2001 OX66 poor fuel mileage

[kingfish]

We have concluded by repeated con*****ed testing (have not yet installed fuel flow meter but have purchased 1), that our 2001 225 HP OX66 on the back of our Parker 2520 SCMV is only getting a shade over 1 MPG in overall usage. We have previously gotten the 2 to 2-1/2 MPG one would expect. Motor seems to run OK with exception that on occasion (and this has happened since new), when running at high planing speed motor will simply stop, as if someone pulled the plug out of the wall socket. Starts back up and runs OK until next time.

Boat and motor are in remote location in Canada with nearest Yamaha dealer new to Yamaha and not very good with diagnosing problems. I am going up there in about a week and would be mighty appreciative for any leads I might follow to track the poor mileage down. Gas prices being what they are, this is a pretty important problem to fix.

Thanks in advance-

[jb123]

For the poor burn:

look at the exhaust, if it is a bad low pressure fuel pump then you should see unburned gas out of exhaust...or it could also be a faulty/dirty O2 sensor.


http://www.boatsetup.com/O2sensor.pdf

when the motor shuts down...it stalls? zero? then fires back up immediately?
does it do this with any consistency?
you may want to use an auxillary tank, ball and hose hooked to motor filter to isolate a fuel delivery issue.

[Captain Bill]

I have a 250 it has 3 low pressure fuel pumps, one of these had a hole in the plastic diaphragm, caused the same problem it is easy to replace and not costly

[kingfish]

Thank you jb123-

Will observe exhaust for unburned fuel.

Have purchased gasket kit for removal and inspection/cleaning of O2 sensor and have downloaded instructions to augment manual already in possession.

The sudden stops occur randomly, or so it seems, except they are always at high planing speeds. When the motor was newer, some time would be required (for cooling?) before we could restart. Lately the motor will restart quite quickly. I can't over-emphasize the suddenness of the stop when it does occur.

[kingfish]

Thanks Captain Bill-

Did the low pressure fuel pump diaphragm cure poor burn or sudden stops?

[Captain Bill]

It did, I had cleaned the o2 sensor before putting in a new fuel pump. After cleaning the 02 sensor the motor ran fine until about 3800 rpm then would like shut off and had to be restarted it was a sudden shut off. It now works fine

[kingfish]

Thanks again Captain Bill-

I understand from jb123's comments that unburned fuel will be diagnostic for a bad diaphragm, but if unburned fuel present, how do I tell which diaphragm is bad? Will visual inspection find the bad diaphragm easy to see?

I have currently ordered gaskets for all 3 pumps, but only the 2 diaphragms for 1 pump, thinking it unlikely that more than 1 is bad, if any. Would love to hear that a bad diaphragm will be easy to spot.

[Captain Bill]

Reading other post it seems that these gaskets should be replaced every 3 years I did not find this anyplace in the service manual but as it’s easy to do I plan on doing it this winter. Will see what the rest of these gaskets look like and go from there.
There is also one gasket where the fuel pump attaches that is not supposed to be reused.
As for finding the bad one you could undo the out fuel hose hook up and hook a small hose to it blow in if it takes air replace the gasket. Good luck to you.

[jb123]

unhook hoses, blow down thru pumps...if ANY air passes, the diaphram is compromised. it is a one way valve and if it works the OTHER way, its bad.

[bljbear]

For what it is worth, I have an OX66 250 on a Parker 2310 walkaround with a flow meter. The best I can do is 1.5 - 1.7 mpg at around 3800 and best trim. Others with similar rigs tell me that that is normal.

[kingfish]

*RESULTS*
I'm back from the trip up north and here is what we've got:

I installed a new fuel flow meter and best I could get under any conditions was 1.8 mpg. Then observed the under-cowling fuel filter clamp had never been secured last time filter was changed, and filter bowl was loose. Probable cause of high speed shut-downs. Changed element, tightened bowl and bolted clamp down. Changed cannister water-fuel separator. Serviced O2 sensor (not nearly as complicated a project as I had anticipated). Changed plugs. New fuel readings showed above 2.0 mpg from 3500 to 4500 rpm and 2.5 mpg at 4000 rpm under optimum conditions. So at this time I am concluding the fuel pump diaphragms are OK, but I'm ready!!

Thanks for the advice.

And bljbear, wondering if your Parker is a deep Vee (ours is modified Vee); those are expensive fuel mileages.