Low WOT, high fuel consumption

[Touch of Madness]

I have twin MY 2000, 5.0L Volvo Penta engines with duo-props. Ever since the end of last season, I've lost 800 rpm off WOT on both engines. My fuel consumption has also increased from 17gph (at cruise) to almost 25gph and I don't have as much power getting on plane. I pulled the plugs from both engines and they were all wet so I suspected spark issues. I replaced the plugs, plug wires, distributor caps, rotors, fuel filters and ignition coils. My TechMate scan tool said I had a bad map sensor on one engine which I replaced. I took the boat out after all the replacements and cleared diagnostic codes to find that I still have the same problem. Came back to dock, checked diagnostic codes and they weren't any. Boat runs fine...no knocks, sputters, etc. Any other things I should check? What ever it is seems to be affecting both engines.

 

[salty87]

Re: Low WOT, high fuel consumption

what's the condition of the bottom of the hull?

 

[Touch of Madness]

Re: Low WOT, high fuel consumption

Condition existed with fresh bottom paint at begining of the season. BTW - it's a Larson 290 if that helps. Wet plugs on both engines has me baffled.

 

[Don S]

Re: Low WOT, high fuel consumption

What is the FULL model number of your engine, or better yet, the serial number?

 

[Touch of Madness]

Re: Low WOT, high fuel consumption

They are 5.0L GSi engines. Unfortunately, I don't have the serial numbers handy.

 

[Don S]

Re: Low WOT, high fuel consumption

Sorry ToM, but I find no information on a 5.0GSi in year 2000 or 99 for that matter. That is why I was wanting the full model number ( Your 5.0L GSi isn't it) or the serial numer.
Look at the Volvo bulletin and see if it will help you find the model number.

Volvo Engine ID

 

[Touch of Madness]

Re: Low WOT, high fuel consumption

Going through the engine Id:
5.0L - 250 Hp each
GM built - Black in color
Fuel injected
Power steering
Built in 2000
Have no idea about random number

Does this help?

 

[Touch of Madness]

Re: Low WOT, high fuel consumption

Finally have serial numbers:
5.0 GI PEFS
4012013531 and 4012013532

500 hours on each engine.

 

[Don S]

Re: Low WOT, high fuel consumption

I obviously wasn't paying attention to your first post about this happening to BOTH engines.
Can I assume the rpms and power was Ok prior to this season?
I doubt you have an engine problem that would be the exact same thing on 2 different engines. I would be looking for an air problem.
Were the air vents were covered up for winter last year and maybe still covered? Did you insulate the engine compartment to make it quieter and cut down on the amount of air getting to the engines?
I would sure do a test run and leave a hatch open to the engine and see if that cures the problem.
What about the flame arrestors? They covered up or anything?

PS, Just an FYI, you Do Not have a 5.0GSi, you have a 5.0Gi, that is what I was haveing problems with. Also, your engine model (Random Letters) are EF

 

[mmainelli]

Re: Low WOT, high fuel consumption

Since the problem is common to both engines, that are otherwise running properly, I suspect that you're using an ethanol fuel (check your marina) that has about 10% less Btu value than the old MDTE fuel (that had about 5% less Btu value than pure gasoline). You're using more gasoline (advancing the throttle) to get the same rpm. Pure gasoline has about 14,400 Btu/gallon. Ethanol-based gasoline (90/10) has just over 13,000 Btu/gallon. Some larger engines actually need to re-jet the carbs for increased fuel flow, for similar performance, in that they couldn't reach WOT on ethanol. I'm surprised I don't see more "ethanol fuel" problems talked about here, especially from older carb boats.

 

[Touch of Madness]

Re: Low WOT, high fuel consumption

Thanks for the advice Don. I've actually had a problem with Larson's hatch design that bathes my engines with salt water every time I take a wave up the stern. I love how they designed it to dump on the engines rather than AROUND the engines. Maybe I sealed it too good...I'll check.

As far as E10 fuel, not every performance problem can be blamed on it. If ethanol fuel has 5% less btu, it doesn't explain my problem. I'm having a 20% loss in top speed and 50% increase in fuel consumption. E10 is not the problem.

 

[Scaaty]

Re: Low WOT, high fuel consumption

On the Ethonol mix, I just did a 26 day, 6766 mile ride. EVERY pump with Ethanol clearly labeled

 

[Reel Poor]

Re: Low WOT, high fuel consumption

Check the Air Temp and the Coolant Temp sensor connections for corrosion.

Run the boat with the engine hatches open to prove an air flow problem.

 

[Touch of Madness]

Re: Low WOT, high fuel consumption

Is it possible for a temp or coolant sensor to fail without getting an ECM error code? Is it possible for them to "stick" or do they fail completely (all or nothing)? I hate to rely on ECM error codes if I need to be running sensor by sensor range tests.

Question 2: I have a MEFI 3 ECM which lists codes Oxygen sensor rich and Oxygen sensor lean. I can't find an oxygen sensor anywhere in the Volvo shop manual or on the engine itself. How does the engine detect running rich or lean without one?

BTW - thanks for all the info!!!!!

 

[Don S]

Re: Low WOT, high fuel consumption

Is it possible for a temp or coolant sensor to fail without getting an ECM error code? Is it possible for them to "stick" or do they fail completely (all or nothing)? I hate to rely on ECM error codes if I need to be running sensor by sensor range tests.

You can watch the actual temps being sent to the Tech-Mate tool, and yes error codes or alarms will go off if they are out of range. But again, you would need the failure to be exact on 2 engines at the same time.

Also, your Techmate tool is designed for use with many engines, but Volvo doesn't use Oxygen sensors on any of the MEFI 3 engines.