DORaymond

I have a pair of 2000 Mercruiser 502 MPI's. The motors start and run well. WOT RPM on the starboard motor is slower than the port by about 300 RPM (4800 vs. 5100). I have new fuel pressure gauges and senders (Gaffrig, this season), and the slower engine fuel pressure is about 38 psi, while the faster port engine reads about 46 psi. I think book value is 43 psi. I am assuming that the gauges are correct, although I have not checked with a mechanical gauge. Could that difference in fuel pressure cause that much RPM drop? If so, does increasing the fuel pressure above book value increase HP? How far can you push it? The engines have MEFI-3 controls.

mats

Increasing the fuel pressure above book will only make the engine run richer, decreasing performance. If the fuel pressure is 38 instead of 43 it may well result in some lost performance even if 300 rpm sounds a lot. Try swapping the leads to the guages and see if you get the same result. If you do not it is the guages. If you do its either the senders or that you actually have different fuel pressure. A good mechanical guage on the schraeder valve is the only way to really know.

mats

Btw, I am in the process of installing a fuel pressure guage. Where did you install the senders? On the fuel rail somewhere?

DORaymond

Yes, on the back end (distributor end) of the fule rail. Had to use a "street" 90 deg elbow and a nipple to get it to clear the intake. This way, the Schrader valve is still in the front end of the rail to check mechanically.

mats

Thanks/Mats

mats

Hi. One simple thing you could try is to removel the vacuum hoses to the fuel pressure regulators and see if /how that affect performance. It is not much vacuum at WOT but maybe you can squeeze out a few more psi and see what happens.

mats

Another thing on the connection of the fuelpressure sender. There seem to be two holes at the backend of the fuelrail. One is for the fuel line. Did you use the other hole.? Is there a plug in it? It is kind of hard to see.

ls1nova

maybe the screens in the vst tank are getting clogged on the motor will low pressure. I would check that out first.

Frank

mats

One way to see if there are any clogged filters or basically any thing wrong in the fuel delivery system, apart from the fuel pressure regulator, is to pull out the vacumhose and check fuel pressure throughout the rpm range. If there are ANY fluctuations, there is something restricting the proper flow;
Fuel pickup, antisiphone valve, filters, pump

If the fuel pressure regulator, sender or guage is faulty it will be a faulty steady reading/pressure at all rpms..

jimjomack

Put a tee in the front of the fuel rail and install a 2 inch liquid filled guage. This way you can see what the real fuel pressure is. I have the gaffrig guages on the dash and the 2 inch guages on the fuel rails. they are always different. I put remote fuel pressure regulators to adjust he pressure to 43#.