I have a 2002 S-Type with the 4.0 V8. The performance seems to have dropped off a bit, but it still starts and runs okay. Checked the fuel pressure with my scanner; 52 psi with the key on prior to starting. 38-42 psi at idle after starting. Same reading while driving at a steady speed with a slight temporary increase on hard acceleration. At no time does ever get near 55 psi which, from my understanding, is where it should be. As a first move I'm going to change the fuel filter but not sure where to go after that. Fuel pressure regulator? Last resort would be the fuel pump. I don't want to just throw parts at the problem so any suggestions would be greatly appreciated. I haven't checked the fuel pressure with a mechanical gauge, but probably should just to conform the pressure readings.

 

Replace the fuel filter using the correct tool to release the connectors. If the fuel pressure is out of specification after replacing the fuel filter, the fuel pump may be about to fail.

Here are the X200/202 fuel system specifications:

 

From the S-Type POWERTRAIN GUIDE (X200)

Fuel Injection
Returnless Fuel System
The function and operation of the returnless fuel
system is the same as that fitted to the V6 engine
but with different fuel pipework to suit the V8
layout. The air assisted, top fed, fuel injectors are
supplied from two separate rails joined by an
interconnecting cross-over pipe (similar to AJ27)
with a single common feed from the fuel tank.
The fuel rail assembly is removeable. An engine
fuel temperature (EFT) sensor and injector
pressure (IP) sensor provide feedback to the PCM
to control the fuel pump flow rate (see S-TYPE
Introduction Technical Guide). Note that the EFT
sensor is not in direct contact with the fuel (dry
type sensor).

From the S-TYPE INTRODUCTION (X200)

Returnless Fuel System
The returnless fuel system delivers the correct
amount of fuel to the engine under all conditions
and at a constant pressure differential with
respect to manifold pressure. This is without the
need for a return line to the tank or a fuel rail
pressure regulator. The use of a return line and
the pressure drop across a regulator contribute to
vapour formation which can affect fuelling
calibration and requires a complex vapour
management system to meet legislative
requirements. Elimination of these components,
D.310.047
together with closed loop control of the variable
speed fuel pump, causes a reduction in the vapour
generated, lower fuel temperatures and better fuel
pressure control. Additional advantages include a
reduced load on the electrical system and
improved fuel economy.

 

Thanks guys, great information! I'm anxious to see what difference a new fuel filter makes.

 

Things get interesting because of how the fuel pressure is controlled and reported back to the computer. The actual pressure at the injector rail is not always what is reported back to the computer.

From what I understand, the 55 PSI is what you'd see at full throttle with a mechanical gauge connected to the fuel rail. At idle, it's around 40 PSI. For those value, the baseline (0 PSI) is ambient air pressure. Think of it as raw data.

Just to make things confusing, the fuel pressure as seen by the computer (and your scanner) is adjusted data, not raw. The sensor looks at fuel pressure on one side of a diaphragm and intake manifold pressure (vacuum) on the other. In other words, the baseline for this value is intake manifold pressure, not ambient air pressure. I'm not entirely sure why the feedback is designed this way, but with the engine running the computer always sees approximately 40 PSI regardless of actual fuel pressure at the injector rail. The computer adjusts the pump output to what it thinks is a constant 40 PSI, but it's really a range of 40 up to 55 PSI (absolute) at the injector rail depending on engine load. Higher engine load (less vacuum) = higher fuel pressure for more fuel flow and increased power.

Seems to me the designers could have used raw data (manifold pressure and fuel pressure) and let the computer tweak the pump output for higher fuel pressure during acceleration. Instead, they are essentially tricking the computer by tweaking the feedback based on manifold pressure. I guess one combination sensor is cheaper and more reliable than two separate sensors. The end result is the same: 40 PSI at idle, 55 PSI at full throttle.

For the numbers you recorded, with the engine off, no vacuum is present in the intake, so manifold pressure is the same as ambient. The diaphragm inside the sensor sees the same value on both sides, so no correction is applied. With wide open throttle, the conditions are very similar. With no restriction from the throttle butterfly, essentially no vacuum is present in the intake manifold, so pressure is the same as ambient. Same as with the engine off, the diaphragm inside the sensor sees the same value on both sides, so no correction is applied. The value you recorded with the engine off (52 PSI) is the same as what you'd see at full throttle with a mechanical gauge connected, so I'd call that good. You could get picky and say it's just a smidge low of the 55 PSI target, but no tolerance range was given.

Head spinning yet? To summarize Proust (bonus points if you know this obscure reference), your numbers look good to me. I'd still verify the sensor is accurate with a mechanical gauge. If the sensor was out of whack, the computer would unknowingly adjust pump output to this inaccurate input, but so far it looks good.

Just for giggles, I played with my scanner on the drive to work this morning. I've got an '02 V6, but the values and general operation are similar. At freeway speeds, I saw a steady 40 PSI, maybe +/- 0.5. At full throttle, the scanner still showed the same value, but if my mental aerobics were correct, that was really 55 PSI at the injector rail. I didn't try recording key on and engine off, but will check that tonight.

 

I tried this and the results were pretty much as expected. Key on and engine off (no manifold vacuum), I saw 51 PSI on my scanner. Soon as I started the engine (manifold vacuum now present), the fuel pressure dropped to 40.

I was thinking I should have seen around 55 instead of 51 before starting, but there was a little lag for the scanner to power up and start reporting.

 

I had a 00 S Type with 4.0 and those numbers are OK. Swap the fuel filter out. Should see a lil improvement. 50 - 55 psi max is the highest you'll see at WOT.

If your constantly dipping below 40 psi when idling your pump may be on its way out.

My $0.02

 

What an incredible analysis of the S-Type fuel system (some of which I even understood)! As complex as the system sounds it seems to work really well. I'm also going to try the mechanical gauge to see if I get the same results that you did.

Thanks for the enlightenment.