Excessive fuel pressure

[Macleesh]

After assembling  my new fuel pressure test rig I finally had the chance to test it. Regrettably immediately prior to this I also fitted a relay for the fuel pump so am not totally sure if my pressure has increased as a result of a better voltage at the pump, but its 116 PSI, so needs reducing. I'm looking at fitting one of the externally adjustable PRVs from TR Trader to save faffing about removing adjusting, testing, removing again, adjusting again of the existing PRV.

 

I'm assuming excess pressure equals a richer than normally rich mixture as the MU opening will be set to allow the correct fuel amount based on a lower pressure than its actually getting?

 

Sean

[TR NIALL]

8 minutes ago, Macleesh said:

After assembling  my new fuel pressure test rig I finally had the chance to test it. Regrettably immediately prior to this I also fitted a relay for the fuel pump so am not totally sure if my pressure has increased as a result of a better voltage at the pump, but its 116 PSI, so needs reducing. I'm looking at fitting one of the externally adjustable PRVs from TR Trader to save faffing about removing adjusting, testing, removing again, adjusting again of the existing PRV.

 

I'm assuming excess pressure equals a richer than normally rich mixture as the MU opening will be set to allow the correct fuel amount based on a lower pressure than its actually getting?

 

Sean

Yes better voltage will mean your Pump is working better and as you see the pressure has increased(Happier Pump)

Yes the New Type PRV will allow for easier adjustment but I’m not sure if the excess pressure will make it run richer I just think that more excess fuel will be returned to the Tank= Hotter Fuel that you don’t need.

[Andy Moltu]

Not necessarily the change in voltage. The PRV should relieve the pressure at the set pressure.

It may be your PRV was stuck or set wrong and the pump is now delivering the higher pressure (or it was all along).

Excess pressure may mean richer running but the metering unit really is designed to work within certain parameters and over pressuring can damage the unit.

 

 

[Waldi]

With 116 psi the MU and injectors work at  a 10% higher pressure (116/105).

In theory this results in a 5% higher amount of fuel injected, that’s theory.

In practice, it is more complex, since the injectors open at a much lower pressure. But that they inject “more” is obvious. Maybe others know more precise?

Waldi

[Macleesh]

18 minutes ago, Waldi said:

With 116 psi the MU and injectors work at  a 10% higher pressure (116/105).

In theory this results in a 5% higher amount of fuel injected, that’s theory.

In practice, it is more complex, since the injectors open at a much lower pressure. But that they inject “more” is obvious. Maybe others know more precise?

Waldi

That was certainly my understanding. In EFI engines you have two ways to alter fueling, vary the injector opening duration and increase the rail pressure so more fuel goes in for a given duration. I think my situation equates to the latter.

[Mike C]

I suspect that the fact that the Lucas PI system is basically driven by manifold depression makes it largely independent of fuel system pressure. 

If the fuel system pressure is higher and this results in more fuel being injected by the MU, the engine speed increases,  the manifold vacuum increases and the control unit then reduces the fuelling rate.

[Macleesh]

5 hours ago, Mike C said:

I suspect that the fact that the Lucas PI system is basically driven by manifold depression makes it largely independent of fuel system pressure. 

If the fuel system pressure is higher and this results in more fuel being injected by the MU, the engine speed increases,  the manifold vacuum increases and the control unit then reduces the fuelling rate.

Not sure it's as cut and dried as that, engine speed can only increase if there's enough air available? As it is the MU will be delivering too much fuel for a given throttle opening/vacuum  level?

[PodOne]

Just a thought.

Before accepting that 116psi is accurate how accurate is the gauge? +/- what %

Andy

[michaelfinnis]

Pressure at the MU is determined by the PRV. The pump delivers more than the required pressure with the PRV relieving the excess back to the tank. A higher voltage to the pump should not increase pressure at the MU, PRV must be stuck or wrongly set?

According to the original Lucas training booklets high line pressure will result in high fuel consumption and may overload the pump causing overheating and possible vapour lock. 

 

 

[Macleesh]

13 hours ago, PodOne said:

Just a thought.

Before accepting that 116psi is accurate how accurate is the gauge? +/- what %

Andy

It's stated as being +/- 2%