Hi all,

I have a problem I can't seem to diagnose. My '87 XJS 5.2 will start and run fine, as long as I don't turn it off. This is intermittent, but happens way too often. What happens is, after driving for awhile and turning it off, the fuel pump won't start - no short run when I turn the ignition on, etc.

The car never dies while running. Sometimes, it will restart and run fine after a couple of hours - other times, it may take a day or two, then start and run fine.

I've swapped fuel pump, filter, relays, cleaned the screen in the surge tank, and it still happens.

Any ideas?

Thanks,
Ron

 

When the problem happens you need to check to see if there is a ground from the ECU to the fuel pump relay. The ECU provides the ground. I am suspecting there is not. You probably need your ECU repaired. Not a cheap prospect so I would do some tests just to be sure. Do you have a wiring diagram for your car? Also the main relay, which is next to the fuel pump relay< sends power to the fuel pump relay and the ECU provides the ground.

Go to the fuel pump relay socket(remove the relay). Put a test light across the Pink/black wire and the orange wire. Now have a helper turn the key to on. You should see if light up for 2 seconds. If it doesn't light up now put the test light from the pink/black wire to the negative battery terminal and turn the key off and back on. Then do the orange wire and the positive battery terminal, turn the key off and back on.

These tests need to happen when the car does not start. Report the results and I can help you sort out what those tests are telling me.

 

Hi Greg,

I actually replaced the ECU about 2000 miles ago, but then the car sat for a couple of years, until I dropped the front suspension to change the upper control arm. I'll take it out for a drive and see if it quits again, and perform the tests you recommend, then let you know the results.

I do have a wiring diagram.

I also have another ECU - it's out of an '86, but I think it should work, if needed.

Thanks for the help!

 

You could run a redundant ground to the f/p relay (orange wire) and see if the symptom disappears.

Be advised that in this configuration the pump will run whenver the key is "on"

Cheers
DD

 

Thanks everyone!

Figures that I took it out this morning and ran about 30 miles, and it didn't happen! I'll update as soon as I get it to fail, again...

 

problem = $299.99

intermittent problem = $2,999.99

just my $0.02

 

I have a 90 xjs 5.3 V12. Do you have a wiring diagram for this model?

 

Do you know where I can access a wiring diagram for the fuel pump/relay for a 90 model 5.3 V12? thanks!

 

How are you suggesting to run the "new" ground wire? from where to where?

 

Greg,
I tried your test scenario on my fuel pump relay. I had no voltage on any of the 3 connections you said to test (pink/black to orange, or the pink/black to neg, or orange to pos). What do you think is my problem? Thanks

 

If there is no power to the pink/black wire with the key on then you need to look at your main relay. it is the relay next to the fuel pump relay. That is what supplies the voltage to the pink/black wire when the key is on.

Key off. Main relay check for power at the brown wire.

Key on not running. Check for power at the white wire.