# Difference between fuel pressure and fuel rail pressure

In OBD PID wiki page, there are 2 PIDs listed: Fuel pressure and Fuel rail pressure. Are both of them the same? Can someone please give an elaborate explanation of it - What exactly are they signifying? Are they proportional to the Intake manifold pressure?

If I don't get the value of the pressures from the PID (say in Hyundai i10), is there any way to calculate them? Do their values remain the same more or less, or it changes with engine load?

https://en.wikipedia.org/wiki/OBD-II_PIDs PIDs: 0A and 22 (mode 1)

• I'd assume they were the same. I've never observed anything other than the fuel rail pressure PID. AFAIK there is no way to determine fuel pressure without a electric or mechanical gauge/transducer. On Fords with FRP sensors they use a Fuel Pump Driver module which receives input from the ECM and adjusts fuel pressure accordingly.
– Ben
Feb 11 '16 at 14:55

On a car with fuel injection there can be two fuel pumps, one high-pressure and one low-pressure. The low-pressure pump moves the gas from the tank up to the high pressure pump. The high pressure pump then raises the pressure to a level that will be useful to the injectors. The two PIDs reflect those two different systems.

``````PID  Description        Min       Max      Units
0A   Fuel pressure        0       765      kPa (gauge)
22   Fuel rail pressure   0     5,177.265  kPa
23   Fuel rail pressure   0   655,350      kPa
``````

You can also see that the range of values returned for each is scaled to be appropriate for the range of pressures to be measured.

(I took those PID values from Wikipedia so trust, but verify.)

Edit: I added an entry for PID 23 as mentioned in comments and adjusted the layout of the max pressure column to highlight the difference in the scales of the data. In OBDII there is no way to specify the units of measure for a given PID, that is part of the reason there are two PIDs for fuel rail pressure. PID 22 is appropriate to the pressures for injection into the intake manifold, while PID 23 is better matched against the pressures in diesel or direct gas injection.

To address the other points in the question. Only PID 22 specifies that it is measured relative to manifold pressure, I would not expect that the other two would thake manifold pressure into account since they are not really affected by it.

These pressures are all controlled by regulators so that any of these PIDs should be fairly constant and not affected by load in a well running engine. Low fuel pressure would be the result of failure or clog in the fuel system.

• In general, there are two fuel pumps on a car with fuel injection this would be a true statement for vehicle with direct injection, but most "fuel injection" cars only have one pump Feb 11 '16 at 23:08
• @Movemorecommentslinktotop it also holds for some older ones as well. My E30 BMW has two pumps, The low pressure one in the tank and then the high pressure one along the driver's rocker panel. But no OBDII on that one to prove out the whole thing.
– Ukko
Feb 11 '16 at 23:11
• Is it gas or diesel? Feb 11 '16 at 23:21