bonesmalon

iTrader: (1)

Saw some fluctuation in fuel pressure this fall. Had just acquired the boat and although previous owner was super meticulous in maintenance I still changed out both water seperators and high pressure filters. Also found both motors had some fuel leakage from mechanical fuel pumps into the gear oil on the water pumps (drained and replaced). Performed fuel pressure regulator test by pulling vac hose off regulator and as spec'd, pressures jumped from about 38 psig up to 43. Ran the boat and although didn't see pressure fall off or fluctuate, pressure doesn't boost to 43 psig at WOT (showed 35-38). I understand the fuel flow during the idle/vac hose test is minimal compared to WOT condition but my question is whether the regulator/pump is capable of delivering the 43 psig at WOT right out of the factory. I understand they use the term "43 pound" injectors so again I assume they're spec'd to operate with the 43 at WOT. Starboard mill doesn't seem to pull as hard when you're up at the top end. Love to hear from merc tech or anybody that knows if fuel pressure is supposed to be bumping up at WOT or just for the idle test.

Raylar

The important thing to remember on fuel pressure is that on flow of fuel in a pressurized system there are two variables. One is pressure and one is volume. If pressure falls, volume will drop and if volume is restricted, pressure will fall, so they are both tied to each other. On a pressure regulator that is vacume adjusted, vacume pulling on the diaphram will drop the fuel pressure to limit fuel flow during idle and on decelleration when full fuel is not required or desired and vacume os high. Hence when you pull off the vacume line at idle, the fuel pressure goes to the max of the regulator on this engine which is 43 psi. When the engine is at WOTat top rpms and is trying to pull a high volume of fuel the engine fuel demand would automatically drop the fuel pressure under load to about 38 or so PSI. Also even at those rpms there would be a little manifold vacume also pulling on the regulator and that would also cause a little pressure drop, but if the guage is accurate dropping to 35psi maybe a bit to much and might indicate either the fuel lines or fittings from the tank tot the lift pump might be a bit restrictive or the high pressure pumps may be getting a little tired. If both systems on both engine match on the guages and the engines are not having issues then you might be OK. I would either crosscheck the gauges between both engines and you can put a calibrated fuel pressure guage on the schraeder valves on the fuel rails and measure the fuel pressure in this way as a cross check against the guages.
Hope this info helps and does not confuse to much.

Best Regards,
Ray @ Raylar

bonesmalon

iTrader: (1)

Ray, I think we're saying the same thing. It's the lack of vacuum that raises the fuel pressure. The vacuum pulls against a diaphram which is loaded by a spring. At idle the vacuum in the intake manifold is at its highest and pulls on the diaphragm which unloads the regulator dumping more fuel back to the tank thereby lowering the fuel pressure set point (38). At WOT the vacuum in the intake is at its lowest esentially because there's this big hole open to ambient (i.e. the throttle butterfly wide open). The idle test where the vacuum line is removed from the connetion at the FP regulator is intended to simulate WOT but without the resulting fuel demand/usage. My question is whether in a BN 525, do the pumps have the volume to overcome the fuel demand at WOT and actually deliver 43 psig to the fuel rail/injectors and if so that tells me i have pump delivery issues to investigate. OR is the boost at idle test simply an indicator that the regulator is attempting to overcome the system loses when flow is at it's maximum??

bonesmalon

iTrader: (1)

In the spring I plan to hook into the low pressure side (mechanical pump output) and monitor it's output at WOT since I did see some fuel in the gear oil reservoir. Want to make sure I'm not starving the input to high pressure electric pump. Also in the process of linking the engines into my in dash Garmin GPS via the NEMA2000 network which should take the gage accuracy out of the picture (displays fuel pressure via engine transducers to virtual gages in GPS). I guess I just need to confirm whether I should be at 43 psig in the fuel rail at WOT and if so, and I don't get there that my high pressure pump might as you suggest, be getting tired (290 hrs on motors)

Raylar

I have never personally seen or heard of the stock Mercury Cool fuel system on the stock HP525 efi engines actually deliver 43 psi at WOT at max 5300-5500rpms and I think Mercury Racing set the fuel curves in the ECM and delivery to the injectors based on the 37-38psi range that we see this engine operate at WOT full load at max rpms. When an engine like this is at full loads its going to be drawing larger volumes of fuel than at idle and the system pressure will drop and the engine is tuned for that state or condition.

I would not worry about getting the injector rail fuel pressure to 43psi at WOT at max rpms unless you are having problems or seeing lean burn condtions. If the one engine is down on power at the same measured fuel pressures of the other engine then I would suspect other issues, ie: compression lost, valve spring issues, spark plug or sparg plug wire miss under load and possible fuel injector delivery issues possibly. You could remove the plugs after some runs and look at their states as well as do a compression and leak down test between the engines and see if that show differences. At 300 hours your differences could be other than fuel pressure.

Best Regards,
Ray @ Raylar