Testing fuel pressure 350 mpi ?
 

I performed a no load test on my fuel rail. Pressure maxed at 20 psi and as soon as the pump quit running pressure fell to 5 psi. I have been chasing a lack of rpm on my port motor and have gone thru all ignition components now checking fuel psi. My adapter started leaking once I connected it to other good running motor so I have nothing to compare to. Merc manual only states 30 psi for this engine but does not indicate if pressure should drop once fuel pump turns off. Any pointers?

Thanks, LC

will testing under no load tell me anything?

If these are like the 6.2s and you have the cool fuel set up, rectangular box that has both pumps in it that sounds low. I believe that injector needs 44 psi to cycle. You may have two pumps if they are like the 6.2s a high pressure and a low pressure,, if so there is a regulator that has a vacuum line connected to it, if I remember correctly there s also a screen under the regulator that gets clogged. There was somebody on here that had a pick up in the tank break on a 292, but not sure if that would cause low pressure would think it would cause no pressure.Load shopuld not matter much as long as you can run the RPMs up

Let me know if you have the pumps that I described, bad news is I don't think you can get them (pumps) anymore and you have to retro to the new style.

i have cool fuel but its mounted in a black box on the lower port side of the motor. not down by the water pump.

LC

just one pump.

Maybe someone will chime in that knows for sure, but like I said pretty sure the injectors need more than 20psi to cycle. You could always switch the fuel pump boxes side to side and see if problem follows, just don't take them apart the rubber that the pumps sit in will be swollen from the ethanol in the fuel, you may nor be able to get it back together without new seals.

You have a returnless fuel system, so it should keep fuel pressure up for a little while it bleeds back through the fuel cooler system.The pressure regulator is sitting atop the round copper cooler and the excess fuel returns to the water separator via a small s/s screen under the pressure regulator.The high pressure pump is in the rectangular black box and the low pressure [ sucking] pump is before the water separator.You should have between 38-42 psi at the Schrader valve at idle.Either the pressure pump is weak or doesn't get enough fuel from the low pressure pump.Check the water separator for water contamination [ just empty it for now and reinstall it] and maybe check the screen for debris [ mesh] on the inlet side of the high pressure pump.

SO,

There is no low pressure pump. Fuel tank to fuel water sep, then on to pump intake, fuel cooler, then either to fuel rail or return from regulator to fuel water sep.

I am gonna check at idle today, yesterday was no load/no run just the 2 second period of run time allowed by the ecm.

Needle in fuel psi gauge is bouncing. Like pump is getting air. Turn off engine gauge goes to zero. Otger motor is rock solid at 28/30 psi and holds 21 when off. I think my problem is on suction side of pump..

I'd get a low pressure fuel pump.If the Bosch unit is too expensive for you, get a cheaper Carter electric [6-7 lbs ] from any auto accessory shop to find out if it fixes the propblem.i used Carter electric pumps as a primer for all my engines with mechanical diaphram type fuel pumps.It was setup as a bypass with switch on the dash. Never had to pump the throttle to start them and it served as a backup, in case the diaghram ruptures.I rather have redundancy than get stuck in the middle of nowhere.

You have 2 pumps. One near the water separator (low pressure) and one inside the cool fuel box (high pressure). Pressure testing is done at the schader valve up top and should be 38-42 psi.

Low pressure pump is under the alternator on this 6.2 .It would be odd if the 350 MPI doesn't have one
http://www3.telus.net/spectrasonly/M...06,2%20L%20MPI

Sn 0m068018. No low psi pump. Service manual 24. 30psi on the rail per merc. Pump will only do 10 now. Relacing pump. Will report back.

Pump and regulator replaced. Regulator was the primary issue. Almost all fuel was returning to the fuel water sep.. I believe this caused the fuel pump to start making noise and eventually lost the ability to keep up with engine demand.

30psi on the rail now, and boat is back running at max performance.

Good to hear. Thanks for letting us know the outcome.

mercury has s/s filter kit to go infront of these pumps, seems to be add-on. make sure you have that to catch trash from getting to pump. the cool fuel pump mounts to copper tube, should come with thermal conductive paste, the bat wing carries heat from pump to copper tube, that pump was designed to be in fuel tank, will overheat if not done.

Ah ha that's what the thermo grease is for. Thanks. I knew there was a reason.