R Addiction

Hook up a fuel pressure gauge and run it. I have replaced the pump on my 99 502 MPI. The symptoms for mine were all was goood until I went over 3800 RPM then the pressure wopuld drop, I'd back off and the pressure would rise up (a bit low) then hammer down and the pressure would drop again. Replaced the pump with a new one and haven't had a problem since. I have a fuel pressure gauge in my dash by the way. And I'm a mechanic so I checked it with a fuel pressure gauge I have at work. Good Luck

Pat McPherson

When I had the leaking problem a couple of years back it was on a 1999 7.4MPI with stock cast iron exhaust.
Toward the end of the season the engine would cough and sputter at about 3500rpm up. It would loose rpm then run OK for a few minutes then loose rpm. I checked the fuel pressure and changed the fuel filter 3 times thinking it was fuel related.
I pulled the plugs and they looked fine. I did a compression test, all good.
After the season was over I pulled the engine to rebuild it with a little more kick.
When I pulled the exhaust off the inner 4 cylinders had rusty exhaust ports. Then I pulled the intake and there was all kinds of gray sludge on the bottom and in the lifter valley. Also the same sludge in the valve covers.
The exhaust riser to exhaust manifold joints had a vary small leaks and water was going into the engine. The harder I pushed the engine, the more water would go in. That is why she seemed to run OK at low rpm and die out at higher rpm.
I'm not saying this is your problem, just something to think about if you do not find a fuel related problem.
Good Luck

mats

R Addiction, I have an electrical fuel pressure guage that I have not installed yet. Where did you hook your sender up? Somewhere back where the fuel line goes into the rail? Did you use some special fittings?

tuffenuff355

its possible your anti siphen valve on your fuel tank isnt working correctly if you have exhausted all other measures try replacing it and see if that works[ very inexpensive too]

mats

OK. I hooked up a mechanical fule pressure guage and a scan tool. The fuelpressure was 43 psi throughout the rpm range. It took 15 minutes of driving before the rpm loss showed up. For a while it was difficult to get the boat on plane. When I hammer the throttle it feels like a loss of power and the rpm goes down according to the tach but somhow the boat slowly comes up on plane. Still the fuel pressure was rock solid, 43 psi. The scan tool showed nothing. No trouble codes, no knock retard , nothing. The plugs look OK. Still not sure if the pressure should be 43 psi or 30. The scan tool says it is MEFI 1 SW in the ECM.
So.... where to go from here? I am completely at loss..

R Addiction

Mats, I was told when I had my problem to check the fuel pick up in the tank. Sounds as if there may be something in the tank getting sucked onto the pickup.

Ted G

If you are not losing fuel pressure then the fuel system is good. I suspect you may have a module going bad in the dist, are you getting tach bounce when or before it starts to screw up??

mats

R Addiction, I actually tried one thing more, and that was to run the engine off a separate plastic gas tank filled up today. Same result.....
Also emptied the water separating fuel filter in a transparant bottle. No water.
So what is left?

mats

The tach bounce follows the loss of power exactly. The whole distributor assembly is new. Using a timing light at idle on the individual plug wires as well as on the high tension lead to the distributor shows very consistent flashes. But I have not been able to check with a timing light at the rpm loss situation.
A funny thing is that when hammering the throttle the tach goes down and the engine feels sluggish, but somehow the speed increases slowly!? The boat does eventually get on plane. It is like I loose a number of cylinders which is reflected in the tach going down but the cylinders that work makes the boat accelerate. Is that possible?

mats

Could it be some intermittent fault in the MAP or TP sensor. That the ECM does not realize that I am hammering and thus it does not provide enough fuel to fire all the cylinders or at least that the combustion becomes insufficient?