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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10802
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Hi, Last summer after warming up the engine took off across

Resolved Question:

Hi, Last summer after warming up the engine took off across the lake @wot within 3min
engine lost power and Icould not get over 2200 rpm started boging big time
took it in to a dealer and they did a lake test with lap top showed no codes they added fuel
created worse bog,compression test 160-170, fuel pressure measuerd 40 psi before tbi
apparently on the high side they suspect bad fuel reg.I changed it and still have same problem.On muffs it runs great in neutral idles fine reved it up 3500 no problem another
dealer had it on dyno pulling 2700 psi @4000 rpm.Put it back on the lake boging still the same problem. any ideas, thanks.
Submitted: 5 years ago.
Category: Boat
Expert:  Tan replied 5 years ago.

Hi my name isXXXXX will assist you.


What you have here is a fuel restriction in the boat side fuel system starving the engine for fuel. Run the engine on another fuel source, if the engine runs better you have a restriction in the fuel tank clogging the pick up tube or a restriction in the fuel supply line from the tank. If the engine runs the same you need to check for restrictions in fuel pumps and filters.

Customer: replied 5 years ago.

fuel pressure is @ 40 psi going to tbi. spec is 30 psi +or- 1 seems to be getting more

than enough fuel, mechanic tried adding more fuel and air created worse bog.

I whatched the injectors as opening throttle wider and there seems to be a

considerable amount of fuel pouring out of the injectors.I myself was squirting fuel in

to tb trying to increase rpm but it would bog worse .

Expert:  Tan replied 5 years ago.
You can have all the pressure in the world but it won't help if you don't have the volume. When you at idle the fuel pressure is the same as at full throttle the difference is volume you use less fuel at idle than at full throttle. So you cannot judge the amount of fuel the engine is getting by looking at the spray pattern. If pressure were the case why wouldn't you fuel pressure be lower at idle than at full throttle. The pressure is constant the volume is what varies. So do the test and you may find that I am right. If not we have eliminated the boat side fuel system as a problem. Now it seems to me you have eliminated everything but the boat side fuel system. Have you done a vacuum test on the suction side of the fuel pump. This will lead you to the same thing as running the engine on another fuel source. If you like I can explain how to do the vacuum test. But if think I am not on the right track I'll gladly do as you ask.
Customer: replied 5 years ago.

Hi Tan,

Could you please explain how to do the vacuum test.

Expert:  Tan replied 5 years ago.
To perform the vacuum test you will need a vacuum gauge a brass tee some fuel hose. Clear fuel hose short piece and brass tee the size of you fuel supply line. Here is a diagram showing the set up. If the vacuum gauge shows more than 2" of vacuum you have a fuel restriction. Use this set up when running the boat under a load. Check that the vent line on the fuel tank is not restricted or have any kinks in the hose. graphic
Tan and other Boat Specialists are ready to help you
Customer: replied 5 years ago.

Hi Tan,

Iwill try this on the week end and let you know the results.



Expert:  Tan replied 5 years ago.
OK, Your welcome.
Customer: replied 5 years ago.

Hello Tan, I ran the boat yesterday with a seperate fuel source, the engine reacts the same still boging under load, no power can not plain out and engine becomes noisy

like its working hard.If I try going WOT the engine want's to stall.I'm thinking of running

a new fuel line from the low pressure pump to the high pressure pump the old line looks

fine from the outside, but it has a braided material on it and you really can't tell what the

line is doing under higher load like perhaps collapsing.I'm totally guessing on this.Is it

worth a try?Do you have any other idea's?



Expert:  Tan replied 5 years ago.
That line is under pressure no vacuum so it wouldn't collapse. Where did you connect the other fuel source to the engine? Do you have a VST # XXXXX (vapor separator tank) if yes then remove the cover and clean the tank most likely full of old fuel and debris.Exploded view / schematic
Customer: replied 5 years ago.

I don't think I have a vst reservoir I have no access to the boat untill later on this evening but what I recall is the inlet is just above the fuel filter and pump is above.

I will check this out and let you know.



Expert:  Tan replied 5 years ago.
Thanks Frank, Post you complete engine model number 5.0Gi ????? there is a series of letters to designate the exact fuel system you have please. We will get this figured out.
Customer: replied 5 years ago.

Hello Tan,

it's a 5.0 gi pwtr volvo penta XXXXXXXXXX year 1999



Expert:  Tan replied 5 years ago.
Your motor does have the VST according to the model number. The VST will be on the rear of the motor with the high pressure fuel pump # 20 on top. Check that tank for debris. Check the cooling line # 48 water out 78 water in if there is blockage the fuel in the VST gets hot you will experience vapor lock.

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