2003 xj8 111,000 miles
Fuel pressure of 33 at idle and 38 with ignition on and engine off. It's about 5-6 psi below spec. Fuel pump replaced 8,000 miles ago. What could be causing this?

 

Did you replace the fuel filter?

 

Yes, 4,000 miles ago. Forgot to mention that.
With no signs of a fuel leak, I'm thinking that the only other things in the fuel system loop are the lines themselves and the regulator. Is it possible that it's a failed regulator producing low fuel pressure and yet still varying the fuel pressure according to engine vacuum? I've never seen that, but then again there's a lot of things that lad hasn't seen. Am I missing anything else?

 

Use a mechanical gauge to read the fuel pressure at the engine fuel rail.

 

I did use a mechanical gauge at the fuel rail. . .

Is there some alternative method to measure fuel pressure?

 

That's pretty much the only way, are you getting faults relating to low fuel pressure because I would think that's in spec' - other than a clogged filter in the tank attached to the fuel pump?

 

No fault codes. I've getting some odd fuel trims (+10 lean on acceleration) and occasionally the engine hot starts and the idle is very uneven and below normal rpm (and lambda sensors say it's rich).
Nothing seems bad enough to actually cause the issue so at this point I'm just grasping for straws.

Probably going to do some routine cleanings: fuel injectors, throttle body.
I'll check for vacuum leaks for the third time too.

Would it be worth it to grab a cheap fuel pressure regulator and see if my pressure goes up, or is it very unlikely to be the issue?

 

There is a rubber diapham in the FPR so they don't last forever

They are available at the local auto parts store for around 75 dollars

There is a coolant temp sensor to change your ECM from open cold engine loop to closed full warm engine regulation loop using the O2 sensors

You can calibrate this sensor without removing it against a chart from Jaguar

Sometimes the thermostat stays open and the coolant does not get hot enough

If your coolant temp sensor reaches above 88 C with a volt / ohm meter, then the thermoset should be working properly

Your fuel pressure regulated is 43 - 47 PSI , this will be at your engine fuel rail test port

Fuel pump relay swap and pump motor connector check ?

 

Water contamination in the fuel can obstruct the fuel filter regardless of its age. I would personally replace the fuel filter again just to rule it out.

Low voltage at the fuel pump could cause low fuel pressure. It would be worth cleaning all the battery power and ground connections in the trunk including the battery cables, fuse box power connection, all the megafuses and terminals at the high power protection module, and all the ground points in the trunk. This is just good preventive maintanence on a Jaguar. You can find the locations of these components and connections in the Electrical Guide:

Jaguar X308 Electrical Guide 2003

Also see Rob's (avt007) excellent electrical troubleshooting guide, which I am attaching.

If your fuel gauge kit has an adapter that can connect to the fuel line threaded fittings where the filter connects, you could test the fuel pressure closer to the fuel pump and with the filter out of circuit.

Cheers,

Don

 

That's a really smart idea mate. My kit doesn't have that adapter, but does anyone know what that adapter would be so I could buy it?
If all else fails I suppose I could chop up my old filter and get the local clown to do some welding.

 

I would be careful about running the pump if its outlet flow is completely blocked by the pressure gauge. These pumps are usually "positive displacement" type which, if the output flow is blocked, may develop extremely high pressure. It may cause the pump to stall (motor to get overloaded) or something inside may even brake.

 

Hi M.,

That is a valid concern if the fuel pump is running into a dead end, but you only need to run the pump for a few seconds to determine whether the pump can supply sufficient pressure. The reading when measured at the pump outlet will typically be above specified system pressure as controlled by the fuel pressure regulator - typically 55-65 psi.. I haven't had any issues testing fuel pumps this way, but I am careful not to run them for long to avoid overheating or stalling the pumps.

This is the type of fuel pressure gauge adapter I was describing:





With a new fuel filter installed, asdf can more easily test the pressure again at the fuel rail with the fuel pressure regulator vacuum hose connected and disconnected.

Cheers,

Don

 

I am not certain I would be overly concerned about that PSI unless there was
some accompanied behavior that is not described,

 

That's a good thought, Randy.

In my home-made fuel injector cleaning apparatus, I have noticed that a difference between pressurizing the fuel injector cleaner solution at 30 psi or 40 psi can be the difference between an injector that just squirts a stream of fuel or one that properly atomizes it. So I do think a deficit of 5-6 psi in fuel pressure could cause suboptimal engine peformance, even if the difference is subtle and no DTCs are triggered.

Cheers,

Don

 

Is the TPS doco valid for pre HE engines as well?