FSE rising rate FPR

[ramon alban]

curious about the 10 psi drop at switch-off

Hello Cross,

My reasoning for the approximate 10psi drop can be found in full, on this page:

http://www.vintagemodelairplane.com/pag ... ply01.html

or in short:-

Note the Efi fuel supply is closed except when injectors open or the FPR dumps excess pressure, so when the pressure is measured, diagnostically, what might be expected when measuring fuel pressure in the fuel rail during or after various conditions?

# by operating the AFM flap with engine not running the pressure will be 36 psi.

# after the engine is swtched off with throttle wide open the pressure will be 36 psi.

# after the engine is swtched off with throttle fully closed the pressure will be 26 psi.

because

* the FPR detects nil or very low vacuum at W.O.T. and engine not running

AND

* high vacuum at idle.

* consequentlly the measured fuel pressure after switch off may be anywhere between 26 and 36 psi depending upon the state of the throttle at the instant the pump stops running and the injectors stop opening.

Well, It makes sense to me, at least!

[r2d2hp]

Hi Ramon,

If revs were slightly raised just before the engine was turned off should the pressure stay at 36psi. If it should then maybe that could be used as a test

[DEVONMAN]

From what 4.6_Crossbolt has written, he has it set up to idle at 36psi. This would normally be about 25 psi due to high vacuum at idle rising to 36 psi at WOT and even higher with the 1.7 factor of the FSE.

I assume his set up requires 36psi at idle to give acceptable performance.
(non standard ecu etc)

Regards Denis

[ramon alban]

If revs were slightly raised just before the engine was turned off should the pressure stay at 36psi. If it should then maybe that could be used as a test

Hello Mate of C3PO, IMO, Yes, but maybe not as high as you suggest - because:

The inlet manifold/plenum vacuum (ignoring overrun) is at it highest at idle, so:

# when the engine is switched off the vacuum causes fuel pressure to be at 26 psi.

With the throttle just a tad open to raise the off-load rpm "slightly" before the engine is switched off, it would reasonable to assume the vacuum may not fall to zero, so:

# a lesser vacuum would indeed leave the fuel pressure reading at higher than 26 psi after the engine is switched off.

But I find it impossible to guess how much (How long is a piece of string?).

So I would have to fall back on my earlier conclusion that:

* consequentlly the measured fuel pressure after switch off may be anywhere between 26 and 36 psi depending upon the state of the throttle at the instant the pump stops running and the injectors stop opening.

Now to your question - Could that be used as a test?

* I think the answer has to be yes, because it can't remain at 26psi.

Just a supplimentary note to anyone who is losing the plot:

The instant the ignition is switched off, so also is the 'Engine Running' signal from coil negative to the ECU so the fuel rail pressure cannot be altered by opening injectors, only by a faulty FPR, a faulty pump non-return valve or a leaking fuel rail.

QED.

[stevieturbo]

So after 3 pages...

What exactly was the problem in the first place ? lol

[DaveEFI]

So after 3 pages...

What exactly was the problem in the first place ? lol

A genuine Lucas/Bosch regulator with a Lucas/Bosch pump will hold fuel pressure overnight when everything is good.

The flapper injection doesn't run the pump for a few seconds at switch on like most modern systems - only gets turned on with the starter motor. The AFM takes over when the engine starts.

One which holds pressure will start more quickly from cold.

[DEVONMAN]

So after 3 pages...

What exactly was the problem in the first place ? lol

Hi Stevie,
There has been plenty of valid comments in respect of a standard flapper system, but the confusion is I believed caused by the fact that the system is hybrid and has a rising rate FSE regulator, an ecu that's been played with, no cold start valve and possibly a non standard fuel pump.
I don't know why the pressure is set at 36psi at idle instead of 36psi with the engine static.

I expect 4.6_crossbolt will comment in due course and get this one into 4 pages.

Regards Denis

[DaveEFI]

I've found on a standard SD1 the fuel pressure changes between engine off/ engine running with vacuum to the regulator disconnected. The pump has a series resistor and this may make it more sensitive to voltage.

[DEVONMAN]

I've found on a standard SD1 the fuel pressure changes between engine off/ engine running with vacuum to the regulator disconnected. The pump has a series resistor and this may make it more sensitive to voltage.

So, are you saying that at switch off the fuel pump stops or slow down before the injectors stop buzzing?

Regards Denis

[DaveEFI]

At switch off both the pump and injectors are powered down at near enough the same time.

I was referring to the pressure you see with the pump running - engine stopped against battery being charged by the alternator. To run the pump with the engine stopped to check the pressure you move the flapper by hand.

[DEVONMAN]

At switch off both the pump and injectors are powered down at near enough the same time.

I was referring to the pressure you see with the pump running - engine stopped against battery being charged by the alternator. To run the pump with the engine stopped to check the pressure you move the flapper by hand.

I assume you are seeing a very small change when comparing effects of battery voltage to alternator voltage, (with vacuum disconnected from regulator) otherwise the regulator is not really doing it's job properly.

I'm still not clear why there is a 10 psi drop at switch off (at idle) on 4.6_crossbolts's engine.
Also why has he set the FSE to 36psi at idle and not set it when stalled with flapper moved?

Regards Denis