Steve might save yourself $60.00
Bosch Motorsports Fuel Pump # 0 580 254 044 [BFP-044] - $199.00 : Jay Racing
I think you can get the in tank pump 040 for about the same price

 

Haven't seen yet a 4.2 pump but the 044 and 040 (which I have) are pretty big pumps in size and I doubt this si used in the 4.2 Jag. I can remember someone had taken the pump out of the plastic unit it sits in, not sure who but if he can chime in, can give the dimensions. That would already be a first check. But if you go for a different pump, you must make sure the output is about the same as the roriginal one. Checkout DTC P0191, that will show what the ECU checks.

 

So the ECM just checks injector pressure.
If as the manual claims the ECM can vary the PWM mark/space ratio from 4 to 50% to control flow that's a pretty generous feedback margin.
Seems to me that it would be easy to disconnect the in-tank pump, jury rig the Bosch pump with a 5L can of petrol and a pipe to the filter and go for a drive.
Maybe, when the weather's a bit more DIY friendly, you'd consider lending me a pump (assuming you're not using it) for a week or so to try. I'd pay return carriage of course.
As I said earlier there's nothing wrong with my pump at the moment but I don't have the Rev Sams facility (or flexibility) any more and a simpler and much less expensive solution is worth exploring. If it works we're all potentially better off.

 

Pumps are in use, wouldn’t want to miss one now.

There’s also another test you could do which will give the same result, you could increase the PWM voltage to the pump (so the wire between the control unit and the pump) as that will increase the flow. You need to be comfortable with electrics, this is not really my area, but my simplistic mind tells me this should be possible.

The ECU will automatically regulate the pressure based on the FPR input, then based on engine load it compares the actual PWM % to the expect value, if outside the predefined threshold, it will trigger the P0191. So you could find here the actual threshold (/margin) the ECU uses. Only way I see to solve this is with a threshold change in the programming. It would be great if you could already find the margins by varying the voltage.

 

Shame about your pump - I'll try to find one locally. The village carsmith is heavily into racing so might have one.
I'm not sure about your idea - according to the manual
The ECM use a frequency of 150 Hz (PWM) signal during average conditions, varying its duty cycle between 4 and 50% to control fuel delivery rate or turning it to 75% to turn off the pump.
so the ECM will be putting out a square wave and (presumably) all the rear control module does is turn that into a 12V high power output to drive the pump.
-or am I missing the point?

 

The pump is getting a 12 volt PWM signal, square as you say, so on/off all the time. So you need to change the voltage that goes to the pump (the on/off), either up or down, although up would be best as most pumps would be able to pump more but then you need to have a converter. Down is easier by place a resistor in the line.

 

Certainly putting a resistor into the PWM feed of a higher rated pump would work if the ECM was bitching about too much flow. Good idea.
I won't try it on the in-tank pump JIC undervoltage kills the windings

 

Presumably if you were to connect an oscilloscope across a known good pump and record the readings in different conditions, you could use these to monitor the condition of the pump from time to time.

Or do the pumps usually just fail without warning?

 

From the posts here they just go open circuit.
The problem with a 'scope would be establishing a baseline for the mark/space ratio measurement. I don't know enough about the system to say the delivery volume should always be constant at no load + 1000 rpm or some such, but it's an interesting thought.
I've got a storage scope package for my laptop so I'll plug it in and drive around to see what I get....when it stops raining!!

 

These little $49 scopes are cheap enough to install permanently

XMEGA Xprotolab | Development Boards | Gabotronics

 

Amazing. The ATMega can be a bit noise prone but maybe they've got that sorted for this application.
I've ordered one for fun.

 

I have one and it works pretty well. The four button user interface is a bit awkward, and my old eyes sometimes need a magnifying glass to read the tiny display, but he packs in an amazing amount of functionality...and continues to upgrade the firmware regularly. I use mine all the time for low speed, non-critical work.