Fuel rail pressure 2003 XKR
 

What should the fuel rail pressure be on a 2003 Jaguar XKR 4.2 supercharged?

Whatever the ECM wants.

The system is 'returnless' so the pump is variable.

bob

A tech at Ford said the Jag specs are 70lbs. However, when I am on running DATA while driving or at idle pressure is 54, 55 steady. When car is off and key in number 2 position I go to fuel pressure and the reading is a steady 70lbs. Start car and goes back to 55lbs. I am told the fuel pump is week and should be replaced. However, car performs perfectly. 55lbs seems to be consistent with non-supercharger. The pressure regulator sensor is new and working correctly.

Welcome to the forums Edwin E,

I've moved your question from General Tech Help to XK8/XKR forum. This is the place to post technical questions about your model.

Please follow this link New Member Area - Intro a MUST - Jaguar Forums - Jaguar Enthusiasts Forum to the New Member Area - Intro a MUST forum and post some information about yourself and your vehicle for all members to see. In return you'll get a proper welcome and some useful advice about posting to the forum.

Graham

Are you getting any codes with this, or is it just what you are being told? Just as a frame of reference I have been getting some lean codes lately so I have been monitoring it pretty close, and while running I'm getting exactly what you are ( with the occasional fluctuation when either really hammering on it, or slowing down quickly). I may be mistaken (I'm still learning around here), but I've been lead to believe that is exactly what we should be seeing while the motor is on. I'm sure someone else will chime in, and confirm, or tell me I'm all wrong.;-)

I think readings have been posted before so Search might find them and they may also be in the workshop manual - free download. Those values sound OK but my memory isn't great...

JTIS Fuel System Specifications section states:

Fuel system pressure - vehicles with supercharger 5.0 bar

Fuel system pressure - vehicles without supercharger 3.8 bar

(That's 72.5 psi and 55 psi)

Careful with psid (differential) and psig (gauge)

Should be ~55psi differential to manifold pressure and this is what you will see on a code reader or obd data logger while the car is running.

If you are reading with an external gauge referenced to atmosphere the number will read lower at idle speeds.

I got a number of codes caused by faulty pressure regulator sensor. Since replaced car runs absolutely great, no codes YET. However, I have been screwed and tattooed by San Diego Jaguar with Bank 1 & 2 lean codes and Performance Restriction. 15 days in shop $1,500.00 and 21 days in shop $1,100.00. Thursday at 11:00 AM I was going back to Dealership for $2,600.00 remove and reseal supercharger. I begged San Diego Superchargers (Bill) for one hour to diagnosis. Bottom line, 40 minutes problem solved and fixed. First thing Bill said 80% of the time with these codes the problem is a dirty mass air flow sensor. I suggest you remove your MAS use Brake Part Cleaner, not Carburetor Spray leaner, and thoroughly clean it. Spray Electrical Cleaner on connector and install it. It's just good maintenance and might solve your lean code problem.

We have same reading 55. 5 bars calls for 72 psi. Does the psi increase based upon demand when supercharger is fully activated?

Yes, fuel pressure is relative to manifold pressure, so, under boost, the actual fuel pressure goes up. If your supercharger delivers 15 psi for example (I do not know the actual pressure an XKR supercharger delivers), the relative pressure is 55 psi, and the absolute pressure is 70 psi.

I think things are a bit more complicated, too. If, under boost, the fuel system delivers 70psi, and you suddenly release the throttle to go in high manifold vacuum, I believe there is a valve at the pump that is supposed to "pop' and let the rail pressure go down. Folks have reported issues with that valve where it does not pop somehow, which maxes past the top of the range on the rail sensor (P0193). Maybe a can of quality fuel system cleaner can help and is a good start anyway. Otherwise, a new (EXPENSIVE) pump is required. There was a post recently where someone disassembled this setup and identified these pop valves, with part#, etc. To my knowledge, the XK8 and XKR parts are different, with the XKR popping at a higher pressure somehow.

Best of luck, keep us posted.

What fmertz said is spot on, and it is worth noting that on an OBD meter reading the fuel rail pressure sensor or with SDD that the pressure ALWAYS reads 55psi +/- regardless of boost or vacuum.

The sensor has a manifold reference and reports the differential pressure. This is the signal that controls the pump speed, trying to hold that 55psi differential at all times.

Thanks for confirming guys, even though it wasn't my thread. I was at harbor freight today picking up some other things (if you get the catalog the click torque wrenches are down to $12 with coupon), and I was eyeing up the fuel pressure gauges, but didn't purchase. Not knowing the difference between gauge and differential pressure would have probably gotten me even more confused. Needless to say with no pressure codes, and reading a pretty consistent differential pressure of 55psi from the OBD scanner, I don't think for me it feels necessary to start measuring gauge pressure at the moment.

John, if you do decide to clean the MAFS use only MAFS cleaner, not brake cleaner.

Thanks for that. I didn't want to clutter up this thread any more, but since you brought it up. I've already been seeking, and getting help, all though no final resolution, in another post on lean codes. I tried cleaning once with MAFS cleaner, with no change. I was actually wondering on the brake cleaner, as my understanding is that was more aggressive. I may give another go, at that, and another go of the smoke test to try and find air leaks. If neither of those produce results, it may be trying a replacement MAFS itself, the Denso ones seem to be running for twice as much as the Delphi, but seem to be more reliable.

Thanks, got enough info supporting 55 psi normal and supercharger psi demand goes up. Since car is running absolute perfectly, I am done.