Hi Moose

If I remember correctly, there was a big discussion on this where those who know concluded that iridium plugs were not the best choice for the XJS V12 that runs better on OEM Spark Plugs

Greg and Grant can hopefully join up the dots, as it's probably got something to do with the Temperature and the operating environment

So since they are the acknowledged experts on this sort of stuff, I don't see anything to be gained for going against their recommendations

I use NGK BR7EF that as I remember were quite hard to get hold of at the time

 

OEM it is then good sir! Thanks for the info! I love this forum haha.

 

One concern given to me about long life iridium plugs is that if you leave them in a V12 for (say) 40,000 miles you might not be able to get them out.

 

Hi all.

I have a new Bosch fuel filter but doubt I will be able to fit it until the weekend. I will of course keep you all updated.

Rob

 

Rob,

You've mentioned that the problem happens under wide throttle openings after c40 miles. However, I don't remember if you've said how long it takes after turning off before the problem disappears and the car runs normally again?

Although it doesn't cover all scenarios, my experience says that air or fuel pressure restrictions dissipate relatively quickly after being released. Whereas heat buildup causing a problem through electrical circuit or component breakdown, takes longer to dissipate. (The failing Opus ignition system on my Rover SD1 would take 4-5 hours after overheating before the car started again)

PAul

 

My fault. It was there....I hate that signatures don't appear on the iPhones.

I apologize!

 

😀 nothing to apologise for Vee. I didn’t know that either

Rob

 

Hi all. Just started pulling it apart and have just pulled one of the relays. This one doesn’t have the red stripe so I assume it’s the fuel pump relay. Red stripe is the main really? What you guys think it looks like it’s been getting hot judging by those spades.

Rob

 

Hi Robbo

Looks like that Fuel Pump Relay in the Black Holder (without the Stripe) could be the cause of the problem

Fuel Pump Relay and Main Relay Location

 

Would replacing the main relay be a good idea as well as it’s the original in there.

Rob.

 

Rob,

You might do it as preventative maintenance but I can't see how it could be the cause of your problem.

Also, whilst your fuel pump relay may be failing, the symptoms don't feel to me like it's a fuel pump issue, as I would expect you to be hearing strange fuel pump noises if it somehow it can't handle the wide throttle load. But, hey, what do I know!

Paul

 

Hi Paul. I was thinking for preventative maintenance as it’s still on the original relay. I have pulled it and all the pins look fine. Just the fuel really been getting hot then and looking at the two pins very hot to discolour them like that.

Rob

 

If the relay contacts look like that, the socket contacts will be just as bad or worse. Use Deoxit or other electrical contact cleaner to attempt to clean them. It may be best to replace the socket as well.

The two relays are different inside. Be sure to get the correct ones.

 

Hi Robbo

As Relays are quite inexpensive, you may as well change both, as the Fuel Pump Relay won't work unless the Main Relay is working as well

The Fuel Pump Relay was the cause of the No Start problem on my XJS 4.0L and so it looks like that could be the problem on your Car as well

The Main Relay with the Red Stripe has a Diode that protects the ECU if someone were to hook the Battery up wrong (or something like that!)

Main Relay and Fuel Pump Relay are not interchangeable

 

Sounds like it may be an ignition coil breaking down when hot to me.

 

Hi guys.

Sorry it’s taken me so long to come back to you all on this thread but I have been busy with other things. Anyway an update for you all. New main and fuel pump relays fitted along with a new Bosch fuel filter. I also refitted my new fuel pump and decided to put my system back to stock by removing the previously fitted pre filter and putting back the original sock filter in the surge tank. Out to a car show tomorrow so will do around 100 miles so will see how she goes and report back.

Rob.

 

Hi all.

Time to report back. After my 100 mile round trip today I’m glad to report the car is now running fine. A combination of things I think caused the problem notably the pre filter I fitted between the main tank and surge tank which I was told had a fairly coarse mesh filter turned out to be paper and this was slowing the fuel flow to the surge tank causing the starvation issue. And also the faulty overheating relay. Moral of this story is sometimes Jaguar did know best and to leave things as standard. 🤣🤣

Rob

 

O