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Egine losing power after 3hr trip

J

joefran

New member
Merc 350 Magnum MPI Bravo 3

On a 3.5 hr trip just after three hrs engine starts to lose power like it is staving for fuel. It will run 2000 rpm or less fine. It never stalled. After running at 2000 rpm for a couple mins I could rev back up to full power for a min or so and it would lose power again. This happened on the way out. So changed water separator and pulled regulator on fuel rail. Took out for short test drive and ran fine. On the way home after three hrs same thing happened loss of power no problem getting home at 2000 rpms.

Engine temp steady about 165, Oil pressure 40+ Voltage good.

My though a filter would stay clogged. Could the fuel pump over heat after three hours? Any other filters I could be missing?

First time making a trip that long steady hard to repeat.

Any help
Thanks
Joe
 
Water in fuel will get trapped in the filter/separator... but it only holds so much! Carefully remove filter so as not to spill any fuel. Upend it into a large glass jar. Wait 20 mins. Fuel should be uniformly "clear"... Any water in the fuel will settle to the bottom of the jar. Cloudy fuel indicative of fuel saturated with water. If there isn't any dirt in the jar, you could just refill the filter with some clean gas and reinstall it and run engine again. Did you put any stabalizer in the fuel???? How old is fuel? If new fuel where did you get it from? Some marinas have notoriously crappy and contaminated fuel...especially early season.
 
I replaced filter and the fuel was clean. The second incodent was after the filter change. Could a eletronics modual be over heating after 3 hrs like the ignition modual. Not getting any soft alarms.

Would electronic issue let me have full power almost right away for another 1 or 2 min.


Still seem like fuel sarvation to me.

Looking for more ideas and testy procedures.

Thanks
Joe
 
I'd be inclined to start with checking the pressure in the fuel rail, across the RPM band, under load.....and dip below spec pressure will cause a lean condition.....
 
I am inclinded to think low fuel pressure as well. But why did it only show up after 3 hrs of run time. Any less time frame and it seems to run fine. Do you tink the fuel pump could start to malfunction only after running for three hrs. Now very hard to recreate the issue because of the time frame of 3 hrs run time at 3500rpms. Do you think the fuel pump would show signs of fatigue even atfter a short run time. Any varations of fuel rail pressure should point to bad fuel pump.

Do you think electronics could be a possability. I am thinking no.

I will take it out without adding any fuel currently 1/2 tank. I will also attach fuel pressure gauge to monitor pressure. Do you know the fuel rail pressure spec for this engine. I would think 40 or 50 psi. Should it vary at all under load?

This is the part number for the pump seems to be the same number for the low and high pressure pump?[h=1]861155A3 PUMP[/h]
Thanks
Joe
 
The 'newer' manual I have says 43PSI in the rail.

It would also be insightful to plumb a vacuum gauge into the suction side of the pump, for testing purposes.....many times, you can use the plug on the head of the fuel filter.

The speculation here is crud building up in the screen of the pickup tube. run it long enough and enough of it accumulates on the screen starving the pump. let it sit long enough and the crud settles back to the bottom.....the vacuum gauge will help if verifying the concept.

Pumps usually work or they don't.....electrical connections can be influenced by the local environment....the connectors are usually solid if they haven't been abused...
 
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