How the hell does the T4 fuel system work?

[ww1dm1]

I was confused by it from the moment I saw that the return line from the fuel pressure regulator on the fuel rail actually goes nowhere. It just begins on the rail and ends after a few inches with a flimsy rubber blocker, which makes me doubt fuel gets there at all.

I've read somewhere on here that it's a Return Lacking Fuel System which has 2 fuel pressure regulators, one at the tank and one on the rail. Though I can't wrap my head around how it's supposed to work. Why is the second regulator even necessary? How does it regulate pressure if there is no return from it?

[960kg]

Originally Posted by ww1dm1

I was confused by it from the moment I saw that the return line from the fuel pressure regulator on the fuel rail actually goes nowhere. It just begins on the rail and ends after a few inches with a flimsy rubber blocker, which makes me doubt fuel gets there at all.

I've read somewhere on here that it's a Return Lacking Fuel System which has 2 fuel pressure regulators, one at the tank and one on the rail. Though I can't wrap my head around how it's supposed to work. Why is the second regulator even necessary? How does it regulate pressure if there is no return from it?

There are two regulators on the T4 because it is a turbo 1855cc only, which is fairly highly tuned to get 200bhp or more from it so therefore that is one reason only Volvo high quality parts should be used when replacing .....so one of the things that add to it`s performance is heat being the thing to avoid the fuel should be kept to under 50 centigrade so the regulator at the tank recirculates the fuel until it goes down the line to the regulator on the fuel rail to be used.

If ever there is too much fuel at the fuel rail it is just let into the engine to burn hence no need for a return system.

[ww1dm1]

Originally Posted by 960kg

There are two regulators on the T4 because it is a turbo 1855cc only, which is fairly highly tuned to get 200bhp or more from it so therefore that is one reason only Volvo high quality parts should be used when replacing .....so one of the things that add to it`s performance is heat being the thing to avoid the fuel should be kept to under 50 centigrade so the regulator at the tank recirculates the fuel until it goes down the line to the regulator on the fuel rail to be used.

If ever there is too much fuel at the fuel rail it is just let into the engine to burn hence no need for a return system.

Neat, the picture helps, though I'm still confused. The fuel pump generates somewhere between 5 and 8 bar. The first regulator keeps the line at 4.5 bar and returns the excess. The second regulator brings the pressure down to 3.1 bar at the rail (or to whatever it is modified to by the manifold pressure), but has no return line. In that case there is always an excess, if it just pours it down into the intake manifold then there is no purpose of the second regulator, as you still get the same 4.5 bar.

How does the system deal with the pressure difference between first regulator at the tank (4.5 bar) and second regulator at the rail (3.1 bar)?

[IainG]

Hi
For a pressure regulator to work, a return line is not needed.
Pressure reducing valves without a return are used all over.
They only need a piston or diaphragm with a spring.
All they do is cut off when the pressure is reached then open again when the pressure drops. This constantly happens to keep the pressure constant. They don't need a return to regulate the pressure.

if a return is utilised it is often used to keep a flow in the pipe at all times which helps with keeping the fuel cooler, otherwise fuel in a pipe under the bonnet can heat up and vaporise.

Iain

[mitchyboy01]

From what I can remember the flow to the rail has a T-piece back to the sender just after the fuel filter. The system is designed like this for emissions reasons and is very restrictive when tuning for being power outputs (300bhp+). It seems to work but no one really knows how lol

If you are wanting to run an aftermarket adjustable FPR you will need a modified rail, return line and the sender FPR removed.

[Antz]

Magic. Witchcraft. Sorcery.

[ww1dm1]

Originally Posted by IainG

Hi
For a pressure regulator to work, a return line is not needed.
Pressure reducing valves without a return are used all over.
They only need a piston or diaphragm with a spring.
All they do is cut off when the pressure is reached then open again when the pressure drops. This constantly happens to keep the pressure constant. They don't need a return to regulate the pressure.

if a return is utilised it is often used to keep a flow in the pipe at all times which helps with keeping the fuel cooler, otherwise fuel in a pipe under the bonnet can heat up and vaporise.

Iain

Ah, this must be the key then, always thought that all FPRs return some fuel, but you're right to say that it doesn't really have to. So they installed such a system to keep the tank fuel temperature below 50 at the cost of higher rail fuel temperatures, to reduce the fuel vapor emissions?

Why not just get rid of one, though? Why not remove the tank regulator, or replace it with the rail regulator? You're still ending up with 3.1 bar at the rail and hot fuel does not return to the tank! I know new cars do the single tank regulator thing (a.k.a. returnless fuel system), is the pressure difference between pump and rail too great for a single regulator of those times? And why does the in-tank regulator return the fuel to the tank if a returnless regulator could work as well?

Originally Posted by mitchyboy01

From what I can remember the flow to the rail has a T-piece back to the sender just after the fuel filter. The system is designed like this for emissions reasons and is very restrictive when tuning for being power outputs (300bhp+). It seems to work but no one really knows how lol

If you are wanting to run an aftermarket adjustable FPR you will need a modified rail, return line and the sender FPR removed.

It really sucks that no one knows how, it can't be that complicated, I want to figure it out. And the more I look into it, the more I want to get rid of it. The non-turbo engines have a regular return system, no? Would the rail fit on a T4?

Originally Posted by Antz

Magic. Witchcraft. Sorcery.

I'll try asking a local gypsy.