Both oil gauge & oil light read no oil pressure

I have a 1974 XJ6 4.2L which has done only 26,000km, it has been started regularly with no oil pressure issues. I had been trying to install a security alarm & central locking. (Another story) I just finished reinstalling the dashboard black leather top the day before.

I started the engine to give it a run. To my horror the oil pressure warning light stay on & the oil gauge read zero pressure.

When last started last week the oil light went out immediately & the oil gauge needle went past halfway.

Earthed out the gauge wire & gauge needle headed towards 80psi & removed oil pressure warning light wire and light went out

Checked the oil dip stick & oil level was spot on. Oil & filter have done less then 100km.

The few seconds that the engine was running there was no indication of no oil, like the first start after doing an oil change.

I am at a loss of what to do next and I am going into hospital for a shoulder replacement on the 15th May,

I am going to borrow a manual gauge from my mate, connect it & see if there is any pressure also look through the filler oil cap to see if there is any fresh oil running

any help would be match appreciated.

The only thing that springs to mind would be the OP relief valve. A manual check with an external gauge is a good idea.

If it were me, I’d start by retracing the things you did since it last worked properly. If you’ve been behind the dash, ideally take the dash top off again and look for a connection that’s been knocked off, not reconnected or connected to the wrong place. Use the wiring diagram from your manual.


First of all, starting and not driving is terrible for cars. For peace of mind you can check if there’s sufficient oil splashing in the cam covers.

A faulty relief valve should still allow enough pressure to turn the light off at a cold start, it would be some wiring that wasn’t connected right but if grounding at the block gives a reaction…
Try the mechanical gauge and give it a little time. Good luck!

Any odds, Thomas; this is related to your security alarm installation. Revisit as Dave D suggests; it may be the connections done/undone - or the ‘fiddle factor’, you may have touched a tender spot under the dash. I don’t know the details of your alarm intents.

Confusedly, early version diagrams doesn’t show warning light and gauge as separated, but sort of in series, while S3 has separate switches for each. Also, there are two oil pressure switches on carbed versions; one switch for fuel safety relay and one for oil pressure gauge/light. The two works differently - and cross connected may cause some confusion?

Certainly; get an external oil pressure gauge to verify - no point in taking chances…:slight_smile:

xj6 86 Sov Europe (UK/NZ)

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as nothing has been done to the engine, I’d also put my money on the electrics. Strange thing is, though, that the circuits for OP warning light and OP gauge are separate, from the senders on.

Even if they are fused together it doesn’t explain why your OP gauge pegs out when grounded.

Maybe, you were so shocked by the apparent loss of oil pressure you missed that other electric circuits were lost as well? How about the fuel gauge? Instrument lights operating?

If everything else fails, remove the antitheft device and buy good insurance instead. Much less trouble -prone;-) and most certainly sufficient as these cars typically are not being stolen.

Good luck


75 XJ6L 4.2 auto (UK spec)

It sounds like the oil pressure relief valve may have stuck open.

This is the second post reporting that issue in the last two weeks and there was another post reporting a collapsed oil filter.

I would be interested to know what viscosity oil these posters are using, as it sounds like the oil pressure is too high at cold start causing the relief valve to open, which on little used engines then sticks.
The collapsed oil filter is also possibly a result of too high oil pressure, which is why engines have a pressure relief valve, which opens to bypass the oil filter.

I suspect all these posters are using 20W/50 oil, which is too viscous, especially at cold start.

What is your location, or current ambient temperature?

That is SO unlikely…

When the valve fails there’s still pressure, just a lot lower. It’s probably not the relief valve, especially not if the cold oil pressure is normally above 40 psi.

20w50 is NOT too viscous. If it was one cold rev would destroy the engine completely :joy::joy:

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Warren’s in Melbourne. Today it’s 24C and overnight it will be 11C. Many on us run 20W50 or 15W60 - without issue all year.

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manual OP gauge is an appropriate course of action

While you wait, if you are up for it, I would check the wiring of the OP gauge and idiot light with a fairly simple test

Now I have not even looked at the circuit diagram, but you should be able to peg the OP gauge, and (I think) alight the idiot light by grounding the sending wires on the block ?

You should look at the diagram, and others can say yay or nay, but I think this is a super easy test.

If they dont do what they are supposed to when these 5min tests are done, your issue is electrical.

poking around with a test light (looking at circuit diagram) will tell if you have + where it should be

He has grounded the wires with appropriate reaction. The wiring and gauge/light seem to work.

Two days ago I had ice on the car and it started and ran perfectly. Dieselman is on to nothing.

The diagrams show the light and gauge in series with the pressure transmitter, Tony - the grounded the connection, bypassing the transmitter; light ‘on’ and gauge peaked. sort of confirming co-connection…?

Highly confusing; with ign ‘on’, transmitter connected - the warning lamp should be lit and the gauge at ‘zero’. How this is done by the transmitter is unclear. I think it was discussed some years ago - but somebody knows, a faulty transmitter…?

That said; checking that there is good oil pressure is certainly a necessity, but something more likely went wrong when installing the alarm. What the alarm was supposed to do may be helpful…

xj6 85 Sov Europe (UK/NZ)

Here’s to hoping this is just a failed pressure sender, but failing on both circuits simultaneously seems a bit unlikely, unless they share a single Gnd connection.

Pressure relief valve sticking at start does appear to be a fairly regulator occurrence.

Oil pressure then noted at between 0 and 5 Psi.
Does this sound like an Oil Pressure Release Valve issue to you? - E-Type - Jag-lovers Forums
Oil Pressure relief valve stuck open - Saloons - Jag-lovers Forums
Oil Pressure Relief Valve Help - E-Type - Jag-lovers Forums

If the relief valve is sticking open, it begs the question why is the oil pressure above 80Psi, especially at cold start…unless the driver is revving the engine to high revs, which seems very unlikely.

Hey Dieselman, click on the very first link and read. It starts out at 60 and falls to 5-10.

Why would I need to click on the link to a post I’ve already read?

There have been more recent posts of erratic oil pressure and one of a collapsed filter.

Because you haven’t read it correctly and I‘m telling you to re-read.

I read and digested the information before linking to the thread.
Investigate the cause and follow the facts.

I think it’s possible the previous time the O/p started the engine, or immediately at start this time, the relief valve stuck so now there is no oil pressure.

Did you know that cows don’t really get their energy and nutrients from the grass they eat? During digestion, bacteria are digesting the grass and they are what actually form most of the cow‘s diet.
If the prv fails, there is oil pressure during start up.

Coincidentally I was lubricating my speedo drive a few months back and when I reassembled the dash the oil pressure warning light didn’t work anymore.

It was clear that I had disturbed the wiring… No.
New bulb, maybe it broke during the work? No.
In the end the sender on the block had jammed and needed a good cleaning. On the very day I was working somewhere under the dash. It may still be possible that the senders are both suspect.

It’s a spring operated valve and seat. If it’s stuck open the oil simply passes through.