Twin Merc 250 fuel burn difference's

[alldodge]

This is my Crapperal twin 250's and the FMS was replaced on starboard because it was burning more fuel than the port. For a couple months all was good, both tracked fairly even. Then the port started burning more fuel than the starboard, not a lot mostly 0.2 GPH on average above 2K to 4K RPM. Above 4K it starts going the other direction, port uses less and starboard uses more.

S/N 3B384246

@muc do you see any issues?

Made a Vid

 

[muc]

This is my Crapperal twin 250's and the FMS was replaced on starboard because it was burning more fuel than the port. For a couple months all was good, both tracked fairly even. Then the port started burning more fuel than the starboard, not a lot mostly 0.2 GPH on average above 2K to 4K RPM. Above 4K it starts going the other direction, port uses less and starboard uses more.

S/N 3B384246

@muc do you see any issues?

Made a Vid

Hi, been away.
I can't really tell much from the vid. A scan tool might say more.
It's been a decade since I was last in the boat business, but back then Merc wouldn't do anything for differences below 5% and it was like pulling teeth to get them to do anything below 10%. I don't know if that's changed.
My experience with twin I/Os was that what you're seeing is very common.
The 100-150 rpm hunting/surging you mentioned in the PM might be cause for inspection. It depends on what the scan tool say's. Is it TPS, MAP, injector on time or ignition timing that is getting changed by the ECM?

 

[alldodge]

Great to see you back again

The reason for the post is this is pretty close to what I was seeing with the starboard motor. The starboard was showing a slight bit more fuel burn and when the Tech was fixing another issue he found the FMS had a problem.

I'm not turning the helm much and note it has 8 turns lock to lock. Just slight adjustments from other boats ahead.

The dealer looked at the fuel burn file (what ever that is) and didn't see anything that advised for any changes. I can connect my scanner but that is just one motor at a time so I don't have a way to compare.

Out of warranty in March so looks like if there is an issue it will be on me when it's found

Thanks for the response, and good to know we might see ya now and then

 

[muc]

This can be a tough one to get a dealer to spend the needed time on. This needs to be diagnosed while out on the water under load. Need to hook up 2 matched fuel pressure gauges and watch them. Need a tech who has the knowledge and be allowed to spend the time to interpret the G3.

Something to remember about fuel flow readings ---- this is a calculation the ECM does based on RPMs and injector on time only. So if there is a fuel pressure issue or a mechanical issue, those numbers will warn you. But a scan tool sometimes won't help.

I once had a new boat with 496's and Bravo 2's that wouldn't run up right in the lake, spent a lot of time with fuel pressure and the scan tool before I hauled it out and saw the factory had forgot to install the thrust washer in front of the port prop. Missed it on my prelaunch inspection and didn't find it until haulout and visual inspection. Couldn't miss the now trashed prop and lower case! Kind of thought the higher ups would be mad, but they let it slide or didn't put 2 and 2 together and know enough to say something?