Hi, I have a 1994 4 liter XJS and having a problem with it. It randomly dies and then attempts to restart but will not stay running. The fuel pump was checked and replaced last week since the pressure was only about 12 what it should be. I drove it about 30 miles after the replacement and then it died again. Pumping the fuel gets it to restart. It runs very rough and then putters out in about 30 seconds. Anybody have any ideas? Thanks in advance for everyone's help. Steve

Submitted by sodium@captain… on Mon, 07/16/2012 - 11:32

I KNOW this problem. Your main power relay is failing.

Here's the good news, you've got a lot of identical relays in the car. So getting unstuck is easy.

The main power relay is on the passenger side fuse panel in the foot well. It's in the middle of the panel and not particularly well labelled. There's 3 relays there in my recollection. But if you start moving relays around this may solve your problem. I pulled one from under the hood during my troubleshoot for the aux fan in my recollection.

Let me hunt around and see if I can find the panel diagram in my manuals.

It definitely acts like a fuel pump issue or fuel pump relay but it's the main power relay. As near as I've been able to determine the problem develops over time by oxidation on the contacts to this relay. It's responsible for energizing both the ECU's logic circuits and the ignition circuits. The ignition power draw will cause the resistance in the contacts to drop the voltage below the ECU's operating threshold and it dies.

Alternately, another possible issue is the crank timing sensor on the front of the engine. These induction sensors can become flaky when hot.

So lots to check. The cheapest, easiest fix/test is to move relays around and see if that resolves the problem. Note: mark the relay with tape so you don't lose track of it.