96' 502 mpi, low fuel pressure
 

Hey guys,

My friends 96 502 mpi mag hasn't been running right. When we get the boat up on plane, and running3500rpm, the boat starts having a misfire which sounds like the ecu retarding the timing. We put a fuel pressure gauge on the boat. At idle it had 36 psi, which is what it should have. As we accelerate up on plane, the fuel pressure never rises above that number which is causing a lean condition. Where should we go from here to see if it is the regulator or the electric fuel pump? It has the vst system and from doing searches here on the forum that we need merc Manuel 16 but we don't have one. Thanks for the help!!

More info: when the boat was put into gear the fuel pressure dropped to 31psi, and never rose at all as we brought the boat up on plane, and up to the 4000rpm mark. Also, we have already been into the vst tank and checked the screen which was clean.

How do you know its going lean?

Is it a subtle misfire? It could be an injector(s) starting to go bad. Pulling, cleaning and flow testing them would alleviate any of these potential problems.

We know it's a lean condition because it didn't build any fuel pressure as we accelerated the throttle. I don't know what the pressure should be because I don't have a Manuel, but I do know it should build fuel pressure.

Without an AFR reading I would be hesitant to say it was lean based upon fuel pressure alone. Could it be? Yes, but we need more information.

Does the fuel pressure go back up to 36 out of gear at idle? I'm pretty sure it should have 36 psi and it will change when putting into gear because the regulator is controlled with vacuum. but it should also increase as when vacuum changes while cruising. I also think the fuel pressure is read with the vacuum disconnected from the regulator. The regulator is mounted right on the front of the fuel rail under the plenum front side of motor. What does the psi change to if you increase rpm at idle? You can check the regulator with a vacuum pump (ie, brake bleed vacuum hand pump). Although it's 30 years old and probably most of the fuel system needs replaced if it never has been. I would probably start with the regulator, but those mechanical lift pumps have had lots of issues with the influence of ethanol and could be bad, starving the VST tank.

At idle it was 36 psi but we never revved the motor in neutral. As soon as we put the boat into gear to leave the dock the pressure dropped to 31 psi and that's where it stayed. We ran the boat under load up on plane and all the way to 4000rpm and the pressure never fluctuated. I understand how to do the vacuum test on the regulator but we did not have a hand pump. The boat is at a marina for repairs, but because yesterday was a holiday, they were closed and we knew they hadn't tested fuel pressure yet so we decided to put the boat in the water and at least do that for ourselves. If the regulator tests ok, would the electric fuel pump be next to test? Also you mentioned the mechanical fuel pump, is there a way to test this? I know they usually run 6-8 psi, and it is a diaphragm that could be leaking or sucking air, but I would have thought it would have crapped out by now completely. Any suggestions? Not that I'm a mechanic, but I'm starting to doubt the marinas capabilities.

29 FEVER -

The fuel psi regulator is hooked to intake vacuum.

Therefore, as you give the boat throttle (in gear) and speed up, vacuum diminishes and fuel psi goes up.

When you pull off throttle, vacuum goes up and fuel psi goes down.

So...if fuel psi does not go up when throttling to go faster, you either have a vacuum line unhooked from fuel psi reg or where it's line attaches to the intake manifold or a slit/cracked line. I've seen several 502MPI's with the vacuum line fallen off cause of vac line rubber softening up/deforming just enough it doesn't stay on.

Or, you have a faulty regulator.

Easy enough to check - especially since you have a guage . So.... Go at it.

+1, and if I remember right the vacuum line is connected to the bottom of the plenum and you almost need to lift the plenum to get to the hose to check it. I would plan on replacing it at that point. Not an easy way to test the mechanical pump, but if it was bad it wouldn't cause a drop in fuel pressure when you slip it into gear, so I doubt that's the problem.

YOU need to post the engine serial # that will determine where the fuel pressure regulator is.

GEN II cool fuel came out mid year with a engine serial number break. The fuel pressure regulator is in the gen II cool fuel sitting on top of the fuel cooler with a special O ring seal and screen under it in the tiny hole of the fuel cooler. Again engine serial # will determine what you have and where. All marine stock engine apps have one fuel pressure regulator.