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1999 502 mpi low fuel pressure help needed.

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Old 05-16-2005, 08:04 PM
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Default 1999 502 mpi low fuel pressure help needed.

boat had a bad sunday. left me stranded fo a while on the river. i think the problem is low fuel pressure. 32psi at the schrader valve at idle. i changed the fuel filter and same values. is there a regulator on this motor? what steps should i take to try to narrow down the problem? would a new fuel pump fix it? thanks....james
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Old 05-16-2005, 11:15 PM
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Default Re: 1999 502 mpi low fuel pressure help needed.

The regulator is non-adjustable on the merc EFI. There are aftermarket regulators available but no real large gains to find with that unless other mods are taking place. I believe Merc spec is 37psi for fuel pressure at all RPMs. I wouldn't think that at 32 you would be stalling or otherwise noticing a huge difference especially at idle but I could be wrong. Don't go changing the fuel pump quite yet. I'm not familiar with the 1999 set up as it is different than mine. Have you checked the ignition...ie cap/rotor and firing order to make sure that the wires are all back on correctly? You could start the boat at night with no lights and make sure there isn't a light show inside the bilge but I would bet it's not the wires themselves. I have read of some guys putting high quality fuel injector cleaner and filling the fuel filter 1/2 full of that and running it through to try and clean...not sure if it really works but some have mentioned that it does. Are you running new gas or old gas? At this point you might invest in a 1/2 hour (or shop minimum charge) to have the scan tool hooked up and check for trouble codes and or other variables...you can find lots of info about your motor when it's connected to the scan tool
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Old 05-16-2005, 11:43 PM
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Default Re: 1999 502 mpi low fuel pressure help needed.

32 psi of fuel presure is plenty to idle your motor. I kept my idle/off boost pressure around 32-35 on my 2000 mpi sys when it was still a 502. Give us some more info,would it idle but not pull or did it not start at all?
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Old 05-17-2005, 06:23 AM
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Default Re: 1999 502 mpi low fuel pressure help needed.

took the boat out sunday. took several tries to start it at the ramp. it normally fires right up. launched boat. all was good. idled good, good oil pressure and coolent temp was normal. crused along at 3500rpm for a few minutes with no problem. stabbed it wide open and it shut off like i turned the key off. would not start for several minutes. got it started again but as soon as themotor hit 3500-4000 it would die back, not stall.wouldnt come up on plane. cap rotor thermostat were just changed along with oil, filter and fuel filter. another member suggested checking the ECT sensor.
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Old 05-18-2005, 06:19 AM
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Default Re: 1999 502 mpi low fuel pressure help needed.

update on progress..
i called mercruser and picked their brains. they said for my year 502 i need to produce and matain 37psi at schrader port. i can get 32 max at idle and it drops to 30 when i bring the rpm to 2000 on the hose. anotherthing. when i turn the key on, the fuel pump dosnt always cycle. it's strange because when i turn the key off and on, when the pump dosnt cycle, i dont get a system check beep like normal. i also turned off my batteries, counted to ten, turned the batterys back on and then turned on the key and the pump still wouldnt cycle. i located the pump in the cool fuel housing. i can't get the cover off of it. i'm taking the boat to my marina and having them look at it(unless you guy's can talk me into doing it myself).....james
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