XJ S2 4.2 fuel pumps stop when swith from 'Start' to 'Run'

Dear all,
After a pause in working on my XJ S2 I resumed work. The car starts OK, with either of both tanks. As I crank, it takes some time to get fuel to the carbs and then it runs. Then I obviously release the key from ‘start’ to ‘run’, and the engine runs for 30 to 40 seconds and stops. It seems to me that the fuel has run out, because when I crank to start again, it takes time again to feed fueol and start. Also: I did a test disconnecting the fuel line and feeding fuel to the carbs from an external container; and in that case the engine keeps running.
I have been unable so far to identify what the electrical condition is that causes this stop feeding the pumps.

  • Both LH and RH pumps work in ‘start’ mode, and both pumps stop feeding as the key goes to ‘run’.
  • when feeding fuel in a way that doesn’t involve the car’s pumps, it keeps running. (the earlier problem with the resistor near the coil has been corrected after kind input froim this forum)
  • The ‘accident gravity switch’ is not in cause: I bridged that cable and the problem is still there.
  • it doesn’t seem to be related to the oil pressure detection: I grounded that wire to make sure.

Could it be the key-contact itself? But shouldn’t then also the ignition stop working?
Are there any other conditions that are being checked in ‘run’ and not yet in ‘start’?
The only condition I can imagine the car checks in ‘run’ but not yet in ‘start’ would be oil pressure. Could I have done my check wrongly?

As always, your experience and insight is very welcome.
Manyt thanks in advance to whomever may take time to chip in.

best regards

Mine is a Series 1 so I can’t offer much. But your mention of a (ballast I presume) resistor and your doing something with it reminds me of something. There is a wire from the starter relay that bypasses that resistor by supplying power to the coil ONLY when starting. If you got your fuel pumps connected there, that would explain your situation.

That shouldn’t be. The selected fuel pump should run with ignition on, and fill the carbies before you hit start. You should hear them clicking. This again is consistent with your powering the fuel pumps from the start relay. Just a thought–probably something else but I thought worth mentioning.

Something is ringing a bell here with me.
When I first tried to start the engine of my series 2 donor car it wouldn’t start.
After while I found that there was a fuel pump inhibiting switch on the block under the carbs switched by the oil pressure switch as a safety feature
I had to fill the carbs manually before the car would start and run. Maybe a red herring but I would check that switch is working correctly or bypass it or check the oil pressure switch and its connection

Yes! That could well be the problem. Starting in 73, the new EGR system caused run-on after the engine was switched off (as EGR did on other cars as well). Jaguar’s solution was to shut off the fuel pump based on the following logic: The driver had turned off the ignition AND there was still oil pressure. This logic was implemented by adding a second oil pressure switch that was OFF with no pressure and ON with pressure–just the opposite of the warning light switch. There was a special wire fed into the engine compartment (under the rear carbs) that was hot (12v) with the ignition OFF and off with ignition on. Don’t know how it was connected,

I thought it was so in a collision or some situation the engine switched off then the oil pressure would fall and the fuel pump cut out and prevent fuel spraying all over the engine bay.
IIRC the switch grounds the fuel pump so you could simply connect the wire to ground.

That came later, on Series 3s I believe. It’s an inertial switch that cuts the fuel in a collision whether the engine is running or not. IMHO.

I’ve got one of those as well in my 1978 series 2 as well. Is it the cylindrical switch on the LH door frame?
TBH I can’t see over-run to be allowed to go on until the carbs ran out of fuel and then having to manually fill the carbs to get the engine started. I can’t remember where I found the info about the switch.
Wasn’t there some sort of vacuum release anti-run on valve like most cars somewhere?

Yes, that may be it. It’s in the archives. The extra OP switch and special 12 volt wire with ignition off are there for sure. The fuel pump ground is definitely to the boot floor next to the pump on my 73. My memory fails with the rest because I “restored” it to Mark X specs within days of bringing it home. Quite a load of stuff like smog pump, EGR valve, strangleberg dual manifolds in the trash can.

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Maybe the switch grounds a fuel pump relay?


I’ve got one also in my early Jan. 1975 XJ6C on the RH side door jamb (LHD car). AFAIK the inertia switch was a safety feature on all S2 XJ’s (and S3 if course) but that would not be the original poster’s problem as if the inertia switch has cut off power the fuel pumps will not run in any position.

I did not know about the later EGR related oil pressure switch but seems logical. Euro spwc cars don’t have air pumps, carbon cannisters etc.


As Anjum says, Peter; it is likely the fuel safety relay - installed to prevent the pumps running with ign ‘on’ and the engine not running…

An oil pressure switch is fitted to the block; with oil pressure (engine running), the switch grounds the relay coil, which is powered with ign ‘on’ - running the pump(s). In ‘crank’ the safety feature is bypassed - and the pump(s) operate.

The most likely fault is the pressure switch; locate it at the block, it should have a green/yellow wire. W -ire might be disconnected, reattach the wire, or the switch may have failed - in the latter case; ground the wire. In either case; try running the engine - if it now runs as it should; trouble is located.

You may test the switch with an ohmmeter; with the engine idling, check resistance between the switch and ground - it should read ‘0’. If not; the switch should be replaced - though it may just be clogged at its oil inlet, and may recover if cleaned.

If the switch is not the problem; it may be the relay itself - to be further investigated…

If the engine is fitted with the anti run-on circuit; there will be two pressure switches on the block. Both switches works the same way; grounded with oil pressure. But the anti run-on switch is powered from the ign switch with ign ‘off’ and grounds the anti run-on valve to let manifold vacuum into the carb bowls. Though additional gadgets, like the EGR mentioned by Robert may complicate matters…:slight_smile:

As an aside; unless the engine has been standing for a (long) while, there should be no need for the pump to feed the carbs for instant starting. Otherwise; the delayed start is not caused by pump action - it is a cold start problem. Either by failing ASC/AED, whatever fitted - or carb adjustments…

xj6 85 Sov Europe (UK/NZ)

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Right, there is no mention that the pumps work with ignition on before trying to start. Maybe the pumps operate ONLY when the key is in Start position. That would also explain the delay in starting whilst turning the engine over.

Frank Anderson’s excellent analysis reminds me of a (now, in retrospect) amusing problem I had more than a decade ago. I had just fit triple HD8s (from a Mark X) to my 1973 Series 1. But the cold weather arrived early, and I needed a quick way to power up the “hisser” starting device. The thermostatic (Otter) switch was defective, so I needed to operate from a switch in the cabin. Being short of time, I decided to remove the wire to the AC compressor clutch and splice it into the hisser’s solenoid. Switching on the AC would then operate the hisser, which I confirmed by hearing the hisser’s click. But the car wouldn’t start. I posted my problem, and a reply from Frank explained what I never thought of. Putting the key in start position disengaged the AC compressor to reduce load on the starter.

Dear Robert, Frank and all other Jag lovers who chipped in,

Thanks for your advice. After reading your input I have a few leads to go on. The first ones I’ll check on will be the relay and the contact from the oil pressure sensor. I have an XJ6C in good order and I’ll borrow the relay as a reference check.
This project is going slow now due to some ‘day job’ priorities, but I’ll post an update as soon as I have news.
Thanks again to all and have a nice evening.