G'day Gents,

Since I put my V12 back on the road recently it has stumbled under hard acceleration.

The V12 X305s and the supercharged XJRs have two intank fuel pumps with normal running on the primary pump and the secondary pump activates on demand.

I thought it might be a clogged fuel filter so I replaced it to no avail.

I then checked the fuel pump relays, the primary pump relay is in the fuse/relay box in the boot/trunk and the secondary pump relay is on the forward wall of the boot/trunk in front of the battery.

Well I found the cause of the stumbling, the primary pump relay was in the wrong socket and so that pump wasn't running at all and the car was operating on the secondary pump only.

The car will start and run on either pump, I checked this several times by removing and replacing the two relays, one at a time. Interesting to note that in the event of a fuel pump failure the car will still run.

With both pump relays in the correct socket the car runs as it should, under hard acceleration the secondary pump activates and man does that thing move!

Cheers,
Jeff.

 

Wow, I didn't know that! Great that all you had to do was put the relay in the right spot. You did well to check the simple things first. The ECU must default to the secondary if it doesn't see the primary. Thanks for sharing.

 

Nice catch!. I'm looking at a 1996 XJ12 this week. They are said to be as quick as an XJR6.
Rob

 

Just as quick and allot smoother.

 

G'day Gents,

More to my story.

My car started to stumble and misfire when operating on the primary fuel pump and ran well under hard acceleration, indicating to me that the primary pump was getting old and tired, like me, so as the car has well over 200,000 miles on it and the primary pump would have been running for every one of those miles I decided to replace it and while I had the tank out I may as well replace both pumps. I didn't want to take the risk of only replacing the tired pump only to have the other one fail at some time in the future.

I didn't want to go cutting holes in the rear shelf of my car so it was going to be a tank out job.

Now as to getting the tank out, removing all the bits and pieces in the boot/trunk was straight forward but the pesky fuel lines were going to be a problem due to extremely tight access especially for me with big hands

I ended up disconnecting the propellor shaft at the diff end and prying it forwards enough to drop it out of the way. That gave me enough room to access where the fuel lines enter the tank. Special tool XYZ didn't work due to access problems so I used a piece of plastic tubing with the same ID as the fuel line OD and split it and fitted it around the fuel line and wiggled it and the fuel line in the correct manner and blow me down it disconnected!

Well one line did. The other line looked like it had been damaged and didn't enter the fuel tank at a good angle and I couldn't get it to release I followed that line back and it was the one that goes into the fuel filter so quite short and with a rubber section and even better I could undo it from the filter. I thought if I undid it from the filter I may be able to remove the tank with that line still attached and pull it through the hole in the car body.

Well with a bit of wiggling and and a few magical incantations I managed to get the tank out with the fuel line attached and once out I could then straighten the bent fuel line and release it from the tank.

Changing the pumps and then re-installing everything should be relatively straight forward.

Cheers,
Jeff.

P.S. I was a pilot for years and many of the aeroplanes I flew had duplicated systems the use of which alternated depending on whether you were heading to or away from home base. Now with the V12 and XJR6 Jags I know the secondary pump is tested when the IGN is first turned on but in general the car runs on the primary pump and hence my primary pump is showing signs of wear whereas the secondary pump would have only minimal operating time on it. As the pumps are computer activated it you would think it would have been relatively simple to program them to operate alternately.

 

G'day Gents,

Well the job is done and the difference in response and performance is astounding. The fuel line I had trouble releasing was twisted badly, it is a wonder any fuel was getting to the engine at all, and the metal fitting on one end was almost straight instead of having a 90 deg bend in it. The OEM hose is NLA so I replaced the flexible bit with high pressure fuel line and re-bent the metal fitting and it fitted just like a bought one.

As mentioned in another post the jet pump in the base of the tank was completely blocked with sediment and scale and once cleaned out and re-installed you could hear the fuel returning to the tank where previously you couldn't.

I'm very happy with the results of my efforts.

Cheers,
Jeff.