Mercruiser MPI motors
 

I have a 2000 7.4L MPI, 310hp Mercruiser. Does this fuel injection system have a return line back to the fuel tank? I am planning on adding a fuel flow gauge but it obviously will measure incorrectly if the extra unused gas goes back into the tank.

Nope, all the Merc EFI motors are self contained.

That is one of the things that really bugs me about Mercs' fuel injection. The fact that they don't use a return line in my opinion is not a good move. I work on cars everyday, some car manufucturers went to this system ( jeep for instance in their 4.0 motors) . The injectors on the end of the line always clog up. I figure that whatever debris finds its way through the filter, ends up in the back of the fuel rail, eventually clogging the rear injectors. If the fuel returned to the tank, it would flow through the filter more often catching any debris. What do you guys think???

Not only that, but the fuel in the line tends to boil into vapor-lock easier because it sits in there longer--especially at idle speeds or restarting shortly after a high-speed (hot) run. I don't know why they don't run a return and cycle the fuel through the tank again.

Corey;

You are right about that. I forgot to mention that problem. I had that exact thing happen to me once. It wouldn't restart for a while, had to open the hatch and let it air out. There is a bulletin on this problem on Mercs' website. But, there is no real fix posted. As mentioned the fix would be a return line.

The fuel in the tank can heat up as much as 20 degrees from running through hot engine fuel rails and lines.the hot fuel has a higher vapor pressure and greater hydrocarbon emmissions.That is why the automakers discontinued the return system, It's an emmissions thing.

I had the vapor-lock thing happen to me a couple of times too (once while drifting towards rocks--got pretty exciting!) and now I idle the engine for a minute or two after running hard--seems to prevent it. I also have removed the outer plastic shell half off the cool fuel/pressure reg which may have helped some. Because there is no return line, turning the key on and off a few times to cycle the fuel pump doesn't help any (tried it before I knew). I think I'll try pumping fuel from the fitting under the intake into a rag if I ever get vapor-lock while drifting towards something ugly again!

I hadn't heard about the emmisions issue before--thats interesting.

There is no return for ease of rigging and making all mercruiser engines the same with regard to fuel line. This way they can rig the boat with a single fuel line and then hook up any engine be it carb or EFI. The fuel line attaches to the fuel separator and bingo- done. Heat and vapor lock are the achilies heel of this setup. No good fix, idle and cool down before shutting down.

Don't the newest merc engines now have a water to gas cooler? I thought I remembered reading that they now have fuel coolers. Seems like a large bandaid to keep from doing the obvious. That tank full of fuel is a great heatsink...

I have a late '98 454 Mag MPI and they do have fuel coolers. The cooler sits low and next to the left engine mount. The fuel between the injectors and the front of the engine is what gets cooked since it is past the cooler and is on a 'dead-end' street until used in the engine.