I own a XJ8 sovereign V8 lwb 1999 registered. Had poor engine performance as though it was running on 6 cylinders only. Jaguar dealer diagnosed 2 faults, new plugs, coils and seals and change both cam cover gaskets. This was done and performance was much better but not perfect. They have now established poor fuel pump pressure so as a temporary measure clamped a jubilee clip around the fuel return pipe (located on the top of the engine by the bulk head) this has constricted the pipe thus increasing the pressure. The engine is now running perfectly with lots of power.
They therefore recommend changing the fuel pump and filter but also the recommend changing the fuel regulator and filter. My question is, is it necessary to change the regulator as well as the pump. Why have they suggested the regulator as well or is it necessary to change anything. I hope someone can advise me. Thank you.

 

I would suggest changing only the filter first and see if that solves your problem. Then go from there.

 

Richard, just my vote. If you've got lots of miles and don't remember when the fuel filter was replaced it's probably time to do that (maybe past time). Mine had 80k miles and the old filter was noticeably heavier than the new one and the gas that came out the input end when I tipped it after removal was pretty dirty. That said, I don't think the filter is your problem as the clamp was what changed your symptoms.
The curious thing is that they were able to boost the pressure with a restriction on the return which suggests to me that the pressure regulator that isn't working correctly. After the filter I'd start by replacing that.
Of course they could be right about the pump but they're a pain to deal with (IMHO).

 

Try the external fuel filter first as it is the easiest/cheapest.

But then think of your next move.

The fuel pressure regulator is supposed to regulate to the
desired pressure. Therefore a function fuel pressure regulator
should allow pressure up to the regulated setting. If it needs
a jubilee clip to increase pressure, then something is wrong.

What you really need to establish is the actual fuel
pressure being seen at the schrader valve on the fuel
rail with a fuel pressure gauge with the jubilee clip
removed and in place.

If the fuel pressure is in spec without the jubilee clip
then the clip is being used to raise the fuel pressure
above spec to overcome some other problem.

That other problem might be clogged fuel injectors
on one or more cylinders. The answer there would be
a fuel injector cleaning, not a new fuel pump or
regulator.

If the fuel pressure without the jubillee clip in place
is out of spec but in spec with the clip, then the fuel
pump is able to build pressure, but the fuel regulator
is not holding it.

So the first order of business is to think through how
the system works, and what might cause the symptoms.

In the UK apparently TerraClean is well regarded as a
fuel injection cleaning system. If no one chimes in,
the administrator here JimC64 is well versed on TerraClean.

 

Many thanks

 

Do you suggest changing both filters, fuel pump and regulator filters. This is the cheapest option to start and I'm not sure when or if they have ever been replaced so is a good idea anyway. With the symptoms and dealer temporary rectification I've already explained is it more likely the regulator is at fault than the fuel pump ? but bearing in mind the age and mileage 108k would it be prudent to replace everything as suggested by the dealer. Is this work for dealer only or is it possible for myself.

 

Your symptoms fit the regulator or the filter, better than a faulty pump. In my experience, the fuel pump has two states : fine, and dead.

 

Do you suggest change regulator and regulator filter

 

Richard,
I was thinking of the can type filter which is above the left side rear axle on my 99 XJR. There are sock filter(s) over the intake(s) of the fuel pump(s) but if you're that far in you might as well fit a new pump, you'll have already done a bunch of work to get to that point.
I don't think the regulator has a filter, at least not a serviceable one. Changing the filter is just good preventive maintenance, especially if you don't know when it was changed last. If a clamp on the return helps your symptoms though I doubt the filter is really at the root of your troubles.
I'd also be suspicious of a partially clogged injector being fixed by the addition of a simple clamp but it could happen. Injector cleaner is an easy thing to add but you don't want to over do additives. Plums is right though, no harm in understanding the system and checking a few things like the fuel pressure with and without the clamp before you start throwing parts at the old girl...

 

Just the filter, as recommended by 2, 3, 4. I agree with Mark: fuel pumps are digital -- yes or no.

Maybe a cleaner for the injectors, as well: BG 44 is the gold standard in the US for in-tank cleaning. I use SeaFoam.

 

Thanks Jim,


I agree with you. After talking to a number of jaguar service specialist they suggest only a filter change at this stage. They confirm that fuel pumps are either working or not and do not progressively get and loose pressure. If there is a loss of pressure then it is more than likely the filter but if not I should investigate the pressure regulator. Also injector cleaning as you have advised is advisable.
Thank you for all you help. Richard

 

The problem continues.
So, in summary. the jag dealer originally said I needed to replace 8 coils 8 plugs. gaskets and associated cam cover and plug seals. This was done and some improvement to engine performance, a jubilee clip was then fitted to the fuel return pipe constricting it. This significantly improved engine running to perfect. So the poor running was cured by the jubilee clip but the problem not resolved. Jag dealer suggested change fuel pump, fuel filter and fuel pressure regulator, but as many forum members suggested I should start with the fuel filter only, bearing in mind the fuel pump was replaced only 3 years ago.
So I have just replaced the fuel filter and removed the jubilee clip. The engine was back to poor running. I put the jubilee clip back and it was still running poorly............so what's the answer.......do I still need to think about the fuel regulator ? Did the jubilee clip need to be put back in a certain way ie when engine off or on. Just seems strange engine was running perfectly with clip on and now put back is running poorly. I need to be sure I'm on the right track. Someone please help !!!!!
I've lost confidence in the jag dealer who seems to just want to replace everything until it runs properly.

 

Please explain what you mean by "installed a jubilee clip"? Was it to unkink a hose? Or to constrict a hose? Either makes little sense and sounds like a cludge to me.

If the return pipe is constricted and that is what makes the car run better, then your problem is clear- the fuel regulator. The regulator works by releasing fuel back to the tank to maintain a pressure. If it is leaking back and not maintaining pressure, then constricting the return would raise the pressure, although not in a controlled way.

Sounds to me like you have a hack working on your car.