Fuel economy issue.

[DK Diver]

2008 Mercruiser 6.2 377 Mag. #1a314659. Fuel economy dropped from 2.5 MPG to 1.8 MPG, instantly while up top and running 3600 RPM. The power loss felt about the same as trimming the motor back down, and the engine sounded like it loaded up a bit. New fuel filters, new plugs, new plugwires, new cap and rotor since the issue. On the next outting, low fuel economy, but it jumped back up to 2.5GPM for a few miles, then back down. You can hear and feel the difference when it happens. There isn't any popping, sputtering or anything like that, it actually runs decent but a normal 40 gallon trip is a 50+ gallon trip. No alarms. 2 fuel tanks, same result on each tank. WOT for 3 minutes @ 5100 RPM's, no fluctuation. Fuel pressure is 39 PSI @ idle. 165 degrees engine temp on the dash. I have spent some time reading the forums, tried to find as much info as I could.

 

[QBhoy]

So is it only at cruising rpm it happens ? You mention about 3600 rpm, but it’s fine at wot 5100 rpm ?
could be a few things, but I’d check that rotor cap again and make sure it’s not been cracked when put on.

but really could be loads of reasons. Good to hear more info about of it’s just at the mid range rpm it happens and not wot.

 

[DK Diver]

It has only came back to life a few times. Once on a plane and once while throttling up from an idle. It usually planes super quick, but it does not now. However it did "kick in" while planing, and it was noticable. My dive buddy actually looked over and said he felt it get better. At WOT it feels closer to normal than at crusing speed.

 

[wahlejim]

Do you have any trim tabs installed on your boat? If so, what kind? Sounds to me like some of these new "smart" tabs don't act very smart at times.

 

[DK Diver]

Tabs are original, they work fine, as well as the drive trim. I have spent hours looking at connections. The injectors all drop 16psi on a pulse test. The ECT ohms out good from cold to hot. My next step is ordering a scan tool, mainly to confirm acurate gauge readings. If I couldn't feel and hear a change in performance, I would investigate Garmin fuel flow, but I can feel the issue. It is as if the timing retards when it acts up. Once again, it runs OK, but not as crisp as it did when the fuel economy was better. Still no alarms. Knock sensors are plugged in. Oil pressure is good. Temp is good. Could an ignition module cause this?

 

[H20Rat]

Do you have any trim tabs installed on your boat? If so, what kind? Sounds to me like some of these new "smart" tabs don't act very smart at times.

tabs wouldn't cause anything remotely related to what the OP is experiencing. Not to mention, smart tabs have been an absolute godsend on the last 4 boats I've had them on. Smart tabs aren't really 'new' either, they have been around for 20 years... There are some hulls that would be better suited for regular hydraulic tabs, but many [smaller] hulls that perform far better with smart tabs compared to fixed hydraulic tabs.


Anyway, squirrel! I'm not a marine EFI expert, but I've spent a lot of time under the hood of EFI cars. OP, it sounds a lot like a temperature sensor that is failing intermittently. Coolant or air, not sure if marine systems have both of them, but that is where I would start. A failing temp sensor will cause the system to use the lowest cold air or water values in its data table, the result is that fueling goes pig rich into a startup condition essentially. Depending on how the sensor is failing, it could actually fail when it warms up, or it could be intermittent. (hit a wave hard, jars the sensor/wire and it cuts out for a while, making the engine think it is cold)

 

[DK Diver]

I have considered the ECT sensor. I did check the wire to the ECT sensor and ohm the sensor itself, everything passed. The boat launch that I use requires a 10 minute idle to the get up and go zone, therefore engine is good and warm at takeoff. 2 launches ago while getting up top the boat did not feel like it was at it's best, then it instantly gained a little more power. I am pulling the spark plugs today, maybe they will tell me something. Thanks for the replies, Merry Christmas.

 

[Grub54891]

Sensor would make sense to me. Had a vehicle that would act up intermittently, changed the temp sender, no difference. Found out there is another sensor that tells the computer what to do. Changed that and the problem went away.

 

[tank1949]

I have considered the ECT sensor. I did check the wire to the ECT sensor and ohm the sensor itself, everything passed. The boat launch that I use requires a 10 minute idle to the get up and go zone, therefore engine is good and warm at takeoff. 2 launches ago while getting up top the boat did not feel like it was at it's best, then it instantly gained a little more power. I am pulling the spark plugs today, maybe they will tell me something. Thanks for the replies, Merry Christmas.

No disrespect, but have you checked to make sure that your spark is advancing??? I am not familiar with EFI timing device advance in boats, but if the correct wire has home loose or corroded and your ignition can not advance, you will have poor performance. Perhaps knock sensor has corrosion or disconnected and advance is restricted to base timing? Simple test... Put a time light on harmonic balance and watch to see is computer advances while friend raises and lowers RPM. Good luck!

 

[DK Diver]

I have not checked the timing. Good idea. I do not know the amount of advance it should read. I will try to locate that info and give it a look.

 

[DK Diver]

All 8 plugs looked like this. Less than 20 hours on the plugs. Seems rich to me.

 

[Scott06]

Agreed way rich, looks like some of my older carbed engines tgat were dumping fuel in. Have you checked the fuel pressure at wot? I think there is a pressure regulator that runs off manifold vacuum may be increasing fuel pressure too much

 

[QBhoy]

I’d be back at the cap and check it’s not cracked when put on. I’ve been through 5 of these in 5 years. One was bad out the box. Known to be a weak point.

 

[DK Diver]

Agreed way rich, looks like some of my older carbed engines tgat were dumping fuel in. Have you checked the fuel pressure at wot? I think there is a pressure regulator that runs off manifold vacuum may be increasing fuel pressure too much

I have checked the fuel pressure, but not at WOT. I did have 38 PSI @ 3600 RPM. My next trip out I will check the WOT pressure. Waiting for my scan tool to come in, hopefully by the end of the week.

 

[DK Diver]

I’d be back at the cap and check it’s not cracked when put on. I’ve been through 5 of these in 5 years. One was bad out the box. Known to be a weak point.

Although I did have this problem with my 3 month old cap, I did replace it again. No runaway sparks in the dark.

 

[DK Diver]

Finally had the chance to run the boat with a Rinda TechMate Pro scanner. The scanner reads 165 degree engine coolant temp, timing is advancing and no alarms. Installed a fuel pressure gauge and the fuel pressure maintains 40PSI through the RPM's. The scanner showed 5100 RPm's at WOT. Still reading poor fuel economy and the plugs are looking pretty rich. Vacuum gauge shows 15HG at idle and 1HG at WOT. Before the issue, cruising @ 3600 RPM, fuel flow usually reading 2.6 MPG, with adequate fuel and dive gear. Now @ 3600 RPM with less fuel, 1 person, no gear or ice, 2.1 MPG. Could the ECM cause this?

 

[Scott Danforth]

40 psi is low for a 3-bar system

what is the pulse width of the injectors reading?

its not an ECM issue

 

[DK Diver]

I do not have the means to measure the pulse width, I don't think I do. I am not running out of fuel, so 40 PSI is too low?

 

[Scott Danforth]

the fuel system should be 43 psi minimum. a healthy system is about 44-46 psi

while the 3 psi doesnt seam like much its below spec, however the ECM is probably compensating by increasing injector pulse width. also, any water or contaminates in the fuel would have an effect on it.

http://boatinfo.no/lib/mercruiser/manuals/mercruiser31.html#/418

 

[DK Diver]

the fuel system should be 43 psi minimum. a healthy system is about 44-46 psi

while the 3 psi doesnt seam like much its below spec, however the ECM is probably compensating by increasing injector pulse width. also, any water or contaminates in the fuel would have an effect on it.

http://boatinfo.no/lib/mercruiser/manuals/mercruiser31.html#/418

Good info. Looks like I am a little low on pressure. I did not know the ECM has the ability to react to LP. Is there a sensor that detects LP? Should I start with a fuel inlet vacuum test?