markmastrangelo;

I work on cars everyday. From what I have seen, for the most part car manufacturers still use a return line. As I said in my other post, Jeep did away with a return line in their 4.0 motors, and I have seen alot of clogged #6 injectors.

CobraMarty;

I am with you. Boat manufacturers and Mercruiser did this to make THEIR lives easier. They could care less if the motor doesn't fire up occasionally. As I said, they have a bulletin on this exact problem, with no fix.

Here is the link to the bulletin.

http://www.sterndrives.com/bulletins/mc/99/EN_007.PDF

The problem with a return line is that it can create non-EFI tank issues when putting fuel back at high pressure.

Now.. I'll tell you that I ran into this post after getting billed $175 for a Mercruiser injector (454 Mag MPI). I was told there is no aftermarket version (which I know isn't true).

I've also owned a 350 Mag MPI - it would vapor lock just as described after a long run with people on the boat. Shut it off even slightly hot and it won't restart. I got in the habit of running the vent and removing the engine cover.

Otherwise, great motors.. Easy to maintain..

not sure what the big deal is here. most comments posted about the detriment of no return line are essentially correct. and having said that, the fix was a piece of cake. took 20 minutes. you will find that your fuel tank almost certainly has a blanked off port . un blank it, put a fitting in it , disconnect the return line from the filter, put a longer one on that dumps back into the tank. that was all it took to fix that.

my boat had junk carbed 454's. took them out and threw them away. plugged in two late 502 mags. plugged in the wiring harness, NOTICED the stupid return line system and changed that as above and fired them up. took no time at all.

as for the original question ... your gage will still be inaccurate as it will not give you the unit fuel per unit time... only the bulk fuel per unit time.... for instance.... if you put it on the inlet to the hi press pump w/ no return line, it will measure how much the pump takes in but NOT how much the motor actually used at that moment because some is retruned and reused ., so to speak... at the end of an HOUR you will know how much fuel you used but not the specific usage at individual throttle openings i figure... on the race cars we had two tranducers... one on the inlet, one on the return and the software took care of the rest.

I'm a GM mechanic. Many of our new cars now do not have a return line. Whats worse, many now dont have a replaceable fuel filter. The only one is the strainer in the tank. Seems like a bad idea to me.

Jerry.

well... the reason for the return line is for fuel pressure control... the spring in the diaphram type regulator along w/ the vaccum line allow very precise fuel pressure control without making the pump do any significan work other than move the maximum fuel nec for max demand , the excess being bled off.
now... im only guessing here but i wonder if your newer gm cars are using some sort of voltage controller to the pump which would very accurately control the pressure/volume delivery and, as such eliminate the need for a return line as the motor is seeing the correct vol/pressure already... it would be VERY easy to do on a street car with electronics available today. a simple software lookup table would do it.

but im just guessing. how do your new cars control the pressure, do you know ?

still have a fuel pressure regulator,its a part of the fuel pump sending unit in the fuel tank.

ah... so it DOES do the press bleed... just does it in the tank so no need for external return... what clever guys those gm engineers.

Steve - My understanding is that although a return line to the tank may be the best "system" and it solves a lot of our electric fuel pump and vapor lock problems, it is not USCG approved at this time. That is probably the main reason manufactures return to the separator and not the tank.

Dead End MPI
 

Requires a sealed fuel tank that runs under a slight positive pressure, and my understanding is that the boat builders could not get USCG approval and demanded that Merc and others modify the MPI set up to a dead end system. Thus the introduction of the Cool Fuel. Volvo, on the other hand just burns up $600 dual pump fuel modules until the rig is out of warrenty.