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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11078
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Trying to figure out why my 96 5.8 efi volvo penta won't get

Customer Question

Trying to figure out why my 96 5.8 efi volvo penta won't get more than 4200 at wot
Submitted: 2 years ago.
Category: Boat
Customer: replied 2 years ago.
Also changed the fuel filter/strainer.
Expert:  Tan replied 2 years ago.

Hello. My name is Tan.

This may take more than one reply to solve your issue.

Now when was the last time you had the engine tuned up?

This could be an ignition issue or a fuel starvation issue. To determine if you have a fuel starvation issue we can run the engine on another fuel source or connect a vacuum gauge to the suction side of the fuel pump.

This will tells us if we have a fuel supply issue or an engine issue.

The vacuum gauge test is installing a vacuum gauge on the fuel supply line going to the engine run the boat watching the vacuum gauge. If the vacuum gauge exceeds 2" of vacuum you have a fuel restriction. This will require locating the issue on the boat side fuel system.

Now have you done any of these tests or done the tune up on your engine?

Customer: replied 2 years ago.
I have done the tune up will work on the fuel vacuum test next.
Expert:  Tan replied 2 years ago.

Ok keep me posted. All you need is a brass tee some fuel line and a vacuum gauge. Tee in at the fuel supply line to the engine. Then run the boat as you always do. Watch the vacuum gauge it should never exceed 2" of vacuum at any rpm.

To eliminate a possible clogged or restricted vent line on the tank remove the gas fill cap.

Post back with findings and we can determine where to go next.

Thank you.....Tan

Customer: replied 2 years ago.
Thanks tan I'll give you a shout after I try this out.
Expert:  Tan replied 2 years ago.

Ok Thank you.

Customer: replied 2 years ago.
Okay I took off the fuel cap when the boat rpm stayed the same about 4200 tops out about 43 miles an hour. How to hook up the vacuum gauge between the fuel strainer and the fuel tank and it never read over to psi even at WOT. It just does not run as strong as it used to. If you put 5 people in this boat it takes forever to get to 35 mile an hour. And it never used to have a problem with that.
Customer: replied 2 years ago.
That's way I shouldn't voice text I see it's all jacked up....I hooked up vacuum guage to the fuel line between the tank and the filter it never read over 2 psi at any setting.
Expert:  Tan replied 2 years ago.

You did the test on the water with boat in gear running as you normally would correct?

What kind of rpms has the boat run in the past?

Does the boat sit in the water?

Customer: replied 2 years ago.
No I pull the boat out of the water hand in the past it seems like it would run right around 4700 - 4800. And yes I did check it on the water this last weekend. When you rev it up without the drive connected you can get it almost 5000. But I do recognize under the load of the boat it's probably not going to get to 5000. Just seems like it slow to get up to speed and up the plane so much so that I notice . I am an aircraft mechanic for ups... you understand
you know how it is you know when somethings wrong with you any of your equipment
Customer: replied 2 years ago.
Been a fuel injected engine I don't have the computer to hook up to it nor do I want to spend 700 bucks to get it lol.
Expert:  Tan replied 2 years ago.

Ok the fuel vacuum test has to be done with boat in the water under a load.

Then engine with no load doesn't require the fuel volume necessary it requires under a load. So the vacuum gauge won't show you if you have a restriction. With no load the engine will run on less than one gallon and hour under a load the engine at full throttle will burn up to 35 gallons and hour. With no load the engine will turn up 5000 rpms if you were only running on 6 cylinders. No load will not help with any diagnostics.

So the test has to be under a load to get the correct results.

Customer: replied 2 years ago.
Maybe I didn't explain my results right. I was testing the boat on Taylorsville the water under a load. WOT 4200 RPM 42 MPH and vacuum was 2 in. Hg. The guage never went over 2 in hg no matter what throttle setting it was at.
Expert:  Tan replied 2 years ago.

Ok we need to do a compression test on all cylinders. Then a fuel pressure test on the fuel rail.

We don't have a restriction and we haven't checked the compression yet. We need to eliminate these as possible causes. I'm trying to do the tests we can with out having to use the computer. If all these tests reveal nothing then you will have to have the engine checked with a computer.

Most of the time we can find an issue with these tests but no always.

Customer: replied 2 years ago.
Ok that might take me a week to get to so be patient gotta round up a compression tester:)
Expert:  Tan replied 2 years ago.

That's fine I'm always around no rush. Thank you for your patience. I know these can be aggravating to trouble shoot but we are eliminating the most common failures. We can without the computer. Most failures are not computer related but it is possible.

Customer: replied 2 years ago.
Thanks for your help! I appreciate it! Saves me lots of money on stuff I can do myself.
Expert:  Tan replied 2 years ago.

Your welcome. Thank you...........Tan

Keep me posted.

Customer: replied 2 years ago.
Sure will!
Expert:  Tan replied 2 years ago.