I don't want to worry you as fuel pressure fluctuating could be a number of things wrong from low 12v power at the pump, bad fuel pumps, partially restricted fuel system anywhere along the line, or restricted fuel regulator at the fuel module, damaged fuel damper up on the fuel rail or possibly fuel a injector problem...Plus one other issue that always seemed to intrigue me if true or not was this guys issue on his Mecruiser 6.2L engine regarding his fuel pressure bouncing,,
FTomazinis;n3673129 said:
Re: Fuel pressure bouncing????
The reason the fuel pressure was bouncing was because of two burned up intake valves. The compression was leaking back into the intake and making the fuel pressure go up and down.
http://forums.iboats.com/forum/engi...gines-outdrives/322189-fuel-pressure-bouncing
With this Gen 3 cool fuel system, 42 psi is within the specifications as seen below from a Merc service book.
Fuel Pressure Regulator Identification (Revised Aug. 6, 2004) • ECM and PCM 555 Fuel Regulator Identification Engine Color System Regulator Model Code psi (kPa) P/N Comments :
V8 5.0L, 5.7L, 6.2L, ECM 555, (MPI, Gen 3 Cool Fuel, GM MPI intake, GMEFI system). 5.0L MPI, 350 Mag, MX6.2 Models. None = No Color Code for regulator, 42.5 psi (288 kpa) 892681 Regulator in top of Gen 3 Cool Fuel Module.
V8 8.1L, PCM 555, (MPI, Gen 3 Cool Fuel, GM intake, plenum, injectors). V8 496 Mag, 8.1S Models. (None = No Color Code), 42.5 psi (288 kpa) 892681 Regulator in top of Gen 3 Cool Fuel Module.
Gen III Cool Fuel System Overview:
Water Separating Filter Element 2 pumps – Lift & Pressure - Lift pump - Gerotor - Pressure pump – Turbine Fuel Pressure Regulator - New 288 kpa (~42 psi) regulator - O-rings seal between OD on regulator and ID in cover, rather than a face seal and return hose - Regulator dumps excess fuel to inlet side of pressure pump - Fuel pressure vs. flow characteristics differ slightly from Gen II, requires minor calibration changes Internal Low Pressure Relief Valve - Limits pressure across lift pump to 10 psi +/- 5 psi - Dumps fuel to inlet side of filter.
System Module Features: Dual Pump Configuration - Improved Vapor Lock resistance compared to engines equipped with current boost pumps which it is not recommended to install a boost pump in front of/inline of the Gen 3 fuel module.
Side Note: Just noticed that the aftermarket Sierra Marine brand regulator part number 18-33211 seems to say on the side of the regulator that it's rating is 300 kilopascals which = 43.5113213 psi.
http://www.marineengine.com/newparts/part_details.php?pnum=MER892681
Even if peeling paint isn't the issue, at this point you might still want to consider opening up the top cover or at the very least remove the regulator and see if you see anything out of the ordinary just in case.
http://forums.iboats.com/forum/engi...ives/417428-mercruiser-5-0l-mpi-fuel-pressure.
The proper way to test fuel pressure on this designed fuel system is to use both a fuel psi gauge along with a vacuum gauge which is outlined within Merc service bulletin number 2004-06 available with a search online. The majority of the testing procedures are quoted below for you though.
Checking Fuel Pressure:
1. Connect a fuel pressure gauge to the shrader valve on the fuel rail. 2. Cycle key switch 2-3 times (OFF to RUN position) at 3 second intervals to reach maximum pressure. 3. Verify that the pressure is within specification. 4. If pressure exceeds 44 psi (303 kPa): a. Replace the Cool Fuel Module. 5. If pressure is equal to or less than 40 psi (276 kPa): a. Use a tee fitting and connect a vacuum gauge to the fuel inlet side of the Cool Fuel Module. Do not remove the fuel inlet fitting adapter. b. Relieve the fuel pressure in the fuel rail. Refer to section 2A for fuel pressure relief procedure. c. Cycle the key switch 2-3 times (OFF to RUN position) at 3 second intervals to reach maximum pressure. d. Verify that the vacuum from the fuel source is within specification. If the vacuum exceeds 2 in. Hg (7 kPa) , excessive fuel restriction exists. Correct the fuel restriction before proceeding. e. With the vessel secured to the dock and the engine running in neutral, restrict the fuel supply and verify that the Cool Fuel Module has the ability to cause a vacuum reading of 11 in. Hg (37 kPa) or greater. If the vacuum reading is less than 11 in. Hg (37 kPa) with the fuel supply restricted, replace the Cool Fuel Module. In-Water Test 1. With a vacuum gauge and a fuel pressure gauge in place, operate the boat throughout the RPM range and record the pressure and vacuum readings. 2. If the fuel the supply vacuum reading is greater than 2 in. Hg (7 kPa) , find and correct the fuel supply restriction. 3. If the fuel supply vacuum is within specification and the fuel pressure is less than 40 psi (276 kPa), replace the Cool Fuel Module. IMPORTANT: It will be necessary to sea-trial the boat following repairs to be sure that the pressure and the fuel system vacuum remain within specification throughout the RPM range.
Hope you find it to be something easy, please let us know what you find and good luck with it.