502 EFI System Problem
 

2000 Merc. 502 EFI (has the fuel heat- exchanger and regulator return line at the fuel pump, not at the fuel rail.) I custom rigged Aeromotive adjustable regulators when I modified the engines. I couldn't figure out why the port eng. ran fine last year but I frequently had to adjust the fuel pressure while the stbd. eng. never varied from +/-40lbs. This year the eng. is noticeably weaker than the stbd. eng. and sometimes misses although its intermittent and not like a single cylinder isn't firing even when it does miss- its more like a general stumble and the tach bounces irregularly. Then I noticed that the fuel pressure drops back to 0 within 15-20 sec's of shutdown so I cycled the pump with the eng. off and then pinched the hose to the fuel rail off- the pressure held until I released the pinch so its not injectors leaking down. Next I pinched off the return line from the regulator, cycled up pressure and it still leaked down. Is it possible to leak back through the pump?? If so, could this in any way be the source of poor performance even though the pressure remains around 40lbs. when the eng. is running?? This one has me stumped so I'd appreciate any input or ideas!

I'm trying to follow but it's confusing. When the engine is stumbling is the fuel pressure steady? Is the leakback different on one engine than another when cycling the key. If you want to spend time not money swap the fuel pumps and see if the symptom moves to the other engine. I assume plugs, wires, cap and rotor are all fresh. Do you have a scan tool or access to one?

Yes, and yes. The pump basically has to have a one way check valve in it or it will have erratic fuel pressure, sounds like it is your problem.

The other motor holds pressure when you turn the key off- a couple of days later it still has 20+ lbs. Plugs, rotor, button all new. Wires 2 yrs. old and like new. Timing 8 deg @ idle, 36 deg. at 2500+ rpm. I removed and freshened up the heads this spring. When this eng. is running the fuel pressure holds fairly steady- not rock steady but around 40 lbs. at all times. I've got another fuel pump I can try but what's making me think I'm still missing something is that its running so weak even though the fuel pressure looks OK (the gauge is mounted on the fuel rail). Maybe fuel bleeding back through the pump is causing some kind of pressure pulsing that the gauge isn't picking up?

Change the pump.

Fixx
 

if that engine had weak fuel pressure the engine would bang and pop out the throttle body.

Are the fuel pumps tied into any kind of low pressure (oil psi switch) that will kill them in the event of oil psi loss? Loose connection, dirty, etc.. Just throwing it out there? I dont really know didly about EFI stuff.

The fuel pump isn't tied to oil pressure as far as I know- Oil Pressure is OK anyway. As far as banging and popping- I've run very low pressure before at idle while making adjustments etc. and never noticed anything like that. Thanks all for your thoughts- this one has me a little stumped. I'll try changing out the pump but if anyone else has any idea I'm all ears-

Fixx
 

it wont do it at idle,,only when a load is applied.

I very rarely see one hold pressure after the key is off, so the bleed down in pressure doesn't concern me. As long as the pump is holding a relatively steady pressure while running, then there is nothing wrong with the pump or supply.
Eddie