type to quote the other forum in my post #32, but it didn't take. The quote is paragraphs 3 and 4 in post #32.

 

[alldodge]

Got it. Does that mean if I test the pump while the engine is running and it remains at 30 PSI from idle to WOT, that I don't have to perform the 60 PSI test you pointed out in post #26? Or do I have to do both?

These are two different points and one has very little to do with the other.

My 60 psi comment was only stating that most marine high pressure pumps CAN (note the word can) produce 60 psi. Now they DO NOT produce 60 psi when being used in the typical EFI/MPI system. I only commented on the 60 psi from your comment of being unable to find a pump which could output a constant 30 psi. I continued to say that the regulator is what is used to reduce the output down to that level.

So for this time forward lets just forget I said anything about it

A comment I just got on another forum follows. What is your opinion about a possible "closed loop."

Is it common practice for the fuel return line to feed directly into the fuel supply line as you showed with the brass T in your diagram? I thought return lines always dump back into the tank.

What I'm wondering is if the pumps are somehow starving themselves for fuel by drawing from the closed loop directly through the TBI rather than drawing from the fuel tank? That low pressure pump is entirely dependent on a good siphon, and if that return loop is interrupting that vacuum it could cause delivery issues into the filter.

Merc has and still does return excess fuel back to the fuel filter inlet side. This is a proven way and has no issues with running a 120 HP 4 cylinder clear up to the 1200 HP V8 motor. The return to the tank only helps in the vapor lock issue, and so far you do not have this issue

 

[PW]

Please don't comment unless you have been following this thread or until you read ALL of it. I been advised on several issues that I've already tested and ruled out. Ignition and ECU issues won't be chased until I'm sure fuel system is working properly. I already know that the system is "wrong" because it's missing the Vapor Separator Tank (VST). I'm working with what I got. I'm not a mechanic. I'm learning as I go. Thanks for yr patience and help.

Vacuum Test A (Old School)
-- Start warmed-up engine and let idle.
-- Spray started fluid around base of TBI adapter plate and base of TBI assembly.
-- No change in idle speed.



Vacuum Test B (Gauge)
-- Warmed up engine. Found a capped nipple on the back of the throttle body. Attached gauge.
-- At idle, needle is steady at bottom (left side) of green "NORMAL MOTOR" zone.
-- At 3000 rpm, needle is steady at top (right side) of green zone.

Low-Pressure Mechanical Fuel Pump SUCTION test
As noted earlier, because the VST is missing, the fuel return from the TBI runs to a tee that dumps the fuel back into the main fuel line just before the filter
/separator.
-- Removed fuel return line from tee and put it in an empty portable gas can
-- Attached vacuum gauge to the tee inlet where I removed the fuel return
-- At any speed from idle to 3000, the needle was steady at a little above the '5' on the left side of the gauge, pulling down counterclockwise from the Zero read.

Low-Pressure Mechanical Fuel Pump PRESSURE test
-- Plumbed a tee in between mechanical pump and high-pressure electric pump
-- At any speed between idle and 3000, the needle swung wildly between 0 and 7-8 on the fuel pressure scale
-- The needle never steadied or reduced the swing. Always between 0 and 7-8.

NOTE: Vacuum and Fuel Pump Test done at the dock, NOT under load. Does this matter?
At lease a couple of gallons of fuel pumped from fuel return into portable gas tank during testing. Is that normal?

 

[alldodge]

-- The needle never steadied or reduced the swing. Always between 0 and 7-8.

If it is quickly pulsing between 0 and 7-8 then this is probably just the pump pulsing. If it smoothly, even though quickly moves then maybe something different.

NOTE: Vacuum and Fuel Pump Test done at the dock, NOT under load. Does this matter?

Since you have an issue reaching top speed then yes, it does matter that your at the dock. It takes very little fuel to raise rpm in neutral, needs a lot more under load.

At lease a couple of gallons of fuel pumped from fuel return into portable gas tank during testing. Is that normal

Not an issue, the pump should be pumping more fuel then needed, even at WOT. This way your ;ump never runs your motor lean.

Please don't comment unless you have been following this thread or until you read ALL of it.

While I do understand the comment, someone with a WAG may notice something that we didn't. Just because someone makes a comment, it doesn't mean it needs to be responded to.

 

[PW]

Did something I shoulda done at first: check the Anti-Siphon Valve (ASV). Put a quart jar in the bilge, pulled the fuel line off the tee and stuck in the jar, which proceeded to fill up. Hardly think 6 ft of 3/8 fuel line could have a quart of gas in it. Re-attached the fuel line, pulled the back seat and removed the ASV. The ball valve was jammed part way open with a thin but stiff wire. So it couldn?t close when it was suppose to, or open all the way either. Your guess is as good as mine how it got there. Yanked the wire, reinstalled the valve, started the engine and let it warm up for a test run. As soon as I tried to throttle up, the engine died and wouldn?t restart. So does this mean the ASV is shot, or that the mechanical fuel pump can?t generate enough suction to pull the valve open? ASV pics below. Pretty sure its original and 23 years old just like the boat. Anybody know what the screw on top is? Any way to bench test the valve? It?s an inexpensive part, so I?ll go ahead and replace it. Looking forward to yr advice.

 

[PW]

Oh. The fuel tank is stainless steel if that makes any difference.

 

[alldodge]

As soon as I tried to throttle up, the engine died and wouldn?t restart

The motor went from not able to reach top speed to it won't start.

If its not getting gas thru the valve, then it needs replacing

 

[PW]

VICTORY! Installed an open test elbow in place of the valve. BINGO! No more surging and WOT is 4450! i think there may still be minor fuel pressure issues, but I have all the gear to test that and wiil do so. New anti-siphon valve arrives Tuesday. But top speed is still only 37mph according to the GPS app on my phone. Maybe tachometer is wack? Anyway, very happy to have solved this with all yr help. THANKS!