picca00

ok heres what i have 96 454mpi (375hrs)that ran fine last year. first day out this year when i started it it was surging up and down alot and ran very rough. had a missfire that would come and go at all rpms...tank was drained at end of season so gas was brand new as well as tank being one yr old as well as fuel lines. new fuel filter as well. i looked at the plugs wires and cap/rotor.. there was some buildup in the cap but the plugs looked pretty good. i sanded down cap and it ran a little better but still going crazy surging. i ran it on the ddt and got code 23 iat sensor. i cleared the code ran again and it didnt show back up and ran better at idle. before i got to run it on the ddt i took it out a few times and i have been down on power and rpms. last year i would get 4800rpms no problem and this year only 3800-4000 rpms and down on speed obviously. i did a compression check and had 135-140 all around. i put it in service mode and pulled some plug wires to see if i had any dead cylinders due to bad injectors or something and maybe had one.. ddt said injectors firing at 2.3 something?
so im thinking i have low fuel pressure(didnt check yet) or bad injector or 2. the reason im thinking injector is because the miss fire comes and goes...and the screens were also removed mid season last year but ran fine till i put it away.. also i removed the vst tank and mechanical fuel pump. replaced with low pressure pump and cool fuel system. i ran the return line from the fuel rail back to tank so i know its no vapor lock but now i have 2 fuel pressure regulator one on fuel rail and one on cool fuel assembly. i have the vacuum lines going from the regulators meeting at a tee fitting and the to the throttle body nipple...let me know where im going wrong if anywhere, i couldnt get alot of info when i did this fuel system but it ran great for 125hrs.
which regulator actually controls the fuel pressure now? and how can i check individually?
what do you guys think is causing me to be down on power and 800 rpms? am i on the right track?
thanks in advance...jay

picca00

open to ideas and suggestions....anybody....

tunertech

take one of the regulators out of the circuit. Use the one on the CF pump as your regulator leave the other open to atmosphere pressure, check your idle FP with the vacuum reference line removed, pressure should be about 40lbs. while idling replace the vac reference line, pressure should drop about 4-5 lbs. Under load you should see the pressure move up to 42-45 lbs. pressure should stay steady. and not vary more than 2 lbs. check your FP at key on engine off..... the prime pressure should go to 45 lbs and should hold pressure for about 15-20 minutes before bleed off. the resulting pressure will more than likely drop to about 30 lbs.

mookymonkey

going thru this as we speak. thinking my electric fuel pump went bad. replacing it tommorow. its an aeromotive that acted up once in the past. uder 100 hours on unit. trying the marine one tommorow. will let you know.. mark

pantera232

what does mpi stand for some type of fuel injection...

picca00

ok got the fuel pressure gauge hooked up, first test was key on pressure went up to 34 psi but started to drop as soon as pump shut off and kept falling to below 20psi in under a min..... at idle the pressure is 32psi and really didnt move alot, i put in gear tied to dock maybe 1100 rpms and pressure stayed the same.....
i dont have a shut off valve for the return line to do anymore testing but i was thinking maybe a leaking injector since the pressure dropped right away? can i remove all my spark plugs dry them off reinstall and then prime the fuel rail a few time and then see if any plugs are wet? if they are that means bad injector right?
the cool fuel pump and regulator were brand new last year and filter under regulator was spotless so im just looking for some advice on how to troubleshoot next to find this problem. also if it was a leaky injector would that cause surging as well?
thanks jay

picca00

want to know if i can test my injectors by removing the top of the intake, and then remove the fuel rail with the injectors still in it. by turning the key to on it will make fuel pump run and prime the rail and show any leaky injector? or will the pressure push the injectors out of the rail dumping fuel everywhere? what do you guys think..anybody ever do this?

mookymonkey

put a longer hose on your fuel gauge and run the boat through the paces. fuel pressure should be a constant 40 lbs on a 502 mpi. mine actually jumped 2 lbs or so when we stabbed the throttle then it went back down to 40.

picca00

i want to make sure my injectors are good first and then do more tests with fuel pressure gauge. just not sure what the best way to test them is. also where do i get a shut off valve for the return fuel line and where do you put it inline?

FIXX

iTrader: (2)

ohme out the injectors with a dvom,i think they should be arounf 12.9-13.2 ohmes. check the fuel filter and make sure its tight and check all the lines, because theirs fuel sucked through the filter and lines they wont leak fuel if any are loose..had one kick my A$$ last year and the tech working on the boat didnt tighten the fuel line after he replaced the impeller..