Losing RPMs in hot weather

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nosner
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Losing RPMs in hot weather

Postby nosner » Mon Jun 12, 2017 11:27 am

Any ideas? Running an 8.1L Marine Power Fuel injected engine on an 18' boat with dual 24 gallon tanks. When the outside temperature gets around 80 degrees I lose RPMs and get a fluctuation at top end. The fluctuation is negligible on tank closer to fuel pump but almost 900 on other tank. But I still lose top end RPMs on both tanks. Here are the readings I took in the last two days: 4 Blade whirlwind props adjusted two weeks ago set for max RPM 5000, Temp was 74
Tank 1 - closest to pump Tank 2 - further away

Saturday - Temp 84
Tank 1 - RPM 4850 - 4800
Tank 2 - RPM 4800 - 3920
*Replaced fuel Filter no significant change in RPMs

Sunday - Temp 54
Tank 1 - 5100 - 5000
Tank 2 - 5130 - 5050

In cooler temps everything seems fine. I changed out all the fuel pressure lines yesterday rubber did seem soft, thinking may have been collapsing during warmer temps. Won't know till next warm front comes thru. Wondering if I'm missing anything. Any thoughts are welcome

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southern safaris
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Re: Losing RPMs in hot weather

Postby southern safaris » Mon Jun 12, 2017 3:29 pm

Is the same fuel pump powering both tanks?i assume so as the way i read your description, And what type of injection ? Aftermarket tbi ,mpi, just trying to get an idea of what your working w , ive been installing a lot of these bolt on style efi kits folks are getting now and all have different little quirks so to speak, also do you have a controller or handheld that may be giving any dtc codes or real time fuel pressure readings maybe?

Give me a call if you get a chance , theres a couple tests i could run you through over the phone ... Anything we can do to help
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br,lc,LA
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Re: Losing RPMs in hot weather

Postby br,lc,LA » Mon Jun 12, 2017 4:29 pm

How is the fuel system set up? Returnless fuel rail or constant regulated return? Water fuel separator or plain fuel filter?
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pirate
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Re: Losing RPMs in hot weather

Postby pirate » Mon Jun 12, 2017 4:55 pm

Air at higher temperatures is less dense that air at colder temperature. Fuel injection is fairly sensitive. If your FI was set up when temp was cooler, the fuel to air ratio will increase as temps warm up. lean it out a little and rpm should increase.
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nosner
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Re: Losing RPMs in hot weather

Postby nosner » Tue Jun 13, 2017 11:10 am

I believe the fuel injection system was factory from Marine Power or GM? I bought the boat 3 years ago. It is a water separator filter cartridge NAPA 3225. The FI is a regulated return system. Copy the air being less dense the flux running off tank 2 is worrisome. It is the same pumps pulling from both tanks.

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Deano
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Re: Losing RPMs in hot weather

Postby Deano » Tue Jun 13, 2017 11:51 am

The whole tank difference thing, sounds like water to me.
You said you changed the fuel filter, what the water separator cartridge?

When and/or where a second tank may be used less, there is a
greater opportunity for complications from condensation. Warmer
daytime temps and wider temp swings can exacerbate that scenario.

If you have already ruled out water as a culprit, then the
indications would allow for the possibility (if not likelihood)
that the number two pickup may not be obstruction free.

If that be the case, I would try blowing some compressed air backwards through
that pickup and see if it subsequently made even a temporary difference.
If so, it would seem that second tank needs a thorough cleaning.
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Re: Losing RPMs in hot weather

Postby swamp_fever » Tue Jun 13, 2017 6:19 pm

My dad was having a problem of not turning up RPMs on his 0540 2500 was just about all he could turn up motor never bogged down he tryed diffrent props with less pitch and still only get about 2500 out of it he had duel tanks with a brass wing valve finally took the valve out hooked up to one tank got 3000 RPM with no problem come to find out it was sucking air through the valve at W O T would'nt run out of fuel just not enought to get the RPMs up just a tought

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JAMES
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Re: Losing RPMs in hot weather

Postby JAMES » Sun Jun 18, 2017 11:17 am

Deano wrote:The whole tank difference thing, sounds like water to me.
You said you changed the fuel filter, what the water separator cartridge?

When and/or where a second tank may be used less, there is a
greater opportunity for complications from condensation. Warmer
daytime temps and wider temp swings can exacerbate that scenario.

If you have already ruled out water as a culprit, then the
indications would allow for the possibility (if not likelihood)
that the number two pickup may not be obstruction free.

If that be the case, I would try blowing some compressed air backwards through
that pickup and see if it subsequently made even a temporary difference.
If so, it would seem that second tank needs a thorough cleaning.

X2
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