mercury 5.7 mpi 350 magnum starts but wont stay running but has spark, fuel etc.

[sinkerswim]

Ok so where to even start with this boat uugh! It was running fine all day then this afternoon after pulling surfers and wake boarders all morning I went to take it out for a second round and it started ok but about 100 feet from the dock it stalled and would not restart. I had it towed in thinking maybe I had flooded it trying to restart. Left it sitting overnight in the water. Went the next day and replaced the water/fuel separator it started once but ran REALLY rough then died. Now it will crank and crank and crank and it sounds like it wants to start but nothing.. unless I push the accelerator almost all the way forward while cranking it, then it will start but LOTS of dark grey smoke billows out of the back and it runs just terrible. I checked for spark and shocked the senses out of myself so I know thats good, removed and checked the new water fuel separator and it was full of new fuel so I know fuel is at least getting that far. I did find the oil pressure switch had become dislodged somehow and reconnected that. The fuel pump does engage but its running for close to 20 seconds which seems a little long. My serial number is 0L6xxxxxxx. HELP ME

 

[Don S]

Re: mercury 5.7 mpi 350 magnum starts but wont stay running but has spark, fuel etc

A partial serial number is totally worthless.

You need to get it hooked up to a diagnostic scan tool to see what is going on and what the sensors are telling the ECM.

 

[sinkerswim]

Re: mercury 5.7 mpi 350 magnum starts but wont stay running but has spark, fuel etc

A partial serial number is totally worthless.

You need to get it hooked up to a diagnostic scan tool to see what is going on and what the sensors are telling the ECM.

So no time to get it out of the water today and the saga continues. I checked for the serial number on the cover where it used to be but I suppose 11 years of wear has worn it off :grumpy: So I took photos of all of the numbers on the engine I could find and they are attached.

I am sure these arent going to help much so I will continue with what I have done so far. Replaced fuel water separator, replaced all spark plugs, verified spark and fuel all the way to the top. I think what I thought was the oil pressure switch may not have been and here is an actual picture of what I found dangling in the dirty hull water post putting it back where it belonged.

I am at a loss besides the last symptom which got worse today. When the batteries are switched on and I enter my ignition code to start the motor the usual double beep happens and when finished I can hear the fuel pump running and it will not shut off unless I turn the batteries off or enter ignition shut down code. I am nervous to jump right into replacing the VERY expensive fuel pump just yet.

 

[Don S]

Re: mercury 5.7 mpi 350 magnum starts but wont stay running but has spark, fuel etc

Not casting numbers, those are GM numbers. Look at the Mercruiser tag just above the starter on the back of the block.

 

[sinkerswim]

Re: mercury 5.7 mpi 350 magnum starts but wont stay running but has spark, fuel etc

I should also add that the block froze in winter of '12 and was fully rebuilt last summer, winterized and stored indoors this winter. When I pulled it out to de-winterize it the gauges werent working (oil pressure specifically) this is a 2002 Gekko G3 sport now renamed to "Revo" I ordered OEM gauges and replaced those on June 28th and all seem to be working fine. But when the engine is cranking the oil pressure gauge shows nothing if/when it starts even for a second it shows 40 "psi?" If I expect any help troubleshooting I want to make sure that everything I have had done recently is listed. Thank you so much!

 

[sinkerswim]

Re: mercury 5.7 mpi 350 magnum starts but wont stay running but has spark, fuel etc

Not casting numbers, those are GM numbers. Look at the Mercruiser tag just above the starter on the back of the block.

I will have a look tomorrow when I have daylight again Thank you so much for the tip! I have been stressing trying to figure out how I was going to recover the number.

 

[Fun Times]

Re: mercury 5.7 mpi 350 magnum starts but wont stay running but has spark, fuel etc

The photo of the sensor with the blue wire is the knock sensor.

Since you had dark smoke, What did the spark plugs look like? fouled? Did you smell fuel?

If the fuel pump is staying on while the ignition key is on or in your case the code is punched in, That could be an indication that a fuel pump relay is sticking to the on position and possibly allowing the fuel system to build to much fuel pressure all the while damaging the fuel pressure regulator enough to allow the engine to run very rich ultimately stalling the engine.

You'll need to check your fuel pressure with a EFI fuel pressure tester gauge to know for sure.

If that's not it then there are four other components that would make it run extremely rich if that's what it's doing. They are the MAP sensor, ECM, stuck open fuel injectors, last but not least the EFI temperature sensor telling the ECM the engine is really cold even if the thermostat is working properly at 163 degrees.

Also the ECM controls key up time frame for the fuel pump, if it's really staying on for twenty seconds and shutting "itself off" and it's not you or the ignition code system you have shutting it down automatically, Then it's either a bad relay, ECM or since someone has worked on the boat it could be a loose or bad wire connection someplace on the engine. Carefully wiggling wires and connections around with the key on may get things back to normal all the while checking the wire connections for corrosion, etc. etc.

Another indication is moving the throttle to almost full open while cranking would actually go into clear flood mode turning off the fuel injectors in turn possibly allowing any excess fuel consumption in the cylinders to burn away for a rough start.

Bear in mind that this is all a guess without actually being there to see what is actually happening.

Hope you get it figured out, Good luck.