I have just purchased a 1977 Series 2 XJ6 4.2. When test driving the car, the oil pressure gauge was recording the recommended pressure of 40psi at 3,000rpm, but on the journey home the oil pressure dropped alarmingly to around 15psi before climbing back to 35psi-40psi after a short while. This cycle then repeated.

I have heard of faulty sender units, but because low oil pressure is quite serious I am unable to use the car until the problem is rectified.

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  • Gitter Accepted Answer
    The electronic sender unit can often be the cause of unnecessary worry, but backup feedback exists in the form of an independently controlled warning light. When the ignition is switched on, it should illuminate in the central cluster between the speedo and tachometer. Having started the engine, the light should extinguish (it does this when the pressure reaches around 8psi). It reacts much faster than the gauge, so if the pressure is really cycling it will probably dip below 8psi causing the light to blink in harmony with its analogue partner.

    However, to find out what is really going on you can have a mechanical gauge temporarily plumbed into the main oil gallery, achieved via one of the many hexagonal plugs found along the right-hand side of the engine block.

    Most garages and many enthusiasts possess such a unit, so it may be possible to acquire the use of one. Your local Jaguar club representative can probably point you in the right direction. Cycling oil pressure could be due to a faulty bypass valve to be found integral with the later canister-type oil filters, or behind the filter-seating disc in the earlier element-type assembly.

    A faulty oil pressure relief valve can also be the culprit (maybe debris wedged behind it). Again, several variants exist, but, as a clue, the valve will be found adjacent to the filter housing end of the rubber sump oil return pipe.
    In the rather more likely event that the sender proves to be faulty, there are early and late versions (the change point is late 1977). Early requires C31177 and late C46272 – do not mix and match because they are not compatible with the respective gauges. These cars are also fitted with an additional failsafe in the form of an oil pressure switch, which cuts power to the fuel pump relay when the pressure is genuinely low.
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  • Started by Jon Cass
  • Wednesday, 17 April 2019
  • 26 Hits
  • 2 Participants
  • 1 reply