Excessive fuel consumption above 4000RPM

[IFish4Tuna]

Hello - first post on this board, but have read through the history of posts over the past year. Have not seen similar problem/recommendations.

The facts real quick:
o 1999 Grady White 306 w/ twin 1999 ox66 250HP
o Slightly over 100 hours on both engines
o Purchased with 72 hours this spring (yes the boat had very little use. It's a short season up in Maine.
o Boat sat for 1 full season unused, and used rarely before that.
o I have put about 30 hours on it w/ out issue
o Single guage Fuel managment guage reports for both engines.
o History of use using ringfree by me & prior owner.
o I have checked, and the 02 Sensor voltage reports normal when plugging & unpluggin #1 hole.
o Thermostats have been checked and are working properly

All season I normally don't drive the boat above 3800-3900 RPMs. I like to keep the fuel consumption to 25 Gallons Per Hour. If I go to 4200 consumption goes to about 30 GPH w/ out a significant speed increase, so I have been staying below that.

8 days ago, I was racing a thunderstorm in from offshore, and so had the need to apply extra throttle slightly above 4200. I first noticed the sound change. Normally when twin engines are synchronized in RPM you can hear it. The engine seemed to lose that proper sound like it was working harder than it needed to. Then I noticed the fuel consumption was going up above 30, like way up to 36, 37, 40. At 4500 RPM it went up to 50 Gallons Per Hour. Unbelievable w/ the price of fuel....

If I applied extra starboard throttle it would noticably bog down. When I pulled it back below and then back up to the RPM of the port engine it would self correct (sound pitch and GPH) sometimes for a short period of time. (seconds). Or it would not self correct at all.

I went out this past weekend after checking sensor and therms, and had a repeat above 3800 RPMS. If I kept it at 3700 it would run at about 24GPH.

I spoke w/ Yamaha Service in Peabody MA who have been helpful in the past w/ a 250 I had on a Grady 263. The advice from the service mgr is this :

Check compression
Check pressure on high pressure tank
Check for blockage in filter on high pressure tank or on injector filters
Check for faulty fuelpump

I know how to, but have not yet checked the fuel pumps by removing and priming bulb. He said I should also look for sheen on water from excessive fuel.

A compression test was run on the engine prior to purchase 30 hours ago. Is another really necessary?

I have checked the plugs. There is no excessive crap on any of them.

I am writing looking for addtion guidance or if you feel that the service manager's advice is sound.

I have ordered the service manual, but it will take a week to get here.

I have read some knowledgable posts here, so look forward to some help. I'm not a great mechanic, but I can follow instructions as long as I have the right tools. I have no idea how to check the high pressure fuel rail though. If someone could identify the item # or part # on the chart (and which chart:Fuel?, Fuel injection nozzle?, fuel injection pump?) that I connect the pressure guage to that would be helpful.

Lastly, while at idle the starboard engine does shake significantly by comparison to port.

Many thanks in advance.

[IFish4Tuna]

Yes I understand they are guzzlers. But the "functions as designed" GPH for 4200 is approximately 28-30 GPH depending upon wind direction/speed. It's the recently more frequent/sporadic jump to 40-50 that's killing me.

Thanks for agreeing w/ recommendations. Some of the local mech's recommendations in the past were good, and some not good. So I wanted to validate. He had led me down some very wrong avenues in the past.

A fuel pressure tester, and compression tester seem like low cost tools worth having.

Can you advise on what I attach the fuel pressure tester to?

[IFish4Tuna]

Picked up the compression tester, and fuel pressure tester today at lunch. I think I now understand what to connect it to.

Something tells me the problem is going to be one of the low pressure fuel pumps is faulty. Dumping too much fuel in. It's probably just beginning to show signs of problem, and that at high rate of flow it's overloaded. A clogged filter would seem to represent not enough fuel getting in. It's a damn science for sure. But at these gas prices it's one that's gotta get resolution.

[Ken]

Tuna,
I'm not familiar with your model outboards, but I'm gonna' put in my 2 cents worth anyway.
I think you first need to find out why the starboard motor is bogging down - I would suspect fuel pump problems.
Secondly, are you sure the fuel management gauge is accurate at the higher fuel flow levels?
Have you confirmed fuel consumption at fill-up?
Thirdly, what is your max rpm at WOT? Maybe going to a higher pitch prop would result in better fuel economy, at the expense of the "hole shot"?
Fourthly, if indeed you are "throwing in" that much more fuel and not seeing any results, then I would suspect spark advance mechanism.
From what JB says he is getting with his 200's, you are doing pretty good below 3900 rpm - I would not suspect anything wrong with compression.
Hope this helps with a little different perspective - I'm not familiar with the "big-blocks" - and I am glad I don't have to feed one [img]tongue.gif[/img] .
Good luck [img]smile.gif[/img] ,
Ken K

[IFish4Tuna]

Thanks for the advice. I welcome it. I'll respond to you questions and throw a couple more at you or whom ever can answer it.

- Agreed the bog down is my priority
- Regarding consumption, yes I am certain it's using more than it should when this symptom occurs. I can tell when I gas it up. But I can't be certain that it's the bogging engine that's consuming the fuel. It could be that the other engine is carrying more load and hence pulling more fuel to maintain speed. I know the FMS says it's burning more fuel. Dunno how the FMS calcualtes its numbers for GPH.
- WOT - well I was trying to keep my issues seperate, that is one at a time diagnosis, but my port engine only gets to 5000 at WOT where the Starboard (the one that sometimes bogs) will go as high as 6000. It will not get to 6000 when bog occurs. It only goes as high as 43-4500 when bog occurs before I need to back it off significantly.
- The port WOT RPM has me less worried or rather was a post for a different day, because the engine sounds fine and performs fine. No shaking/bogging.

- I am an engineer (software) so I do appreciate the details of how things work, and would rather avoid the swap this swap that mode of troublshooting, but I do understand it's value/necessity. I'm trying to think of the logical items that would cause fuel consumption/bogdown to be higher with a high rpm situation. I don't understand the mechanics and relationship between the high pressure fuel pump and the low pressure diaphram pumps. If anyone cares to elaborate further then I'm sure I'm not the only one that can benefit.

Sorry for all the writing and no data points yet. I'm only able to do the troubleshooting on the weekend.

[IFish4Tuna]

2 words. O2 Sensor

Not faulty...not dirty...LOOSE!!!

For those of you who haven't yet dove in to the mechanics of your engine, the O2 sensor screws in like a sparkplug. Mine had approximately 1/10 of a turn of play in it. That's why when I tested it (using the top butterfly hole mehtod) the voltage read as normal.

I did yank the sensor, and clean it and changed the plugs. I also did the seafoam engine treatment for carbon build up. I did the 1 hour method, and the overnight method. BTW, the seafoam website has the how to listed right on their website. I did put it right into the filters. Good idea.

So I now have box of new tools, and a service manual for the next adventure. I'm sure there will be one. It's a boat.

Also did the other engine while I was at it.

So for comparison purposes should anyone have any interest

At idle (both engines)
700 RPMS
3.6 GPH

Flat seas:
3700 RPM
27 MPH
22-24 GPH
1.3 MPG

2-3 ft chop
3800 RPMS
22-24 MPH
22-24 GPH
.09-1.0 MPG

1-2 ft chop
4900 RPMS
38 GPH
1.1 MPG

[Ken]

Tuna,
Thanks for the feedback - hard to believe that O2 sensor being loose 1/10 turn would make that much difference.
Was the sensor loose on one motor only?
Can you get the port engine up to 6000 now?
Is starboard showing any signs of bogging down?
Thanks [img]smile.gif[/img] ,
Ken K

[IFish4Tuna]

Hello Ken, I did all 3 steps at once (carbon flush, o2 cleaning, plugs) but I'm pretty sure it was the o2 sensor. The inside of the cover was oil filled. So I had to replace the reflective padding also. The other sensor was clean inside and tight.

No bogging.

I just got my service manual via ups yesterday and was reading it over a bowl of frosted mini wheats [img]smile.gif[/img] , so next step will be to take a look at the throttle cable. The throttle cable seems to have a rather precise set of instructions for validating its set properly (or adjusting it) The throttle valves will need to be inspected also. I have looked at the throttle valve positions in comparison to the other engine. They do look like there is a tiny gap where the others don't have it. More research...