I have a 2002 410 Express with...

[dannysale1]

I have a 2002 410 Express with 8.1s motors. My starboard motor having issues when taking at least a 20 mile trip. Take short trips no problem. It will be running fine then it will start loosing rpm. It goes from 2500 to 600 and back and forth. The tack is going crazy but the motor usually will not die. If it would die it will start right back up but still have the same problem. Limp back to my dock , take it out the next day problem is gone on short trips, long trips same issue accrues.  

[wingless]

A good first step is to attach a fuel pressure gauge to the fuel rail.

The Factory Service Manual (FSM) defines a few tests around the fuel pump and regulator, measured in the range of 34-60 PSI.

[gregk]

My 2003 350MAG did something similar, but lower RPM range. It eventually threw a TPS code, which solved it.

[dannysale1]

Wingless, thanks I hope to test pressure tomorrow. gregk, thanks for responding, I thought it would of threw a code but hasn’t .

[Double_trouble1]

It could be your cool fuel module that, there is a service bulletin out on this where the paint delamination on the inside of the cap and clogs regulator and injectors this could be giving you the intermittent problem but if left could lean to a lean fuel mixture and potentially engine piston problems. Currently mercruiser is covering parts ( new injectors & new cover & new orings & gaskets) and customers are required to cover the labor. Currently the covers are on backorder. I just currently bought 2 complete module assembly's because I can't wait. Good luck

[Ronald]

I assume you’ve checked all of your fuel filters. I had similar issues with my 1989 Sundancer 340.

[dannysale1]

Thanks for all the ideas.I installed pressure gauges on both motors. I check the fuel pressure, which ran about 40,on both motors. Then after running pretty hard starboard motor started acting up check pressure gauge while running hard and it dropped down to 10 and back and forth. What I find it is a heat issue. After just a bit with the access hatch open it stopped acting up when watching gauge, ran fine till I closed hatch. I usually never run my blowers so I turned them on and left them on solved issue except for one time, slowed down rpm.
Stopped problem. Don’t know to much about the fuel cool, or is the fuel pump getting week or should I add more blowers and run blowers all the time?

[wingless]

Good info.

Note that gauges are good for diagnostics but are an extra point that could fail so might be good to restore original configuration.

Look at my heat extraction system, on a similar topic in The Stream, drawing heat from above the engine, instead of the required exhaust drawing from the bottom.

Heat rises so the existing system is better than nothing, but not 100% effective, as you determined. Mine is 100% effective.

[dannysale1]

I removed the pressure gauges and back to original. Could I hook up two more blowers to the factory switch ? Should I run factory blowers all the time? Will there be enough intake air coming in when adding two more blowers like you show?

[wingless]

According to the manual the 4" Shurflow draws 5.2A and requires a 7A fuse.

Factor that when modifying existing wiring. My boat has a blower module that monitors the blowers, providing an indicator during a fault and provides appropriate circuit protection for each.

My new blowers are isolated from the OEM blowers.

The big concern is the hull cracking like an egg shell when existing blowers are operating, with the engines also sucking air, plus the new blowers sucking air. (That is my best attempt at humor...)

https://images.boatid.com/shurflo/items/pdf/2774110-2-installation-instructions.pdf?_ga=2.134946866.2129889443.1597677182-43195883.1597677182

There has been zero issue of airflow w/ all four systems operating concurrently on my boat. Mine also has the high-volume, high-pressure air compressor w/ forced fresh air cylinder head cooling.

[wingless]

When operating my boat I enable the automatic thermostatically controlled heat exhaust during and following operation and only use the OEM blowers prior to starting to exhaust (non-existent) fuel vapors, per recommended starting procedures.

My experience is the blowers kick on infrequently during low speed operation and cycle on and off once stopped, on about 50% of the time. I haven't noticed the heat exhaust operating during higher speeds, while operating on plane.

[wingless]

Note that OEM blowers and heat exhaust blowers should be off while refueling.

When refueling my boat, I shut off engines and blowers, close the salon door, windows and hatches, until refueling completed.

The blowers are then turned on while completing the transaction and paying for fuel, prior to starting the engines for departure.

[dannysale1]

Another thought, would it work having fans mounted at top front of motors and blowing air across them instead of adding two more blowers to suck the heat out?

[wingless]

Yes, internal circulation will decrease the existing thermal stratification, hottest on top, cooler lower and in the corners.

It may be that increasing the thermal uniformity within the engine compartment may provide enough benefit, possibly combined with usage of the existing exhaust blowers. An interested party could quantify those effects using thermocouple thermometers.

But, the engine compartment is insulated, retaining heat. The engine block has massive thermal mass, causing a huge spike in temperature, once the cooling water flow has abated.

If that method is explored, then my suggestion would be to have a hose suck from the top and the fan dump somewhere else, like low on the sides, to establish a mixing airflow, for greater uniformity.

[dannysale1]

I want to thank you for your advice and I will let you know the outcome.

[SeaMax]

If it were ambient temperature affecting one engine, one would think it would affect both. I would be looking at the coolant system in the problem engine, perhaps flush and fill with new ELC. You could check the ambient temperature while running with a remote meat thermometer. Move the probe from one side to the other to get a comparison. But as I said, I think its highly unlike that is the problem.

[PeterG1]

It is always good to have a laser heat gun. cheep at Harbor freight. with it you can check all surfaces for uniformity.