Cold start, 10-15 C
I drive away shortly after starting the engine and gets to a stop after maybe 200 meters. I have now twice noticed that the (noisy) fuel pump stops running for a fraction of a second. Engine runs without problems, but the signal telling the fuel pump it shall run must be missing for a short while.

Where do I start my search ?

 

Hi Leo

First you need to test the Fuel Pump Relay.

(1) Remove the Two Wires from the Fuel Pump

(2) Connect a Test Light to these Two Wires

(3) Put the Test Light somewhere you can see it while you are sitting in the Car

(4) Turn on the Ignition while you are Watching the Test Light (But don't attempt to Start it)

(5) The Test Light should Light for about 3 Seconds and then go off (of its own accord)

(6) In your case, repeat this at least half a dozen times.

(7) The Test Light should Light for about 3 Seconds and then go out of its own accord (every single time)

(8) If that happens then the Fuel Pump Relay should be OK

(9) Any variation, may point to a Faulty Fuel Pump Relay but 'Could' also point to a Fault with the Main Relay which controls the Fuel Pump Relay.

(10) These Two Relays are behind a Plastic Cover in the Boot/Trunk (see photo)

The Black one is the Fuel Pump Relay and The Red One is the Main Relay.

If possible try fitting a new relay if you have a Spare one somewhere.

DON'T Swap the One with the Diagonal Stripe for one without as the One with the Stripe is fitted with a Diode.

Even if you don't have a Spare, take the Relays out of their Holders and thoroughly Clean the Pins and Spray not only the Pins but also their

Sockets with Electrical Contact Cleaner.

That on its own might be enough to cure the Fault.

So Remove the Leads from the Test Light, reconnect them to the Fuel Pump and then Try the Car.

If not report back, as it could be a lazy Fuel Pump or even an FPR or something else.



Location of Fuel Pump Relay.
Black is the Fuel Pump Relay.
Red is the Main Relay.
The Main Relay also Controls the Fuel Pump Relay.

 

Can you test for power to the pump? If the pump is noisy I'd suspect that the pump is bad and about to fail.

 

Leo
The intermittent pump will most probably be an electrical supply problem. Clean all the connectors to the pump, clean the boot earths, replace any tired or old looking wiring. Be particularly attentive to the main feed to the pump. As OB said, a relay problem is quite possible. I think replacing the fuel pump relay is a very sensible idea after 40 years!
Greg

 

Thanks all

As it (so far) is only happening at very specific conditions: When I lift of after 200 meter of driving from a cold start, I was thinking, that the system that allows the pump to run was failing somehow ?

The pump runs for around three seconds when I turn on ignition, then it cuts out. This is normal and as it should be, but what is controlling this cut-out system and allows the pump to run, when the engine is running ? Is it an ignition signal, vacuum or something else ?

And yes,I should just change the noisy pump, but I'm pretty sure it's not the pump, that fails as it always works, except from under these exact conditions. The cutout is short, but noticeable.

Changing the pump will make sure I dont notice the problem anymore.
Problem solved ? Probably not

Cheaper aftermarket pumps has been mentioned. Could someone post a link to one of those or the specs I should go for ?

 

I would do as Greg said first, clean all the connections, remove the main and pump relays and clean the terminals and sockets.

There are a bunch of earths down beside the battery, pull all of these off and clean the metal of any paint and/or corrosion, clean the terminals and put them back with star washers between the terminal and body. Then cover the whole lot with seam sealer or marine terminal sealer.

 

Maybe its old age, or I am losing it.

You said that the "usual noisy pump goes silent, and the engine runs no problem", so If I am not losing it, the engine does NOT cut out, just the pump goes silent from noisy.

NO problems, just all the rollers lining up as designed, and the pump goes silent.

On any EFI engine, it the pump even "stutters" so does the engine. If the pump stops rotating, the engine also ceases rotation.

Unlike carby cars, where the supply in the carbies will maintain engine activity for about 5 minutes after the pump stops.

The item that switches OFF the pump after that few seconds, is the timer circuit inside the ECU. If you "think" this is the issue, simply Earth the Orange wire from the relay in the Black socket, and that will remove the timer circuit, BUT, the pump will run whenever the Ign is in the ON position.

 

The ECU (I have no idea what sort it is in an early XJS) controls the fuel pump relay signal. It is just possible your ECU does not like the cold! It is also possible that the signal the ECU receives from the ignition that says the engine is running, so keeps the pump running, is iffy in the cold. But this is an HE diagnosis, not a pre HE.
Grant's the guy to explain to us how that works!
But do the obvious basics first, as Warren confirms.

 

You got this part right, Greant, but the pump noise returns a fraction of a second after disappearing. My pump is always noisy when it runs, and the maybe 3/10 of a second without noise does in my opinion mean that the pump is cut out for a brief moment. I have then just learned, that this is controlled by the ECU, thanks.

The very short cut-out of the pump is not enough to stop the engine, so it runs fine afterwards.

I'm just afraid this will get worse (longer cutout) and actually cause the engine to stop

You're back in the game: My car has been updated with an '89 HE driveline, and the ECU is the 16-something. Perfectly common

 

Leo,

That 1977 with the 1989 HE is I assume a Lucas system????.

In my time, even a heartbeat stutter of the EFI pump, does show up in the engine also stuttering.

I would be more concerned with a tired old pump actually stopping for good, more than whatever is keeping it going.

 

Yes, it's Lucas

I will get that pump changed. CBC5657 as far as I could find. David Manners has pumps at 35, 92 and 263 GBP.
35 seems too cheap. Any experience ?

 

Same pump as a lot of BMW, Merc, Ford, GM (mainly Aussie pretend cars, ha), so NOTHING special in any meaning of that word.

1/2" inlet, 8mm (5/16) outlet, external mounting.

Just watch the polarity when reinstalling. Some have the "large" terminal as the +ve, and some have it as the -ve. Pump will work either way. The wrong way will push fuel back to the tank, whoopee, and the right way will make you smile.

The sell here in decent brands for $120 or thereabouts.

 

The 92 one is OEM manufacturer without the box! Buy that one. Just fitted one to my mate's car.
Thanks for putting me right about the upgrade to HE
Greg

 

A few months later.

Fuel has been emptied out, fuel pumps (new and old) is on the work bench, and the new Bosch CBC5657# is not very similar to the old Lucas pump.
Old pump has larger diameter (ø57mm opposed to ø52mm), different electric connectors and most importantly: The outlet is straight instead of angled 90° as the old one.
Could you show me pics of your install with this pump with inlet/outlet inline with the pump axis ?
It cannot just sit in same place as the outlet goes directly into the boot floor even before I start thinking of mounting a fuel hose.
As the outlet is plastic I would very much like it to be fairly well protected against anything in the boot, so I guess the best solution is to still point it towards the boot floor and then just move the pump up, angle it a bit and make some cover plates to prevent it from being damaged in case spare wheel needs to be used.
I do by the way have my fuel filter in engine bay as the car is (mostly) PreHE, so a PreHE picture of fuel pump area would be really great

The old setup was just nice, protected and compact. Annoying it is !

 

Got it mounted using a small custom bracket, but I’m not really happy. Suction line does now protrude further back than before.

View from spare wheel location looking right

 

I can't recall ever seeing a fuel pump with a 90 like that. You might be able to use a short piece of hose and then a 90 degree fitting to make the turn?

 

you may have seen my pump,and filter assembly+ fuel tank Plastic!

when i got my XJS it was a mess of rusty junk did not work , every thing was damaged!

it also runs continuous, 23years all OK !

 

Thanks Ronbros

I was planning for something with a slightly more stock look, but it surely looks great

I actually got feedback on Facebook on same question, and I will of course share the pictures with you all here:

 

Leo,

Very early PreHE fuel pump you had in taht car, well done on one still working.

The replacement is the replacement, and in the early days, the suppliers included a thicker foam mounting pad, to compensate for the diameter change.

The HE also had a supply Hi pressure hose with a moulded 90 deg bend, so the pump sat in the same place and the 90deg allowed the suction hose etc to remain as was.

I worked on mine, HE, until I got the pump in its OE bracket and the hose "just right" so the bend was tight enough without kinking, and I am referring to standard EFI replacement hose off the roll. It will "curve" more than you think if you take time with it.

The HE also a tin cover arrangement to cover the pump, and keep the spare wheel from clobbering it.

 

Thanks Grant

I will save the old pump. It will be worth a fortune in 15-20 years...

I had some leftovers from the padding used under the fuel tank, so I added a strip of that to fill the gap. Works just fine.

I don't see that it could be mounted in the original position as the outlet from the pump touches the boot floor, and that's before we start talking about mounting a hose on the outlet.
It sits OK now except from protruding too far back, fuel line routing is good. I will look into placing it as the blue pump I showed above, I just need a new (and longer) fuel hose to be able to re-position it like that

BR Leo