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

[PW]

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

 

[harleyman1975]

Check fuel pressure throttle position sensor, spark arrestor, fuel/water separator?

 

[Scott Danforth]

dirt and crud in the fuel tank is indiscriminate if you have a carb or EFI. it plugs them up either way.

as harleyman indicated, check your fuel pressure AND the contents of your fuel/water separator. I bet you find cloudy fluid that isnt 100% clean and fresh gas

 

[Rick Stephens]

If the issue is plugged up fuel system then you are running lean, hence the surging. Must be careful and address that quickly as you can burn through pistons and other useful components.

 

[PW]

Great info, guys. Thanks. I will start checking.

 

[PW]

I got the OEM Fue Filter/Water Separator, a bottle of Sea Foam and a can of Throttle Body Cleaner. Any tricks I should know about using them? Step-by-step instructions on testing fuel pressure and throttle position sensor would also be a great help. Thank u!

 

[harleyman1975]

take and pour contents of fuel/water separator into a clean glass jar and look at it...then wait an hour and look again...should be perfectly clear with just a hint of color...not cloudy at all no bubbles in the bottom nothing floating around in it at all. either dump sea foam in tank or if there is a vacuum line you can pull and stick it in the can and suck it through intake that way. not sure what to tell you about the throttle body cleaner...is it a spray?

 

[PW]

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, I'll have to tee in a pressure gauge to check the fuel pressure. The photo 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

 

[Rick Stephens]

Electric fuel pump - (connected with automotive fuel line. - a definite no no in a boat) I have no idea if the fuel pump is marine rated or not.

Seafoam will not fix anything.

 

[bentle]

Just a thought here. Could your altitude be higher where you live? If so this will slow you down a bit.
Just a thought.

 

[PW]

If that's an electric fuel pump, why does my engine have a mechanical fuel pump too? Why does this pump have what looks like a factory electrical connection from a wiring harness? My lake is 955' above sea level. The engine is surging. If altitude was a factor, wouldn't the symptom be constant?

 

[Rick Stephens]

I wonder if someone replumbed the vapor separator. Supposed to be a single unit, but there are automotive separators where the pump is separate from the tank. You'd have to trace the lines around.

What the vapor separator should look like.

 

[PW]

Comment on another forum 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. According to serial number search on another site, my engine, 95 350 EFI Magnum Tournament Ski Ser. #OF562552, is supposed to have a mechanical lift 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). The VST is missing. All of which indicates that the fuel system has, for some reason, been modified. I'll take end-to-end photos of the fuel run and post them soon. I'll probably also replace the fuel lines with USCG rated. Thanks for pointing that out Rick Stephens. Then I'll test the system for correct pressure at the lift pump and electric pump. 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 on certain versions of this engine. Haven't shopped it yet.

 

[Rick Stephens]

$1700! Holy Smoly.

Custom mods by previous owners are a bear.

 

[PW]

Diggin around and found a great thread on this site: http://forums.iboats.com/showthread.php?t=283269
I'm going to start there.

 

[PW]

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.
These are the readouts 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?

 

[alldodge]

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

 

[PW]

Mercruiser 350 Magnum Tournament Ski Serial #OF562552. Definitely Throttle Body EFI per my observation and parts lookup on this site. The fuel outlet is plumbed per the diagram below.

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 currently sealed with teflon tape 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.

 

[alldodge]

My bad did see in another post you listed the s/n. If you have the ability to do so, I would run the return line back to the tank. Doing this gets rid of the vapor lock issue. Going to the filter it can still happen because of the anti-siphon valve

 

[PW]

@AllDodge: While the electric fuel pump MAY be out of a GM truck, the rest of the Mercruiser engine, including the MEFI1 ECU, is original to the boat. The parts lookup by serial number proves it. My buddy has the same boat with the same engine, though his still has the VST. I will be replacing all of the automotive fuel line with USCG A1. If the electric fuel pump provides specified pressure, it stays unless you have a very good reason why it shouldn't.