So I bought this car almost four years ago now and until today, it has had neither a working oil pressure gauge nor a working oil pressure light.

I'm preparing for a long trip in the car over July 4. I had the oil, oil filters and air filters changed today. I also bought both oil pressure sending units and had them changed.

The shop called this morning in a half-panic to tell me I had low oil pressure and wanted to know what I wanted done.

The oil pressure warning light comes on whenever the car idles. It goes out under acceleration, it comes back on during deceleration.

That's better than the gauge itself. The gauge barely moves off "0". It moves up a little at startup and never budges up or down after that.

Also, the shop found out the reason the light didn't work before: Someone had unplugged it because, I guess, they were tired of the car using it to notify them that they'd gotten off the gas and/or were idling.

Let me reiterate, I've had the car four years. In those four years, I've put about 33,000 miles on it and it has been, at times, the daily driver in my household. That's the role it fills now, too, and has for a couple of months now that my XK8 is (sort of) for sale.

Not once has the car's engine ever rattled, grinded, or exhibited any other symptoms of low oil pressure. The engine is smooth and accelerates strongly. I do not abuse this car, either.

I have been advised "if your car has exhibited low oil pressure for years without any problems, do nothing." I'm inclined to follow that advice, because otherwise I'm going to worry myself sick. There's also the issue of money -- as in, I cannot afford to have this engine rebuilt right now, in no way, shape or form. Oil pumps by themselves are almost $2,000 and unless one could be changed without the engine coming out, it's moot. If the engine in this car needs a rebuild, I will be forced to garage it for probably 2-3 years, minimum, while funds accrue.

We did not check the pressure with a manual gauge before putting it back together, because we just didn't expect this outcome, given the amount of use the car has had the last four years. I suppose the gauge could be shot, but that doesn't explain the light. Obviously the previous owner felt comfortable enough about things to just disconnect the light and move on.

What say ye?

Jess

 

Do nothing of course.

Ignorance has been bliss. Unplug the light if you can't stand it.

This reminds me of when I replaced the spark plugs on my non-running car and broke a spark plug and had to use an extractor. Somehow I became convinced that I'd dropped piece of porcelain into the cylinder and so I bought a endoscope to look inside the cylinder. That was a TERRIBLE mistake!! The cylinder walls looked like Mars and i was certain that the engine would be an oil burning monster.

Nothing further from truth.

So, don't get full body MRI's or genetic testing, don't look inside your cylinder heads and don't do anything to your car that has run fine for 33k miles.

 

The V12 does typically have low oil pressure at idle. I would verify with a proper mechanics oil pressure gauge what the pressure is at idle. I think spec is 5 psi. The v12's I have had do have low pressure at idle, but the light does not come on.

Other thing you could do is go to a slightly heavier grade of oil. What oil are you using now?

 

I have 6.0 V12, but not aware of difference in oil pressure specifications.
.at normal running temperature minimum oil pressure listed as 0.5 bar (7psi) at idle.
At 4000rpm it should be 4 bar (60psi) minimum.

There was a Tsb (technical service bulletin)

88-20 for V12 saloon (sedan)
Details as follows

ISSUE: The low oil pressure warning light on XJ12 Sedan vehicles may illuminate at idle with the engine at normal operating temperature and idle oil pressure within specification (minimum idle oil pressure - 7 psi [0.5 bar]). This condition is caused by thinner, energy efficient engine oils. ACTION: If a customer complains of a low oil pressure warning at idle and the oil pressure is within specification, replace the existing oil pressure transducer with an oil pressure sensor switch. NOTE: The oil pressure switch will affect the operation of the oil pressure gauge. The oil pressure gauge will read at mid-scale under all normal driving conditions. It will only move (to zero) if a low oil pressure warning occurs.

 

hmmm, I might like the solution.

After 20+ years of XJSs, the oil pressure gauge has always struck me as next to worthless. Once the oil is up to operating temperature, it barely registers on the gauge and a sudden drop would likely go unnoticed.

 

Normal from down here also.

The light switch is possibly a too high ON setting. I run a Toyota switch there and it is ON at 4psi.

The gauge is a best guess at any time. Bad earth circuitry in the dash cluster is the most common reason, and a seperate earth wire to the bulkhead bolt nearby usually sorts all the gauges.

Both my 5.3 engines run 18-20psi HOT at idle, and 75+ at 3000rpm, HOT. This is with a mechanical pressure gauge screwed into the light switch hole on that pedestal. I run 10W40 in both. The PreHE has now topped 550000kms and untouched.

The Jaguar (Smiths) gauge sender is unique to the Jaguar system, and substitutes with the "reverse" ohm readings are rare. Datsun 280ZX is one such sender.

 

I bet it's 10-40 rather than 20-50. I'm virtually certain that's what the shop put in it, looking back.

However, its last oil cycle was 20-50 and there was no difference in the gauge reading. Of course, that was also before changing out the sender.

I feel like you guys have at least talked me off the ledge here, for which I'm thankful.

Jess

 

OK guys, this has been a problem on the XJS since day one. Searching the archives, Jess isn't the first on the ledge.

Can we do better? Maybe come up with a good solution and make it a sticky.

The Jaguar solution mentioned above was to turn the gauge into a glorified idiot light, which is better than nothing I guess.

Is there another sender from a different car with the same threads that would be more accurate?

Perhaps someone with EE skills could figure a way to recalibrate the signal from the sender so it's more helpful?

Any other suggestions?

 

I put 20w50 in my car and it is usually bottomed out while idling. The gauge behavior you are describing is normal but can be worse with a lighter weight oil.

My oil light came on once, it was because the sender was leaking. The idiot light sensor is right on the sending unit, so the difference in pressure was very noticeable.

If your sending unit or nearby lines and banjos are leaking at all, it will get picked up very easily by the idiot light, but won't have a big affect on the car as a whole. All you have to do is scope it all out to look for leaks.

If you REALLY need to, pop the sender off and check pressure with a gauge. If you see proper pressure with your own two eyes, you can disregard the light. My brake fluid light comes on sometimes and I refused to believe my car was actually staying cool until I checked temps with an IR thermometer.

 

The oil pump is 100% bulletproof on the V12. You have perfectly good oil pressure or the engine would have seized miles ago. If you undo the oil filler on the cam cover you will see plenty of oil up on the cams. If you rev it with the cap off, you will see more ! If you go for a blast the oil will be hot, and it did not get hot by lying in the sump!

I am prepared to bet that: a change of senders to OEM spec, a thorough cleaning and general R&R of the wiring and connectors including the huge very fragile ones that go into the gauge cluster, a full dis-assembly of the actual gauge and a full clean of the electrical path through the gauge itself (refer to the Great Palm for how to do this), and as the WoOz says, an extra gauge earth and a full clean of all the earth paths and the earth metal strap in the gauge cluster, will fix the visuals!
Just a question of whether to bother, what is NOT happening is lack of oil pressure! Even if the pickup pipes' O rings in the sump are going home, the pump has huge over-capacity so nothing to worry about.
Greg

 

I wait to get into the rev's until the oil warms up and the pressure does come down...

 

You could buy one of these off ebay (about $60 from memory) for peace of mind.
Mine always showed 0-20 psi and the light came on at idle.



Not a problem now with new engine and senders, gauge always shows a solid 70 psi except at idle now.

 

I would try a mechanical gauge to verify acual oil pressure. Remove the idiot light sender and place there and compare the two. You then have a reference. Then remove and reinstall idiot light sensor.