DORaymond

I have 2000 502 MPI mercruisers that I just installed new fuel pumps and regulators (cool fuel system). With key on, and engine not started, read 30-33 psi on both engines (agrees with shop manual). With engines running on the hose (no load), the pressure remains about the same. Shop manual states 43 psi when running. I see the regulators are connected with a vaccuum line to the intake plenum - will the regulators adjust when under load, or is the pressure still too low?

Thanks

GPM

43psi, engine running and vacuum line disconnected and plugged,is what I heard. Injectors are sized at 43.5 psi. The vacuum should pull the pressure down 1 pound per inch. Under load vacuum decreases and pressure will increase.

DORaymond

Thanks GPM, I will check with the engine running and the vaccuum line disconected and plugged.

stevesxm

the shop manual is correct and so are your readings ( assuming accurate) . only change i would suggest if it applies to your set up is that stock cool fuel deal has the regulator return line dumping back into the fuel filter and you will get cooler and less aereated fuel if you replumb that line so it dumps istraigt back into the tank instead. unless you are doing other things to the motor , i would suggest not changing fuel pressure... that will just richen it up for the same pulse width and give you a linear offset to the fuel curve.... which... unless you have some genuine reason to BELIEVE you are lean, isn't going to do anything except use more fuel for less power.

more isn't always better.

DORaymond

Stevesxm - thanks for the reply. The only thing that bothers me is why don't I see 43 psi when I first turn on the key with the engine not running? Vaccuum at rest is zero, which should give full fuel pressure, just like under high speed heavy load. Only after the engine starts, and there is vaccuum in the intake should the pressure adjust downward (?).

stevesxm

would ask where you are seeing this 43 psi number... i don't see anything like that in my manual...pg 5c -60 in manual number 16 ( don't know the page for the later one...thats on the boat) regulator is a spring preloaded diaphram... at bet 34 and 38 the spring gives up and bleeds off excess to the return line... the diaphram is such that at high vaccum it actually REDUCES the preload on the spring and REDUCES the fuel pressure.... ( as you say)

the max press your system will see ( as you observe and surmise) is with engine shut off and pump running... and that is going to be in the 33 to 38 range ...give or take and the accuracy of your gage. thats what my manuals say and thats what my two 502's do... (having gone thru a whole fuel press exercise a month or two ago chasing unknown prob). mine are box stock and run great at mid /hi 30's

is this intellectual question/curiosity or is there some gremlin you are chasing ?

DORaymond

Stevesxm,

The SELOC Mercruiser manual I have states that the pressure should be 43 psi for 2000 502 MPI. The new fuel pressure regulators I purchased #861126A state 43 psi in the parts breakdown, such as found at mercruiserparts.com. I changed both regulators AND put in new fuel pumps (#861156A1). Now both pressures with the engines stopped are about 33 psi, maybe pushes 36 psi, if I cylcle the ignition on and off. I thought I had higher pressure before I changed them. (I changed them both because I did have low pressure on the starboard engine, 25psi). I see that there is a 30 psi regulator that looks exactly identical from the outside. Still just trying to determine if all is well.

Thanks.

stevesxm

manuals i have are factory mercruiser. they say 33 -38. my best professional opinion... and thats all it is ... is that you are ok and the aftermarket manual etc may be sending you down the garden path with a wrong figure. simple empirical fact is that my motors are box stock, on a 10,000 lb boat and go to within 100 revs of the rev limiter at 67 mph... so i think they are ok... plugs , when i had them out while chasing that other problem (one bad injector on the same day i got suspicious fuel) looked perfect... i would think that if the fuel pressure was down 6 psi which would be the better part of 14 % the motor would give some significant indication either by not making the power to run up top or getting into detonation/knock sensor trigger... none of which is true in my case... so i say drop the hatch and get a beer and go home early.

and for the record... i do this lunch thing to an island some 15 miles away... and i run the thing flat out all the way there... temps and pressures perfect on arrival.... if it was lean it would eat a plug or slow down as the timing kicked back... no evidence of that at all... so i figure 33-38 is fine...

DORaymond

I guess I will move on to the next project - thanks for your help. Won't know for sure until I get it wet in May. Just hate to wreck anything by being lean, or loosing a mph or two