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[addy]

Could anyone tell me what Fuel pressure readings I am looking for, on the 2.0 16V X20XEV petrol engine?

Ignition key to the on position engine not running.

Engine running

Engine under load (was told transmission in reverse (automatic) holding the brake while gently giving the engine a slight amount of throttle)

The one where FPR vac pipe is removed, done with engine running for a few seconds, what should the pressure jump upto?

Also how long is the pressure in the fuel rail, supposed to stay from turning off the engine. And what sort of drop should I expect?


I think the FPR is on the way out, as the vac pipe is dry, but there is a strong smell of fuel vapour in it. But want to check it before I purchase one. Also hopefully will tell me if injectors are working ok.

There is still a smell of being rich when idling, but not as bad as it used to be.  Changed Coolant temp sensor, new complete Idle Air Control Valve. Got a Throttle position sensor. plus other bits. Genuine O2 changed, new exhaust from downpipe to back box. Type approved Cat.

Thanks for any help
Addy

[Doctor Gollum]

Pump pressure is 4.0 Bar iirc.

FPR should be 3.0/3.5 Bar (2.6/3.2 are 3.8 Bar).

Pressure drop, after running should be zero. ie no drop

A weak pump will give the same symptoms as a failed FPR.

FPRs are generally reliable, Fuel pump less so

[Marks DTM Calib]

3.0 Bar on 2.0, 2.5 and 3.0 

[Doctor Gollum]

3.0 Bar on 2.0, 2.5 and 3.0 

That's the FPR pressure?

Pump should be significantly higher, although not enough to run an LS1 at more than 2/3 throttle

[LC0112G]

I would expect it to be 3 bar *above* manifold pressure. Manifold pressure will be close to atmospheric (1 bar) at WOT or when the engine isn't running, and be quite a big vacuum at warm tickover/overrun (perhaps 0.1-0.2 bar). Therefore (assuming the basic FPR is 3bar)  the pressure in the fuel rail will be between 3.1 (overrun/tickover) and 4.1 bar(WOT/stalled) absolute. If you've got a mity-vac, you can apply vacuum to the FPR and monitor the fuel pressure in the rail. The fuel pressure in the rail should vary linearly exactly in line with mity-vac pressure.

As DG says, the Fuel pump has to be able to supply fuel at a higher pressure than the FPR needs. So if the engine might need 4.1bar absolute then the fuel pump will probably need to be capable of 5bar.

Most EFI systems try to ensure a constant pressure differential across the injectors - it makes the injector pulse width calculations much easier. The squirty side of the injector is squirting into the manifold, and the air pressure there depends on all sorts of things - engine load, engine RPM and throttle position to name but three.

[deviator]

I fully accept I could be wrong here, but on a normally aspirated engine, wouldn't the manifold pressure be a negative value? Well, less than 1 bar. As the engine is sucking.

[Doctor Gollum]

FPR is on the return end of the fuel rail so as to maintain a working pressure for the injectors.

A weak pump or a weak FPR will result in a fuel rail pressure below the marked FPR pressure. This in turn will reduce the available pressure for injector opening durations (which are controlled by the ECU based on tables programmed into it using a predetermined amount of air flow, load and the marked FPR pressure).

Intake manifold pressure has no bearing on fuel pressure as long as the pump and FPR can maintain the required fuel pressure for the injectors maximum opening durations. AIUI.

[LC0112G]

I fully accept I could be wrong here, but on a normally aspirated engine, wouldn't the manifold pressure be a negative value? Well, less than 1 bar. As the engine is sucking.

You're correct, ish. There are two ways of measuring pressure - absolute and relative (sometimes called gauge).

Absolute pressure can never be negative (well perhaps in the world of Einstein and Hawkins it can, but not in the real world). Atmospheric pressure varies on a day to day basis - you can read your own local pressure on a barometer, or watch a TV weather girlie. It's normally in the 950-1050 miliBar region. First order approximation is to assume it's 1Bar absolute.

Relative/Gauge pressure is measured with respect to atmospheric. So yes, a running engine at tickover will be producing vacuum. What that means is the pressure in the engine manifold is less than atmospheric, but it won't be less than zero absolute. A good engine (good rings, well sealed valves, close fitting cylinders) might be capable of 'sucking' 0.9bar of vacuum, but if atmospheric is 1bar, then the absolute pressure is still +0.1 bar.

A proper EFI management system uses both a Barometric pressure sensor (BARO) and a Manifold pressure sensor (MAP) - none of this MAF rubbish :-)

[Doctor Gollum]

All very interesting, but has zero bearing on fuel pressure (pump or rail) in a closed system

[LC0112G]

Intake manifold pressure has no bearing on fuel pressure as long as the pump and FPR can maintain the required fuel pressure for the injectors maximum opening durations. AIUI.

No. The main reason for the FPR is to try and maintain a constant pressure differential across the fuel injectors.

There is an air pipe from the middle of the FPR body to some point on the plenium/manifold. This pipe applies manifold pressure to a bellows inside the FPR, and that bellows modifies the fuel pressure setting of the FPR. The usual setting of fuel pressure is 3 bar above manifold pressure.

Take for example a turbo car which runs up to 2.5 bar of boost. At tickover/overrun, it'll be sucking perhaps 0.9bar of vacuum. Assuming atmospheric pressure is 1bar, then we have +0.1bar of absolute air pressure in the inlet manifold. If the fuel pressure were a constant 3 bar absolute, then when the injectors open you would have 2.9bar of pressure difference over the injector forcing the fuel through into the manifold/cylinder.

Then giving it some wellie, boosting to atmospheric+2.5 bar, so 3.5 bar absolute pressure in the manifold. If the fuel pressure remained at a constant 3 bar absolute, then when the injectors opened the fuel wouldn't go into the manifold - the manifold air would go into the fuel tank!

Sure you could raise fuel pressure to a constant (say) 4 bar. But that would give you 3.9bar over the injectors at tickover, and only 0.5 bar over the injectors at full boost. It's a nightmare to map the fuel flow through an injector if you can't assume a constant fuel pressure differential across it. So what you actually do is put a FPR in which regulates the fuel rail pressure to manifold pressure + 3bar. On a turbo car, that means fuel rail pressure may need to be 5.5 bar, and therefore fuel pump pressure 7-9 bar.

On a normally aspirated engine - like the Omega - the same rules apply. Minimum manifold pressure will be 0.1 bar absolute, but maximum pressure will rarely exceed atmospheric 1 bar. (Except for very highly tuned engines which use resonance in the inlet manifold). So with a manifold pressure from 0.1 to 1bar absolute, and a typical 3 bar FPR, fuel pressure in the rail will vary from 3.1bar absolute at high manifold vacuum (tickover/overrun) to 4.0 bar absolute at WOT/Stalled.

 

[Doctor Gollum]

I stand corrected

I had forgotten about the vac connection, runs to the TB on the Omega