Just a sanity check really. I've checked the diagrams and warmed up the the grey matter. If I plumb my fuel pressure guage in between the fuel accumulator and the fuel filter it'll read system pressure, right? c.f. K-Jet Diagram. This seems the simplest way for me to go about doing this test, as its just a case of undoing a jubilee clip. I'll be testing control pressure too, but I don't want to have to buy a high pressure fuel tap, or mess about blocking off fuel lines, so I'll test them separately. Edited by: GTI
It is recommended you test the pressure at the suggested point in the k-jet diagram. You will be able to check the cold start (warm up reg). You will watch the fuel pressure rise as the car warms up. You will also be able to measure the pressure drop, usefull when looking for leaking injectors/accumulator probs. All the values in the books are measured at the above point. I have a set of fuel pressure guages with the correct fittings, it also has a tap on the unit to measure the fuel flow from the pump. The kit was not that expensive. Certanly paid for it self......
where can i get a tap that'll take 5+bar then? the people who sold me the guage (www.fuelsystem.co.uk) don't sell one. according the the man on the phone (who seemed very knoweldgable) theirs are only suatable for about 10psi. seems like a waste of time/money to try and find one if i can test system pressure in the place i suggest??? i will be testing control pressure at the noted point (on the diagram), but that won't require a tap.
i'm just finishing off replacing the entire fuel delivery system (from tank to metering head anyway) with all piping and connections replaced with push fit banjos (except for the accumulator connectors, from which i've had to salvage the originals, but these are push fit too) onto braided hoses with annodised ends... ...which is why i'm thinking about testing fuel pressures now, while i'm at it. just a case of ondoing the annodised (jubille clip internal) end from one of my banjos to fit the guage in line (using one extra piece of pipe and another jubilee clip) where the main fuel line from the accumulator attaches to the fuel filter.
The place you suggest will only test the fuel pump pressure, which is also recirculated back into the tank. You need to measure it between the warm up reg and the metering head for the reasons I suggested in my last post. Snap On is the make of my set, you can buy to fittings from them.
Philip, are you misunderstanding what I'm trying to test? The 'fuel pump pressure' as you put it is the system pressure. To quote a very detailed site: "Systems Pressure This is the pressure that is seen within the system between the fuel pump and the metering head. This pressure is determined by the primary pressure regulator, situated within the metering head." The pump is capable of generating much more that system prtessure, but it never actually gets the fuel to over system pressure because of the bleed off at the primary pressure regulator.
btw, am i right in thinking there is an unlabeled non-return valve on the diagram: separating the control pressure from the system pressure?
You have 3 pressures all together as far as im aware. Fuel pump pressure, system pressure and control pressure. The system pressure is determined by the pressure regulator you can see above, I measured this on the supply line to the WUR, which I believe is the best place to measure it. Lost of useful info in my recent post about it HERE
thatnks smithy (i've read your posts in detail), but the non-return valve separating 'fuel pump pressure' from 'system pressure' is located in the pump - not the metering head: so by the time you get as far as the fuel filter, you're looking at system pressure.
Well as far as I understood, its the regulator in the metering head that determines the system pressure. Adding/removing shims to this regulator alters the system pressure, anything before that is just the pressure from the fuel pump.
pretty sure it don't work like that dude. have a think about it. blowing up a leaking baloon is a good analogy, with atmosphere being the tank, the leak being the pressure regulator, your mouth being the non-return valve and your lungs being the pump ... ... i'm off for a job interview, back laters
I've added a shim in the metering head and measured the change myself, so it definately works like that. If it works the way your saying (system pressure determined at the pump) then there would be no point in the pressure regulator in the metering head.
no, no, no, that's not what i meant! i knew this would happen. you are absolutely right that system pressure is set by the shims in the metering head. the point i was making was that because the non-return valve for this part of the system is at the pump, the part of the fuel system that is kept under system pressure extents right back to that non-return valve - i.e. right back to the pump - and isn't just within the metering head. i'm drawing a little diagram ...
Ok, I understand a little more now you've posted some more detail. Firstly if you are using ali banjos and hose clips, are they and the hose rated for nearly 6 bar? Let me remind you that that's 87 psi. Secondly Smithy is right, the pressure you measure down at the pump is pump pressure not system pressure. If you measure it there it makes no allowance for any losses in the pipework or filter. There is no point in measuring the system pressure anywhere other than between the metering head and the WUR as the control pressure is dictated by the system pressure. Finally there is no non return valve where you've marked it. I think why you're getting confused is because control and system pressure are the same. The WUR bleeds off system pressure to make it control pressure, the WUR is quite a basic differential pressure regulator.
What is this? "GTI is a spaz" day or something? Of course I've got correct hose and fittings! I'm not at all confused either. I completely understand how control pressure is attained by bleeding off from system pressure. The reason I asked about the non-return valve in the metering head was because if there isn't one there I have to ask how the part of the system under control pressure is kept separate from the part under system pressure? If you don't have a non-return valve, then as soon as you bleed off from the line going to the WUR you'll cause the system pressure to drop along with the control pressure. They must be separated somehow, or they would be exactly the same all the time. I can see how you might get a variation in pressure in various places throughout the system due to fluid dynamics and viscosity, but unless you have a leak, the difference should be fairly minimal. From my understanding, taking a reading just before the filter should give exactly the same reading as taking it from just after the pump, or from a tapped off WUR feed line for that matter. Here's how I understand the system to work: The pump pushes fuel out past its internal non-return valve. When the pressure on the tank side of the pump internal non-return valve gets too high (over 15 bar iirc) the pump's internal pressure limiter begins to be forced open and the excess pressure is bled directly back into the tank. This keeps the pressure inside the pump ('pump pressure' well above that specified for either system pressure or control pressure. It doesn't matter what happens to the fuel which leaves the pump away from the tank (with regard to maintaining the pressure inside the pump), because of the pump-internal non-return valve. On the metering head side of the non-return valve, the fuel passes through some pipework, through the accumulator (which is there to keep the pressure in the system up when the pump is switched off), through the filter and on to the metering head. At the metering head there is a pressure regulator and (I assume, although haven't seen it on the diagrams yet) another non-return valve. The pressure regulator begins to open when the fuel entering the metering head goes above a certain value (set by the shims) and thus limits the pressure in this part of the system, from the non-return valve in the pump through the pipework, accumulator and filter, to the metering head. The excess pressure is bled back to the tank. This part of the system is what I'm referring to as being under 'system pressure', roughly 5 bar. Control pressure is bled off from system pressure in exactly the same way as system pressure is bled off from pump pressure. Fuel passes the non-return valve inside the metering head into the control pressure side of the circuit. This part of the system would reach system pressure (along with the fuel in the rest of the metering head) it it was not for the pressure regulator inside the WUR, which bleeds excess pressure from this part of the system back to the tank. It regulates the control pressure by letting some of the pressure out. As I said, if there wasn't non-return valve between part of the circuit under system pressure and that under control pressure, the the bleed off from the WUR would affect the entire system (right back to the pump). Which doesn't make any sense. Phew!
Wait a minute ... ... cogs turning ... ... oooh dear ... bugger! I've got it wrong haven't I? Non-return valves have nothing to do with it do they. Oh dear oh dear. That's not how it works is it. No, no, no.
If you pressurise a tube by forcing liguid (or gas for that matter) into the left hand side of it, then make a hole in the middle of it, the pressure on the left hand side of the hole will be maintaind, whilst the pressure on the right hand side will drop. I'll get me coat ...
