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Matt
Matt, Engineer
Category: Jaguar
Satisfied Customers: 20590
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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I am having issues with a Jaguar X300 XJR. Originally

Customer Question

Hi, I am having issues with a Jaguar X300 XJR. Originally wouldnt run better than "utterly terrible". Replaced all coils & CPS, some improvement. Then replaced Idle air valve, air temp sensor & replaced some inlet hoses which were leaking, and also replaced the MAF. Now runs better still, slight misfire, but wont rev over about 2000 rpm under load as when accelerating, result in kind of back-fire. After this, the engine revs remain high (pulling the car along like the throttle is depressed) or if in neutral the revs are high (about 2k).
JA: Have you checked the connectors on the sensor itself? It's important that they not be loose or corroded. What year is your XJR?
Customer: At the time I did the idle valve, I re-wired the engine sensors to bring them all around the back of the engine, instead of through the inlet manifold (future-proofing in case I need to get to the sensors again) at the same time I replaced any suspect wiring and connectors. It is a 1996
JA: Are you fixing your XJR yourself? What have you tried so far?
Customer: Replaced all coils & CPS, some improvement. Then replaced Idle air valve, air temp sensor & replaced some inlet hoses which were leaking, and also replaced the MAF. Now runs better still, slight misfire, but wont rev over about 2000 rpm under load as when accelerating, result in kind of back-fire. After this, the engine revs remain high (pulling the car along like the throttle is depressed) or if in neutral the revs are high (about 2k).
JA: Anything else you want the mechanic to know before I connect you?
Customer: no
Submitted: 15 days ago.
Category: Jaguar
Expert:  Matt replied 15 days ago.

Hello

This sounds like a lack of fuel and could be a lack of fuel pressure from the electrical pump at the tank.

1st thing to try is to remove the fuel filler cap and then flick the ignition on whilst listening for the brief ‘buzz’ of the pump

If the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,

Ideally measure the pressure at the injector rail (should usually be around 3Bar / 45psi +) and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.

You can also get this issue with contaminated fuel , if there’s water or dirt in the tank then either the petrol floats on top of the water or the filter sock on the pump is choked up – so if the other things check out then I’d consider pulling out the fuel pump and cleaning the filter sock

And one other thing to try is to loosen off the fuel filler cap, as there’s a 1 way valve built into the cap to let air into the tank and so prevent a vacuum from being created as the fuel leaves the tank to the engine

If this helps then replace the cap with a new item

Customer: replied 15 days ago.
I'll certainly try this in the next few days, however, I don't think this explains the high-idle condition after the backfire. Any thoughts on this?
Expert:  Matt replied 15 days ago.

hello

I've known a high idle to be created post misfire due to the throttle blade being 'popped' open and it then needs a throttle blip to resettle it

another possibility is the 1 way valve in the brake booster line is faulty and its drawing air in that way

Customer: replied 12 days ago.
Hi Matt,
Just to let you know where I'm up to:
Opened fuel tank & emptied contents of all old fuel & dirt, cleaned pickup pre-filter (seemed ok) inspected both fuel pumps, tested them on bench & they both run at the same speed & pull same amperage when running.
Replaced main fuel filter, appeared to be reasonably clean & not all too old, but there's a new one in now.
I have checked visually the pump solenoid, which seems to work as intended, all wiring, connectors & fuse are clean & functioning. Have ordered a spare solenoid so can replace that when it arrives & keep the old as a backup.
Took your advice & checked for any leaks on the brake servo, all seems good. To eliminate an internal diaphragm leak I pinched off the vac line before attempting to start..
SO!
Fresh fuel in car... start her up &... no change. Still running as before.
I have not been able to test fuel rail pressure, hopefully my fuel pressure tester will be with me by the end of the week, so I can eliminate this soon. I will keep you informed of further activity.
Regards
Expert:  Matt replied 12 days ago.

Hello

lots of good progress there, OK assuming that the fuel pressure may be OK then it would be worth pulling out the O2 sensors on each precat so you can peer inside at the surface of the cat as it should be even with no lumps or bumps and the 'honeycomb' a regular shape

Customer: replied 5 days ago.
Hi, quick update, though there's not too much to report, the weather over here is freezing and working outside in these temperatures with rheumatic hands is slow at best. One thing I did manage to check was the fuel pressure, is around 36-38 between tickover and higher revs (with car stationery, cannot check whilst under real load) One thing I notice is that the car will rev ok if you allow it time to get there, but, if you floor the throttle revs might rise quickly to 2-3000 then the engine cuts-back to idle, engine light comes on momentarily. Fuel pressure ok whilst this occurs.
I am having issues with checking the condition of the pre-cats with the method you suggested. the position and orientation of the lambda bung will not provide easy visual access to enable any meaningful observations to be made. Is there another way of checking? I've heard of people using non-contact thermometers to check temps at pre/post cat, are you aware this might work? Other than removing the assembly from the car entirely, I am at a loss. Do you know if the catalyst core can be internally removed without initiating an engine diagnostic condition?
Regards, Simon.
Expert:  Matt replied 4 days ago.

Hello

Ok so the fuel pressure is fine, the other option to check the cat condition is to use a 'borescope' to poke down the exhaust and through the sensor holes and so peer inside

they used to be very expensive but these days you can get a cheaper version which are perfectly good enough for the home mechanic

here's an example

https://www.tester.co.uk/testsafe-9mm-borescope-camera?fee=3&fep=6647&gclid=CO_Y_vbA0tACFfUW0wodMIgMRw

or even ones that plug into your phone or laptop

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