How does the fuel rail in an EFI petrol engine retain the pressure with the pump not running? Obviously some form of valves - but where? One in the pump and one in the pressure regulator? And do all pressure regulators include one?
"Duncan Wood" gurgled happily, sounding much like they were saying:
Not all the time. The pump runs for a few seconds when the engine's first started, then runs whilst the engine's turning.
AIUI, the pressure's held in the rail by the pump not allowing pressure to flow back, and by the regulator not allowing fuel back to the tank return until it exceeds the rail pressure.
A vacuum operated pressure regulator will stop leakback for a while, so acts like a check valve.
The regulator's job is to keep the pressure in the fuel system to a constant psi. It does this with a spring-loaded diaphragm that controls a valve. The valve, when opened by excessive pressure in the fuel lines, uncovers a fuel line that returns excess fuel to the fuel tank. The vacuum connection is there to help reduce emissions during deceleration. During deceleration, the vacuum connection serves to open the fuel return valve wider, which reduces pressure in the system and prevents excess hydrocarbon emissions due to less fuel being injected as a result of the lower pressure in the system. When the engine's switched off, you'd expect the fuel to be retained at pressure behind the spring-loaded diaphram- for a while at least.
The pump usually has a check valve integral or just outboard of it, so between it and the pressure regulator will keep pressure up for a short while, but I wouldn't expect that to last for more than few hours. Most modern systems will switch on the pump for a short time as soon as the ignition is turned on, switching it off again after a few seconds if the engine's not started.
That's got me confused for a start as they are fully open with no vacuum - ie engine stopped.
Yes.
Think you've got the wrong end of the stick. The primary purpose is not to do with emissions since they existed on fuel injected cars before regulations. Their primary purpose is to keep the flow rate of the injectors independant of manifold pressure. Manifold vacuum tends to suck in fuel so the pressure is reduced in proportion to compensate.
Except that the valve is fully open with no vacuum...
Right. On mine it must be integral.
Yes. Mine doesn't but I know some do.
Basically the fuel rail on mine is depressuring rapidly and I'd like some guesses on what to check first. Obvious one would be leaking injector(s) but I'd expect this to make some difference to the idling at least. The parts are rather expensive to replace using a shotgun approach. So any method of testing in situ would be welcome.
The SD1 is losing pressure rapidly - goes to zero as soon as the engine is stopped. It didn't use to - it would maintain pressure for several hours. I recently had the tank repaired and wondered if some dirt had got into the pump. Or just coincidence. Basically I was wondering about a test method for all the components. The pump in particular isn't easy to get at. I could easily disconnect the reg in and out and plug those ports - suppose that would eliminate the injectors? Or any thoughts?
The ports could be plugged at either end as suggested while monitoring the pressure in the rail. Another suggestion-
Suck on the vacuum port of the regulator. It shouldn't leak vacuum. If you get petrol down the vacuum port, the diaphram's gone and that would explain the fuel leak-down.
Not on modern systems at least - as was stated above, it's to keep a more constant pressure across the injector nozzle regardless of manifold pressure. Some FI systems don't bother, though: it can be compensated for by other means and manifold pressure is only one factor that can affect the pressure across the injector nozzles.
For fuel efficiency, quite often the FI simply cuts out altogether on the overrun, provided the engine speed's above a certain RPM.
As long as it's holding the right pressure with the engine running and there are no other symptoms I wouldn't worry too much. If you must investigate then you'll need to rig up an air pump and pressure guage and connect it into different parts of the system. It should be easy enough to connect a foot pump into a fuel line. My tyre pump has a little plastic adaptor that would do that although I think it's meant for airbeds and the like. Otherwise an old tyre valve glued into a bit of tube would do or you can buy proprietary Schrader valve hose adaptors.
Disconnect the line to the pump on one side of the injectors and test the pump. Do the same with the pressure regulator on the other side. If both of those are ok it must be in the fuel rail and injectors. I doubt if any fuel line will hold system pressure for long though and that's why normally the pump triggers for a couple for seconds when you turn the key before the starter motor operates. It will only take a tiny imperfection in a seal somewhere to lose pressure very quickly once the pump stops, far smaller than would manifest as a running problem, so personally I wouldn't even bother about it. I bet most older cars don't hold system pressure in the fuel line without the owner even being aware of it. I'm sure you can find plenty of other things to spend your time on.
That's certainly true reference Bosch K injection, (hydromechanical - but the theory holds good) fuel pressure is maintained by a dedicated accumulator, ISTR pressure holds up for hours. Once the accumulator becomes shagged, pressure drops off too quickly after engine stop and gives rise to potential hard start and long cranking due to engine heat causing vapour locks.
On the Bosch K (the only system that I have a good working knowledge of) you can test the injectors for leakage simply by pulling them from the manifold and seeing if they dribble or weep with residual system pressure. Testing of flow is easy too - just hot wire the fuel pump and lift the metering plate by hand. I normally let them spray into an old blanket so as to lessen the chances of a fire.
It depends on the system, it normally holds for somewhere between several minutes & a few hours. If you're trying to find out what's leaking then normally you just pinch off the relevant pipes, you can see if the injectors are leaking by just powering the pump up without starting the engine, if they're good pinch off the fuel feed & see if it's back flowing through the pump.
Wuss. On 'The Secret Life of Machines' (which was all available properly free online last time I looked, for those who remember it) they demonstrated injectors by having a head held in a vice and firing all four injectors through a flame. Inside a wooden shed.
It makes it need far more churning over than normal on a cold start. The pump only runs with the starter. I can make it run by putting the car in drive and turning the starter - the gearbox inhibitor switch stops the starter motor but not the circuit to the pump. If I do this from cold then move the lever to neutral it starts almost instantly.
Of course I could make up a circuit to run the pump for a fixed time at switch on - but I'd rather get to the bottom of the problem.
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