Logo
FULL SITE MENU

4.3L Fuel Economy

A

akhinton

New member
I'm getting horrible*fuel economy (1.43 to 1.75 mpg) from my inboard motor.**Here is what I'm operating.* I have a 1997 GM 4.3L. motor pushing a 21' full cabined Bayliner.* The Motor was installed in 2006.* It has a Holly 2 barrel carburetor (Model 2300) which I replaced last year,*the cooling system is enclosed (raw water through a heat exchanger then out the exhaust manifolds). I replaced both exhaust manifolds in 2008.*I'm running*the boat*in the cold--cold waters of Prince William Sound, Alaska.* Water temp is probably close to 42 degrees.* Temp indicator (for the motor) reads 175 degrees when warmed up and doesn't change too much no matter where the throttle is placed.*
*
*Last year I was having a horrible time getting the motor started but after*warming it up, it ran fine.* I changed the carburetor which helped some and I estimated my mileage at approximately 2.2 to 2.5 mpg. *I further had (and still have) a problem with run-on when I try to shut*down the motor from a normal cursing speed.* --like, I have to let*the motor idle for approximately 2 or 3 minutes before I shut it off*to keep it from "dieseling"**Last fall I took the motor to a mechanic who adjusted the carb and further noted to me that the timing was off.* He adjusted the*distributor and carb which made the engine start easier but the run-on at shutdown has persisted.* I further noticed that when I start the motor cold, I get a lot of smoke which*lasts for about 30 seconds.* Once the motor is warmed up, no smoke and it appears to run fine, except for the mileage and run-on at shutdown.* I have not noticed any decrease in oil level over a 107 mile trip and don't believe I'm burning oil.* Rather, I think maybe the carb is adjusted to rich...maybe?
*
*Does anyone know what the mileage for a 4.3L should be?* I'm cursing the boat at approximately*3700 RPM @ 25 to 26 mph, calm seas, with no more than 2' waves for the last couple outings and*I've managed to stay on step while cursing for the fuel estimates.* I can full throttle the motor*at 4600 RPM and get 29 mph on a dead calm day.** ...with the price of gas these days, this mileage thing is killing me.
*
 
I would think you should get about 3 to 4 mpg (5.5 gph pretty typical fuel flow for 175HP V6 on cruise)
29mph seems a bit slow for top end... have you ever weighed the boat? could it be storing water somewhere? wet floatation foam etc?
a dirty hull is a serious issue for top end speed and fuel consumption...bottom growth is a wallet emptier
is the choke opening fully ?
have you ever tried removing the flame arrester while under way to see if you gain rpm's?
or have you tried opening up the engine compartment (wouldn't be the first engine starving for air in these forums)
start with the basics - e.g. a compression test
 
Last edited:
I had a similar problem, if I ran 87 octane gas the engine would "diesel" for awhile even with the timing adjusted spot on. It sounds like the cylinder temps at cruising are high enough to ignite with out a spark and when you idle the engine it allows them to cool enough to prevent it.
 
I don't think there is any water in the boat. I've never seen any foam that would be able to soke up water...not to say their isn't any foam, just that I've never seen it. The boat is always removed from the water so no growth on the bottom either. Vents leading to the engine compartment are clear, I'll try removing--cleaning the spark arrester. Will also check compression.. I was running last years gas. Both tanks are topped off with freash fuel now, maybe that was it... I'll let you know... Last year I was estimating 2.2 to 2.5 mpg. Not that that is great, but it was still better than now...
 
you don't see the foam unless you remove the floor
Every 21' Bayliner has flotation foam. It's part of the build process.

You can sometimes use a hole saw underneath a seat box etc to explore
 
Last edited:
What type of fuel are you using around here all that I can get is an E 10 blended fuel. Even at the marina> E 10 fuels do not deliever the same btu's that pure gasoline does and the fuel consumption goes up.
 
might have to do with the engine model.... when I fuel up on the lake I buy 87 octane E10, when I trailer, I usually buy the Sunoco 94 or some 93 premium unblended
I can't see a measurable difference on my fuel flow meter.
 
Don't know how you guys work out fuel consumption for boats in the US, but here in Austalia we work on litres per hour, or gallons per hour on imperial system. I had a 4.3l V6 that used around 30 litres / hour, and that was considered fairly economical. Petrol here is $1.60 litre at marina, thats $7.20 gallon!!!!! You guys have it good over there in terms of "gas" prices.​
 
Back
Top