[Diy_efi] fuel injection problem, cont'd (2)

[charlesmorris at direcway.com]

Adam Wade <espresso_doppio at yahoo.com> wrote:
>A big clue is the rust and varnish throughout the
>system.  The one thing I didn't see you say you'd
>checked was the fuel pressure regulator.  If there's
>crap in the regulator, it may well be bleeding fuel
>back through the return.

Good idea. In one of my posts, though, it says I measured the return line flow with the injectors/engine off and the pump running at 43 psi. This represents the total amount of fuel available to run the engine at that pressure... and it is way too small. It should be about 4 liters per minute or so (255 lph rating at that pressure) and it's not anywhere near that. Don't remember 
the exact figure but it wasn't even 100% injector duty cycle (less than 2 liters per minute).

So I said to myself, what happens if the regulator is partially obstructed and not allowing fuel to return? The pressure would get higher as the fuel demand drops. It doesn't do this - it is never *higher* than 43 psi with 0 vacuum at the regulator hose, but it is often lower. Pulling off and reconnecting the hose at idle changes the pressure repeatably from 36 to 43 and back, as it should. 

What if the regulator is stuck completely closed, allowing no fuel to return at all? Then with the injectors off, I would expect the rail pressure to rise to the relief valve opening in the pump, typically 100 psi or more.

Lastly, what if the regulator can't close down enough to maintain pressure with the engine pulling fuel, as you speculate above? Then the pressure would be normal at idle and low load, and drop at higher loads, which is what it does. BUT - then there should be lots of return flow when it's not going into the injectors, and there is not.

Based on that logic I didn't replace the regulator ($70+ since it's a Eurospec part) but I may "shotgun" it anyway - nothing else is left!

>You should also clean the injectors in an ultrasonic
>tank, as they are likely all running lean from
>varnish, if not from crud plugging their intake screens.

Unfortunately that is already done, they were actually pretty clean according to the shop.
Thanks for the ideas though.
:(
-Charles


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