Mercruiser 496 Mag Ho low RPM

[Radiowave]

My port motor is running about 900 rpms less than the starboard. Impellers and plugs have b been replaced. Top speed is only 15 mph when both motors throttled up. However, one time for about two minutes while I was returning to my slip, the port side rpm's rose to normal (I did not touch the throttle) and ran at 23 mph. Then as suddenly as it powered up the power on the port motor dropped and I was off plane and plowing. Someone mentioned that Merc had issues with paint clogging the fuel injectors from powder coating fuel modules. The motors are from 2009. Does the problem sound like a fuel module replacement issue? Has anyone experienced the same thing?

 

[Espos4]

It could very well be a paint peeling issue with the cool fuel 3 modules.
A fuel pressure test may give a better indication as to whether or not CF3 could be the issue.

Give us some maintenance history and you’ll get some better feedback.

How long have you owned the boat and what was you highest top speed at WOT?
I would think twin 496’s in a 330DA would be well over 40 mph.
If you only hit 23 mph WOT, you may have more than one issue causing lack of power.

Maybe someone with a similar boat will chime in with some info on what you should be seeing performance wise.

 

[Radiowave]

It could very well be a paint peeling issue with the cool fuel 3 modules.
A fuel pressure test may give a better indication as to whether or not CF3 could be the issue.

Give us some maintenance history and you’ll get some better feedback.

How long have you owned the boat and what was you highest top speed at WOT?
I would think twin 496’s in a 330DA would be well over 40 mph.
If you only hit 23 mph WOT, you may have more than one issue causing lack of power.

Maybe someone with a similar boat will chime in with some info on what you should be seeing performance wise.

Thanks for the feedback. I wasn’t very clear regarding the 23 mph scenario. I was not at WOT. I wasn’t sure if I would damage something trying to push the starboard motor hard while the port was under performing so I limped back at 3000 rpm on the starboard and 2100 on the port when all of a sudden the port rose to 3000 (I was using one stick) and I got out of the hole and made it to 23 mph before the port’s rpm’s dropped back to 2100. The gas filters and oil were changed last year (I did not used the boat much in 2020). Impellers and plugs are recently changed. I’ve had the boat since 2013. I was able to get it to 42 mph at WOT. I like to cruise at 27 ish. I have just about 400 hours logged.

 

[paulswagelock]

Had a very similar issue years ago with a single motor application. New fuel injectors were the fix.

 

[mnm99]

I bet its as said before, Injectors or check your cool fuel. Did you change the inline filters also?

 

[Jeremygavin]

Is the gas from last season? You said you didn’t use it Much last season, could the fuel be still be from 2 seasons ago? Was it treated for the winter? Even it is was, fuel does not have a very long shelf life these days. If you don’t have a lot of fuel on board then I would think about pumping it out and refilling with fresh fuel and some Techron Marine formula fuel treatment.

https://www.amazon.com/dp/B07K95SQR5/ref=cm_sw_r_cp_awdb_imm_9V5FW6SM8KFG071132QC

 

[Radiowave]

Is the gas from last season? You said you didn’t use it Much last season, could the fuel be still be from 2 seasons ago? Was it treated for the winter? Even it is was, fuel does not have a very long shelf life these days. If you don’t have a lot of fuel on board then I would think about pumping it out and refilling with fresh fuel and some Techron Marine formula fuel treatment.

https://www.amazon.com/dp/B07K95SQR5/ref=cm_sw_r_cp_awdb_imm_9V5FW6SM8KFG071132QC

I do treat the fuel but I agree with you.

 

[Radiowave]

I bet its as said before, Injectors or check your cool fuel. Did you change the inline filters also?

I did not change the in line filters. Thanks