Mercruiser 5.7L EFI engine, fuel pump air block?

[Ballistix]

Had a bizarre situation yesterday trying to fit this "now pain in the arse Floscan fuel sensor" in my boat. I put it in line again, thinking I had it right, engine wouldn't start...so thought darn it, took it out and put the normal line straight back into the water separator. Wouldn't start.

Much pain and head scratching. Tried everything, resulting in me taking the pipe off the inlet manifold on top of the engine and sucking hard as the key was turned one click to fire the pump up, after 5 minutes of this and one last hard suck I got a gob full of petrol and it was back!

Anyone heard of this sort of problem? I thought I had buggered the pump to be honest.../phew.

NB. thinking about it later, I should have asked the wife to do the sucking thing, isn't hind site a wonderful thing. Of course she wouldn't do it not liking the taste of pertrol...

 

[Alrob]

so the moral is - dont fit flow meter

I wouldnt fit one, load of work & money for what I already know - 2 MPG if you take it easy

Boattest.com have already done all the tests
/forums/images/graemlins/tongue.gif /forums/images/graemlins/tongue.gif

 

[alan006]

I have come accross a simular problem with the v6 Mercruiser and perhaps it is caused in the same way.
The newer v6 engines have electric fuel pumps and these do not seem to have the power to pump fuel through if the system needs priming. The older v6 engines had a mechanical pump which did not suffer from this problem. I asked an engineer why they changed from the mechanical pump to the electric pump and he told me that this was because they had changed from carbs to fuel injection.
This worried me and to get round it I put an outboard priming pump in the circuit so if it needs priming I can do it. I have found that other people have done this as well.
Hope this helps.

 

[Ballistix]

Thanks, yeah I heard that was quite a good idea. Thanks
T

 

[William_H]

I find these posts very confusing. Perhaps someone can explain.

A car EFI system has an electric fuel pump in the tank it delivers about 15 PSI at quite a large flow. The pump draws several amps of current.
A carburettored engine will start on fuel in the bowl so then mechanical pump can do its thing. An EFI engine will not start without fuel pressure hence electric pump needed.

This pump is controlled by the elctronics such that it will run on cranking the engine but will be disconnected if the engine is not running. In case of crash or similar.

The fuel manifold rail has a regulator at one end which maintains a correct pressure by bypassing much of the fuel back to the tank. This keeps fuel cool avoiding vapour blocks.

So I assume the Mercruiser system is similar hence complexity of fitting fuel flow meter. I understood they have 2 sensors one measuring fuel in and one measuring fuel returned.
On cars fuel flow is measured on the basis of on time of an injector. Predicated on the constant flow rate of the injector and constant pressure. very simple.
I have always felt that a vital device on EFI is either a fuel pressure gauge or a pressure sensor switch to a light. Fuel pressure is the ultimate check of fuel system integrity.
As an aside our club recently put a new engine in the rescue boat. It6 was 140HP Mercruiser i/b /o/b. I was disgusted to find the new engine had a carburetor when to my mind fuel injection is just so much better and safer.

So am I way off the mark assuming Mercruiser is similar system to acar or have I given some insight into the problems? olewill