MPI fogging mixture vs fuel return

[Stinnett21]

Let me see if I understand this correctly. On my 2002 5.0 MPI (serial #0M345638) the fuel pump puts pressure on the injectors, so much so that all of the fuel pushed to the injectors is not used but some is returned to the fuel filter. So when Merc recommends a 10:1 fogging mixture, 10:1 is not what is getting to the cylinders. It would be a higher ratio maybe 20:1?

 

[Scott06]

Let me see if I understand this correctly. On my 2002 5.0 MPI (serial #0M345638) the fuel pump puts pressure on the injectors, so much so that all of the fuel pushed to the injectors is not used but some is returned to the fuel filter. So when Merc recommends a 10:1 fogging mixture, 10:1 is not what is getting to the cylinders. It would be a higher ratio maybe 20:1?

Yes there is a pressure regulator that bleeds off the excess pressure and fuel volume, but you are returning 10:1 and mixing it with 10:1 so it will be the same ratio not get more concentrated....

 

[Stinnett21]

Yes there is a pressure regulator that bleeds off the excess pressure and fuel volume, but you are returning 10:1 and mixing it with 10:1 so it will be the same ratio not get more concentrated....

Well...yes duh. Thanks.

 

[Scott06]

Well...yes duh. Thanks.

Ot sure if that was sarcasm or i answered your question ?

 

[flashback]

Ot sure if that was sarcasm or i answered your question ?

Don't sweat it, I think he really had a moment.

 

[tpenfield]

Essentially you have to purge the non-mixed fuel from the fuel system. So, you attach a 10:1 mix to the incoming fuel line. It goes to the fuel filter where it meets up with a bunch of straight gasoline, on onto the pressure pump and fuel rail. So,initially the engine sees very little of the 10:1 ratio. As time goes on the 10:1 purges the existing fuel.

That is why the service manual says to run the engine for about 15 minutes. So,there has been plenty of time (fuel flow) to purge the straight gasoline from the fuel system.

The reverse process occurs in the pre-season prep, as your engine purges the 10:1 mix that was in the fuel system during the off-season.

I used the engine fuel flow indication on the VesselView to run 1/2 gallon of 'fuel' through the system, figuring 1/2 gallon would more than purge the straight gasoline.

 

[Stinnett21]

Ot sure if that was sarcasm or i answered your question ?

Lol you answered my question. Mad at myself for not thinking of that. Seems like in the past someone made a point that what goes in the filter is not what actually goes through from there but I either mis-understood or was bad info.

 

[Stinnett21]

Essentially you have to purge the non-mixed fuel from the fuel system. So, you attach a 10:1 mix to the incoming fuel line. It goes to the fuel filter where it meets up with a bunch of straight gasoline, on onto the pressure pump and fuel rail. So,initially the engine sees very little of the 10:1 ratio. As time goes on the 10:1 purges the existing fuel.

That is why the service manual says to run the engine for about 15 minutes. So,there has been plenty of time (fuel flow) to purge nthe staight gasoline from the fuel system.

The reverse process occurs in the pre-season prep, as your engine purges the 10:1 mix that was in the fuel system during the off-season.

I used the engine fuel flow indication on the VesselView to run 1/2 gallon of 'fuel' through the system, figuring 1/2 gallon would more than purge the straight gasoline.

Tbh what's driving my thinking about this is your suggestion of simply adding 2 stroke to the boat's tank instead using the remote tank and disconnecting the fuel supply year after year. My boat is stored at the lake 2 hours from the house so it's advantageous to do everything I can on my last outing. This would make this process much simpler. I can add 2 stroke relative to what Vessel View says is remaining in the tank. Drive around for bit and head to the ramp.

 

[Scott06]

Lol you answered my question. Mad at myself for not thinking of that. Seems like in the past someone made a point that what goes in the filter is not what actually goes through from there but I either mis-understood or was bad info.

I think ted explained it better And his method of adding oil to remainder of fuel makes sense.

 

[tpenfield]

Just be mindful, that I drain the fuel tank and then add back in about 8 gallons of the 10:1 mix for the winterizing process.

If one were to add oil to the remaining fuel at the end of the season, it may only be practical if the tank were fairly low.

Since I hardly ever let the fuel tank get much below 1/4 full, which is about 40 gallons, I choose to empty the tank and add back a smaller amount of fuel. Since my engines are CAT exhaust, it is a bad thing to run out of fuel. So, I add back about 8 gallons just to be sure it does not. The winterization uses about 1.5 gallons of fuel, since in total I run the engines for a bit over and hour.

 

[Stinnett21]

Always appreciate the input!

 

[QBhoy]

Tbh what's driving my thinking about this is your suggestion of simply adding 2 stroke to the boat's tank instead using the remote tank and disconnecting the fuel supply year after year. My boat is stored at the lake 2 hours from the house so it's advantageous to do everything I can on my last outing. This would make this process much simpler. I can add 2 stroke relative to what Vessel View says is remaining in the tank. Drive around for bit and head to the ramp.

Exactly what I do. Run it low on last outing then add a Jerry can of mix to the tank. Just enough to get to the slipway and make sure enough ram through to smell it out the back. Also means that the tcw3 in the tank helps keep things decent in there too, with the additives a good product usually has

 

[RaceCarRich]

My old mechanic used to fill the cool fuel module with the mix when he changed filters and then run it briefly to get to injectors. That combined with some stabilizer in the tank. I followed this when I did myself one year with no adverse effects. And downside to this method?

 

[tpenfield]

My old mechanic . . . Any downside to this method?

Just as long as the engine is run long enough so that the fuel mix makes its way to the fuel rail and purges the straight gasoline. That may be a couple of minutes vs. the 15 mins or so that the service manuals describe.

 

[tpenfield]

FWIW - my engines (6.2 Merc) each burn 1.5 gallons per hour at idle when cold and go down to about 1.1 gallons per hour when warm. So, I'm running the engines (separately) for a full 30 minutes to make sure enough mix has gone to/through the system to purge the straight gasoline.

 

[RaceCarRich]

Just as long as the engine is run long enough so that the fuel mix makes its way to the fuel rail and purges the straight gasoline. That may be a couple of minutes vs. the 15 mins or so that the service manuals describe.

Yeah that was my challenge. I calculated how much mix went into the cool fuel module (less than a quart if I recall correctly) vs the gph at idle and tried to nail a tight running window between when it started drinking the mix and when it had drank all of it and got back to straight gas but had no way to calculate how long it would take to burn what was already in the rails. I incorrectly assumed they were empty after not running for a week or two and opening the fuel module but now it sounds like there is a check valve that would keep them full.

In reality, wouldn’t treating a boat like any other gas engine that sits 6 months be sufficient? I mean lawn equipment, race car, generator, etc. (all carb’ed) never seemed to be a problem with add stabilizer and run for 15 minutes. I’m sure many injected vehicles sit for 6 months with just a dose of Stabil. Why do we need to make it harder?