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

pwskicat

Regular Contributor
I recently bought a new-to-me 1995 Ski Sanger direct-drive tournament towboat with a Mercruiser GM 350 EFI Magnum Skier V8. It starts and runs fine. None of us skis faster than 30, so I've never taken it to WOT until last night. It topped out at 35mph @ 3500 rpm. The label on the spark arrester says RPM at WOT should be 4400-4800, depending on prop. The boat is bone stock, with the factory 13x13 OJ Legend prop and 350 hours. It "surges" a bit while cruising at 30, with the RPM revving very slightly between 3200-3300. It isn't even enough for my GPS speedo to register a change in speed. Other than this, nothing out of the ordinary. I bought the boat from a party in AZ and had it shipped to MI, so please don't ask why I didn't notice it on a test drive. I had a mechanic in AZ check it out. He gave it the OK, and so far, it works for my purposes. My buddy has the exact same boat in CA and I've driven it at WOT to 43mph. Checked 1:1 Velvet Drive trans fluid, and it's full. Oil pressure and operating temp appear to be normal. I will be checking throttle cable/linkage to make sure it's opening all the way. Any other checks you can give are most appreciated. Remember, its EFI -- no carburetor. Many thanks for yr help, PW
 
Ayuh,..... Check the Contents of the fuel filter for anything but clean fresh gasoline,.....

Yer probably gonna find water, 'n crud,...
 
Ayuh,..... Check the Contents of the fuel filter for anything but clean fresh gasoline,.....

Yer probably gonna find water, 'n crud,...

I got the OEM fuel filter water separator, a bottle of Sea Foam and a can of throttle body cleaner. Any tricks I need to know about putting them to use?
 
Tee in a fuel pressure gage and have someone eyeball it. There's usually a Schrader valve connection to tap into. Not sure what that motor uses--probably 40 to 50 psi. Better look it up.

Jeff
 
fill the new element with fresh fuel and oil the gasket....

if it has an injector for each cylinder, there will be a schrader valve on the rail.....if its the throttle body EFI, checking the pressure is a PITA as there's no factory adapter....there is a test fitting but since they were only needed for a couple years, they are hard to find....if you know somebody that works at a hydraulic shop, buy them a case of beer....
 
Changed fuel filter/water separator today, cleaned the flame arrestor and put a bottle of SeaFoam in the tank. The surging problem still persists. Did not try WOT as I figured if the surging was still there, WOT wasn't going to happen either. This is a throttle body engine, so per fastjeff's comment, I'll have to tee in a pressure gauge to check the fuel pressure. The following is something that's in the fuel line between the fuel pump and the throttle body. It's the size of a Red Bull can cut in half and has an electrical connection. Anybody know what it is? How to test to see if it's working or needs replacement? Thanks again, PW
MercRedBullCan.jpg
 
You can put your engine serial number in the engine parts section of this website will give you a complete breakdown of parts. To me it looks like a fuel pump, but regardless with hose clamp used on the right, someone's messed with this before
 
TBI should be around 14psi. That's a fuel pump. There are two fuel pumps on this type of system. One is a lift pump and the other a pressure pump.
 
This firckin' site is NOT posting my stuff!!! o2batsea finally told me what no other search or info did, including the factory engine manual I downloaded yesterday: There is a lift fuel pump and a pressure fuel pump. My engine: 95 350 EFI Magnum Tournament Ski Ser. #OF562552, is supposed to have a mechanical pump mounted to the seawater cooling pump (mine's mounted on the engine block). It's also supposed to have an electric pump as part of a Vapor Separator Tank (VST), which is missing. All of which indicates, per an earlier comment, that the fuel system has indeed been "messed with." I'll take end-to-end photos of the fuel line run and post them soon. I'll probably also replace the fuel lines, as a commenter on another forum indicated that the current lines are automotive and not ISCG rated. Then I'll test the system at the lift pump and electric pump for pressure. Hope there's a work around for the missing VST, as I have found exactly one one line vendor that had one, and it's $1,700! There is a later factory system called "Cool Fuel" that was developed to deal with vapor lock problems an certain versions of this engine. Haven't shopped it yet.
 
I wish I still had all my TBI stuff from back in the day. I think you can pass on the vapor separator for now. The thing you want more is a fuel pressure gauge. You may also want an adjustable fuel pressure regulator but I think that's overcomplicating things. Get the Shields Flex-acol fuel line from Jamestowndistributors.com. Pop for some nice AWAB hose clamps while you're there. The ones you have are, well, not good.
If, after you have sorted out the fuel delivery, you experience vapor lock, then you can try some simple remediation. This can be as easy as just rerouting the lines.

And this is just me, but seafoam is about useless.
 
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Looks like the pump in the vapor separator must have died, and they didn't like $1700 either. Verify the fuel pressure is ok, if it is your problem could be elsewhere. Was there any crud in the fuel filter? What about condition of ignition system ? Cap rotor wires and plugs.
 
No crud in fuel filter, but lots of other signs that fuel delivery might be a prob. I took pics last nite and will post with a diagram and description soon.
 
On this thread or on another forum, someone commented that after chasing a similar issue through multiple tests, the problem turned out to be the coil. So I tested it per the following instructions from the Mercruiser shop manual.


The numbers from my digital multimeter:


Test 1 (meter @ 2000 ohms): 1 on the left side of the display
Test 2 (meter @ 200 ohms): 1.2 on the right side of the display
Test 3 (meter @ 2000 ohms): 009 on the right side of the display. However, if the probes weren’t making solid contact, the number jumped as high as the 500s.


Question: The manual listed a removal procedure before the test instruction. I tested the coil in-place after disconnect the wiring harness and coil wire to the distributer. Would NOT pulling the coil from the engine affect the test results?


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Progress! A diagram of my fuel system from tank to throttle-body is attached. We know that it’s wrong because the Vapor Separator (VST) is missing. A previous owner decided to do without it, a new one is $1,700, which I’m not willing to spend. So this is what I have to work with.

This past weekend, I tightened all the fuel unions that had hose clamps. Then I took the boat out and ran it under load with the fuel tank filler cap off and on to make sure it wasn't a tank venting issue. The boat ran the same either way and the surging is still there, but now WOT is up to 3900!

The next test I will due is to hook up a portable fuel tank with fresh gas to the input side of the Fuel Filter/Water Separator to rule out any fuel tank/bad gas issues. If the conditions remain the same, then:

1) Disconnect/Clean/Reconnect all threaded unions using 592 thread sealer per the engine manual
2) Test the mechanical pump for correct pressure (3-7 PSI)
3) Test the electric pump for correct pressure (30 PSI at all throttle settings)

I know there are other tests as well. They will come in time after I eliminate fuel pressure issues. 20+ years ago, I gave up wrenching on automotive engines because they got too complex, and I've never worked on a fuel-injected engine before. As long as I see progress, I'll keep going. I'd forgotten how fun and satisfying it can be :)

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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 last post. I think I'm chasing a vacuum leak or fuel pressure issue. How do I test for vacuum leak(s)?
 
I'll show my ignorance of TBI/common rail injection fuel systems here......

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.

Do you have another fitting on your fuel tank where the return line can dump into? If not, can you T into the fuel tank vent line and dump the return into that? Maybe you could re-plumb the return line to the tank and eliminate the T in your supply line and give it a try. At bare minimum, rerouting the return line is a pretty low cost, low effort thing to try. Of course, this is assuming that this type of plumbing is non-standard. If others chime in and say it's normal, then don't bother making the change.

Also,that 'Tube To Nowhere' concerns me. I don't know TBI well enough to give much advice, but from a tidy standpoint, I don't like seeing hoses just capped off. If this fitting is there it has (had) a purpose, so you have to wonder why it's been modified.

KJ
 
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Agreed. Drag racers often don't bother returning fuel to the tank, they just let it "pile up" at the regulator. No problem since they don't run too long, and the fuel flow is not wasted (assuming they need the greater flow rate).

Jeff
 
@kevinj: The fuel return used to dump into the Vapor Separator Tank (VST). As noted earlier in the thread. Also noted, this is a direct-drive tournament ski boat. I don't know where fuel tanks are located in relation to a a stern-drive Mercruiser, but my engine is dead center in the boat, while the fuel tank and its vent line are all the way in the stern, presenting a major plumbing problem. To reach them, the fuel return line would have to run down into the bilge, under the floor and back up to the top of the fuel tank. The line isn't pressurized. What would pump the fuel all that way? I'm guessing the boat's previous owner plumbed it the way it is now when the VST died and they decided not to spend $1,700 to replace it. So it's either never run right since, or it ran right for some time and now it doesn't. I'm going to run the the first two paragraphs of your comment by a guy on another forum who has given me a lot of good info, and see what he has to say. Let's see if anybody else on here chimes in, too. Thanks for yr comment!
 
@Jeff: There is an available input on the fuel filter/water separator mounting plate. Would running the fuel outlet there solve the problem?
 
@kevinj: The fuel return used to dump into the Vapor Separator Tank (VST). As noted earlier in the thread.....the fuel tank and its vent line are all the way in the stern, presenting a major plumbing problem. To reach them.....The line isn't pressurized. What would pump the fuel all that way? I'm guessing the boat's previous owner plumbed it the way it is now when the VST died and they decided not to spend $1,700 to replace it. ....
Okay that clears it up. So I looked up a VST to educate myself and it pretty much looks like a tank with a reservoir, float valve to control filling, and a low pressure fuel pump. In other words, a miniature fuel tank to keep a ready supply of liquid gasoline available to the engine.

Again, I"m not speaking from experience but just reading what I see in the diagram. I really have to wonder if that low pressure pump is creating vacuum from the path of least resistance which would be the return line. Maybe pulling that vacuum against the fuel pressure regulator is creating an issue and as I said it's trying to pull from the loop rather than from the tank.

Do you have access to a 3 gallon or 6 gallon portable fuel tank? How about it you plumb the return line into one of those and either cap off the return connection or replace the T with a brass hose barb butt connector? You'll have to loosen the lid on the portable tank to allow the fuel to flow in. If my hypothesis is correct, when you give it full throttle the low pressure pump should pull fuel straight from the main fuel tank and you'll have full power. If not, then you're only out a few dollars for fittings and a little time.

One reason that points me to this is all of the electrical connections and sensors seem perfectly fine as you stated earlier so outside of it being an ignition problem it really sounds like a fuel delivery issue. Add in the completely non-standard fuel routing (along with the mystery hose) done by the previous owner and I'd really have to question how correct the setup was done. Since you don't have a full history on the boat it might be a good guess that it has never run right since the fuel system work was done.

One thing to ask is do you truly know the health of the ignition system. My thread here explains my situation:http://www.marineengine.com/boat-forum/showthread.php?433827-Mercruiser-5-0-Intermittent-Surge
My carb'd 5.0 was doing a weird surge thing intermittently and just didn't sound like a healthy V8. In 7 years of ownership (I know, call me a dunce :p) I hadn't done any PM on the ignition system. So, rather than drive myself crazy chasing gremlins I did a complete tune up and it solved my issue. I can't say which component was bad, but a complete swap of plugs, wires, cap, rotor, and pickup sensor got it fixed. I'm not suggesting you drop $250 to do the same thing, but you might verify all of that is in good condition while you're troubleshooting.

KJ
 
@kevinj: The only part you may be confused about is that the missing VST was the high-pressure fuel pump in the system. If you scroll back to post #8 in this thread, you'll see what a previous owner installed to replace the VST; a high-pressure pump similar to the type used in the "Cool Fuel" system that the factory developed to replace the VST, which was having vapor lock problems. The low-pressure pump on this engine is just a plain ol' mechanical hung off the side of the block. I'll do yr test dumping the return into a portable gas tank. Can't get to it til the weekend. If that solves the prob, could I run the return into an available intake on the fuel filter mounting fixture? Review my diagram and tell me what you think. Won't start chasing ignition til I rule out fuel and vacuum. Thanks for yr help.
 
A vacuum leak you can hook a vacuum gauge up to a port on the intake manifold or at base of throttle body - I think above 15 in hg and steady at idle is good, will drop as the throttle is open . The gauges are available on line or at auto parts stores around $20. They also can test low pressure (<10 psi) fuel pressure so its not a bad tool to have. Old school method is to spray starting fluid around the intake gaskets and throttle body- if the rpms increase then you're sucking in starting fluid through a bad gasket.

I agree it makes sense to stay focused on fuel, but wouldn't hurt to take a look under the distributor cap and ohm out a few plug wires to ensure they are at least ok to rule out any low hanging fruit. You can buy a simple spark gap tester for like $12 and put it in place of a spark plug. Should be a nice blue hot spark. if orange or cant jump much of a gap you need to look for ignition issues. On an old outboard I went through carb rebuild, fuel pump, adjusting points only to lastly check the spark and find out my coils were basically dead. Now I check this first to get a baseline...
 
@kevinj: The only part you may be confused about is that the missing VST was the high-pressure fuel pump in the system. If you scroll back to post #8 in this thread, you'll see what a previous owner installed to replace the VST; a high-pressure pump similar to the type used in the "Cool Fuel" system that the factory developed to replace the VST,
Yep, missed that one. I thought the lower pressure pump was in the VST. I got it now.
 
Please don't comment unless you have been following this thread or until you read ALL of it. I been advised repeatedly about ignition and ECU issues. I won't chase them 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.


To review: Engine surges at steady throttle and engine won't rev past 3500 at WOT


-- Put Seafoam in gas tank
-- Changed fuel filter/separator
NO IMPROVEMENT of surging and WOT only 3500


-- Tested coil: OK
-- Tested Throttle Position Sensor: OK
-- Tested Idle Air Control: OK
-- Tightened all fuel hose clamps
STILL SURGING. WOT improves to 3900


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.


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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?
 

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you really need to measure the fuel pressures under load....that will ensure the engine has enough fuel when it really needs it.

Thats a significant pressure drop on the suction side of the first pump...may want to check for anti syphon valve at top of fuel tank's outlet....the usually look like a 'pipe stretcher' and typically are made of aluminum. a check valve inside usually catches any debris in the tank.

The jittery low pressure output isn't 'normal' but could be due to the 'high pressure substitute' pump you have. really curious what's going on fuel pressure wise, on the high side, under load.

The hose to nowhere could be the regulator hose from the removed $1700 plumbing. Sounds like there have been many changes to that engine before you got it...
 
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 resvior between the two pumps this would lead to a fuel starvation and surging.

In post 18 you reference 30 psi on the high pressure side, but I couldn't tell if this was actually measured or your target value. Bottom line if under load you are getting a constant hi pressure supply to the throttle body id think that's all you need. If the pressure cycles like you see here on the low pressure side, if think you've found your issue
 
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