I recently converted a carbed 502 to Merc EFI. Ran fine except for the dreaded VST vapor lock issues. So I eleiminated the VST and installed return line to the suction side of low pressure pump through a fuel cooler. Engine now runs flawless under all conditions so far.

http://www.offshoreonly.com/forums/g...apor-lock.html


But I ran into another issue. Before the swap I was burning approx 40gph overall with carbs. After the swap the burn went to 32gph (yeah baby). Then after I eliminated the VST's and added a fuel cooler the burn went up to approx 38gph.

I am using floscans, and double checked the calibration on the last trip out and they are working correctly. I am perplexed on the next step. What should I do? Is it possible that they were running too lean with the VST's? I do recall that the risers did seem quite a bit hotter with the VST's vs the carbs. What about a adjustable fuel regulator? What is the easiest way to monitor the mixture without spending a ton of time and money? This boat is used primarily for long distance cruising. Range is very important, but at the same time I don't want to cause any lean run issues.

thanks in advance for any help.

Steve

I wonder if it's possible that the return from the regulator is pushing fuel back through the Floscan under certain conditions and making them register higher? You could verify this by putting a one way valve into the line between the floscan and the low pressure pump.

That's exactly what I thought at first. So filled one tank and set the totalizer to zero, and topped it off. After I burned about 100 gallons, refilled that tank and it was within 2%.

That what it has been for the last five years that I have been running the floscans. So I'm relatively certain that the measured gph is correct.

i am not familiar w/ the flow scan but am very familiar with fuel consumption data recording in efi. because the efi system uses a fuel return we always had to run two sets of transducers... one measuring the fuel going into the log and the other measuring the return and the recorder did the math and it was 2 decimal place accurate. if your system is just measuring the fuel in and not accounting for the fuel return then your numbers are wrong by a lot.

The return lines are connected to the suction side of the low pressure pumps, which are down stream from the fuel flow sensors.

The only reasonable theory I have, is that the current high pressure pump is force fed directly from the low pressure pump increasing fuel rail pressure. Where as the VST pump pumped out of the vst tank with no pressure directly feeding it. However: I did check the fuel pressure at idle and it was within specs. I have not checked fuel pressure while under way, maybe I should. I'm not sure if it would matter?

thanks for all the responses, keep them coming.

Steve

think about it... if you measure the fuel flow before the return which then recirculates then all the system knows is the total volume that enters the loop even tho some is "used" twice because it all doesn't get burned... so the number is wrong.

or

if the transducer is located IN the loop its exactly the same problem.

after about a million hours of working out fuel calculations with various systems, i believe i can say that if you aren't running a two transducer differential system, then your numbers are meaningfully wrong to a significant degree. not to mention that recirulating the hot return fuel is a bad idea across the board. that needs to go back to the tank .

Hi Steve,

Thanks for throwing your hat in here, but I don't think we are on the same page. Let me explain
The fuel comes from one of three tanks, through a manifold to the water seperator. Then flows from there to the fuel flow sensor. Then on to the low pressure pump. From there it feeds fuel to the high pressure pump which in turn supplies the fuel rail. The return line is then routed thru a cooler (small oil cooler) back to the suction side of the low pressure pump. The fuel lines are cold to the touch even after ideling for extended periods. As mentioned above, the fuel totalizer is within 2%. It was checked by topping off one tank, setting totalizer to zero, going for a ride, then refilling tank.
At first I thought it had something to do with the return line possibly sending a pressure pulse up stream to the sensor. That's why I checked it's accuracy right away. If that was the case I could have calibrated the sensors to handle that. But no such luck. With three tanks and two engine a return to the tank is just not practical.

thanks again,

Steve