454 mag mpi running rich and using a lot of fuel

[Fun Times]

Wow now we are talking good info!!! Its a 1996 454 MAG MPI serial 0F764338.
standing behind the boat smells like raw fuel, it uses a gallon of gas within 20 minutes or less just at idle. What can cause this. Fuel pressure is at like 30 psi and no fuel leaking anywhere.

You in my opinion probably have an internal VST tank issue (Vapor Separator Tank) as what you described was sort of a common issue with these models and if not then you'd probably have a MAP sensor issues or a fuel regulator problem as those are "some of" the biggest fuel flow contributors for your fuel system for an over rich condition as you described... Which if you do have a VST issue, the scan tool won't necessarily tell you pinpoint-ally the issue but more by way of you really knowing what the normal injector flow rates usually flows at in the live data for your engine model under normal conditions plus a few other data readings too in certain combinations....< And bear in mind that the same condition could accrue with the MAP sensor depending on the range of the MAP senor is putting out to the ECM vs actual problem.

The low fuel psi may be because the excess fuel is escaping out the engine/VST too fast to show the correct psi at the fuel rail.

The most probable causes would be the diaphragm is bad inside the VST, debris inside the VST causing fuel float type issues inside the VST such as fuel soaked float, sticky float valve kit which is basically the needle and seat kit like found in carburetors getting stuck or possibly the fuel regulator is bad...Again sensor wise ought to be the MAP being that much fuel going through the system.

Check the clear or yellow tygon type tubing on top of the VST fuel vent system for fuel inside the tubing...There shouldn't be any fuel inside... Item number 26,http://www.mercruiserparts.com/bam/s...31796/1614/150

Also check the two rear engine cylinders # 8 first and # 7 if needed for excessive raw gas as again those are signs of a failed diaphragm or needle and seat in the VST.

http://ww2.iboats.com/502-EFI-GEN-V-...iew_id.1693335 ... https://www.perfprotech.com/mercury-...g/umodel/10489

 

[cableguy1979]

I just rebuilt the VST with new fuel pump seals and even diaphragm but I guess I could have gotten something messed up but there is no fuel in the tygon fuel hose. As far a fuel pressure I just remember it was over 30 something. I will have to look to see if it's closer to 34 or 30. Considering I just pulled half the engine apart I'll need to check my sensor connections because when I put it all back together I had a hard time getting the vst to run. The fix was to unplug and plug it back up so maybe the sensors can benefit from the same treatment.

Here's the thing. I dont trust other mechanics with my boats unless they are reputable but those are $150 a hour and I only know one as they are authrized mercruiser dealer and mechanics. The last mechanic I took mine to be was revving the engine with the outdrive in trailer position. It broke off the spanner nut flanges and I figured that will be the last time.

I do have someone nearby that may let me look over their shoulder IF he has the right scanner. I'll call him today and find out if he can check diagnostics on a mefi1. First I'll buy the led and see if any codes come up because if a sensor has malfunctioned it should show up. If that doesn't do it I'll pull it to the mechanic