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Oil pressure drops to zero suddenly--oil light comes on

XJ6 & XJ12 Series I, II & III 1968-1992

Oil pressure drops to zero suddenly--oil light comes on

 
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Old 01-28-2012, 02:13 PM
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Default Oil pressure drops to zero suddenly--oil light comes on

I've been noticing a couple times when driving the XJ6, the oil pressure reading will drop to zero for several seconds (seems to happen when sitting at a light) and the oil light comes on. Then it will eventually jump back to 40 or so (what are the units?!?). The oil level is normal according to the dipstick.

Any idea what's going on? Is it just a loose ground or something like that?

Throughout this whole ordeal, the engine runs exactly the same--leading me to believe it is not really an oil pressure problem.
 
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Old 01-28-2012, 02:20 PM
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The oil pressure transmitter situated near the oil filter housing is notorious for failing or for transmitting the incorrect pressure. However, it could be telling the truth so some tests are needed.

The only real way to test is to subsititute with another transmitter, and/or to remove and substititute with a pressure gauge and look directly at the reading on the gauge.

However, from what you describe, it sounds like the transmitter is faulty, or there is a bad connection betwee it and the gauge. Do any other gauges show odd readings, because there is also a voltage stabiliser somewhere in the system ?
 
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Old 01-28-2012, 06:32 PM
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The oil gauge and oil warning light are separate circuits with separate sending units. A double fault is certainly possible but.....

.....yes, you could also have an actual oil pressure problem.

Out of curiosity what weight oil are you using?

When was the oil and filter last changed?

Cheers
DD
 
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Old 01-28-2012, 06:55 PM
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Originally Posted by Fraser Mitchell View Post
The oil pressure transmitter situated near the oil filter housing is notorious for failing or for transmitting the incorrect pressure. However, it could be telling the truth so some tests are needed.

The only real way to test is to subsititute with another transmitter, and/or to remove and substititute with a pressure gauge and look directly at the reading on the gauge.

However, from what you describe, it sounds like the transmitter is faulty, or there is a bad connection betwee it and the gauge. Do any other gauges show odd readings, because there is also a voltage stabiliser somewhere in the system ?
Hmmm.. yes, there are a lot of other odd things going on in the dash. When the headlights are on, the headlight indicator goes on and off intermittently. The battery reading is also occasionally abnormal. Of course my gas gauge doesn't work either!

Unless it were out of oil, why would pressure drop to zero completely? Now that I think about it, the first time I noticed it (maybe it's been happening more often when I'm not watching the gauges) I wasn't at a stop--I was coming to a stop. The second time I noticed it I was stopped.
 
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Old 01-28-2012, 06:57 PM
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Originally Posted by Doug View Post
The oil gauge and oil warning light are separate circuits with separate sending units. A double fault is certainly possible but.....

.....yes, you could also have an actual oil pressure problem.

Out of curiosity what weight oil are you using?

When was the oil and filter last changed?

Cheers
DD
I am not sure about either of those questions. I am planning on getting the oil changed sometime next month (I don't plan to really drive it between now and then) at the dealership after putting some SeaFoam in the crank case.

I'm also hoping to do the tappet guide stakedown kit installation before I take it in.
 
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Old 01-29-2012, 04:25 AM
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Sounds electrical to me. Check earths (grounds), in particular the main engine earth, (found near the bell housing at bottom of the bay on LH side of the car, at least it was there in mine.
 
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Old 01-30-2012, 10:01 PM
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Depending on the viscosity of the oil being used and the level in the engine, the oil warning lamp will come on under deceleration as there is a momentary interruption in the oil supply to the oil pump. What is the oil pressure gauge reading when that happens? A healthy engine with good main bearings should read around the '40' (40 pounds/square inch) mark at idle when warm.

If the engine has been neglected or has been sitting for some time, the pickup screen for the oil pump can become clogged causing the volume of oil drawn up by the pump to be less than normal. Given the age of these cars, I found removing the lower sump and cleaning the pump pickup and sump itself helps maintain improved oil pressure. I replaced the oil pump on the last XJ6 I owned as the vehicle had only 120 000km, but sat unused for several months at a time.

Remember, Jaguar's initial design parameters for the XJ6 were for the vehicles to have a service life of 35 years. Any unmolested Series I and Series II cars have exceeded that mark already.

NBCat
 
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Old 01-30-2012, 10:05 PM
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The engine at idle has about 40 lb/in2. But when the symptom I described happens, it drops to zero and the oil light comes on.

I checked the oil level today cold and level, the level is slightly low according to the dipstick--maybe 1/4" below the full mark. I'll probably add a little bit of oil until I get the oil changed.
 
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Old 01-30-2012, 10:35 PM
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In an earlier post, you mentioned getting the oil changed sometime in February; you may want to post results of oil pressure readings under the same driving conditions following an oil and filter change.

I found using Castrol GTX 20W-50 worked best for my car here in southern California, but you must use oil with viscosity appropriate for your local climate.

NBCat
 
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Old 02-04-2012, 01:22 PM
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Haven't changed the oil yet, but I attempted to top it off by putting a quart of fresh 10W-40 in it.

The car must be really low on oil, because the oil level was maybe a hair lower than the low level reading on the dipstick, and it only budged a little bit! I'd estimate the difference between the "low" and "full" levels on the dipstick (the hash mark) is probably a good 4 quarts of oil!

This would definitely explain why the oil pressure would suddenly read zero, especially when coming to a stop!
 
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Old 02-04-2012, 04:32 PM
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The system will hold 7 quarts; a bit more if the oil cooler is drained. The difference between the high and low marks is about one quart. So if the dipstick is still not showing any oil as being present after a few minutes of you adding some, it was probably extremely low on oil to begin with, which now explains the drop in oil pressure after a sudden stop and the accompanying oil pressure warning lamp.

My suggestion would be to have the oil changed as soon as possible and to check for external leaks.

NBCat
 
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Old 02-04-2012, 06:01 PM
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Hmmmm... the dipstick was showing oil present before I added oil--but it was a little bit below the start of the hashed area on the dipstick. I swear I put a quart in and it only moved maybe 1/8-1/4" up the dipstick! Unless the oil is pouring right out to the floor!

I have idled the car for probably a total of 3-4 hours in my parking spot and have never seen any oil on the ground.
 
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Old 02-04-2012, 07:31 PM
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After you shut off the engine, let it sit for about 15 minutes and recheck the oil level again. That will allow sufficient time for oil in the cylinder head to drain back into the sump so you have a more accurate idea as to the true oil level.

NBCat
 
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Old 02-05-2012, 05:05 AM
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From what you describe, you are very low on oil.
 
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Old 03-01-2012, 03:01 PM
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Should've followed up sooner. I was indeed low on oil--probably about two quarts or so. Although this sounds low, considering that the car normally holds over 7 quarts, I don't think I did any damage. But definitely wise for me to keep it topped off from now on!
 
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Old 03-01-2012, 10:26 PM
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Originally Posted by amcdonal86 View Post
... I don't think I did any damage. But definitely wise for me to keep it topped off from now on!
Most likely you will be okay with regard to the oil level; be sure to check it frequently.

NBCat
 
 
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