How many miles are on it? Does it run fine?

 

This may seem like a dumb answer to your problem, but it's worth checking... Check the oil level. If you are low on oil (very low), the pressure guage will read really low at idle, then jump up as soon as the rpm increases and the pressure builds. Maybe the place you had the oil changed at forgot to fill it and you are running on empty. I've seen worse at those places... Forgetting to replace the oil plug, etc.

 

I agree, sounds like it's just low. Remember the Series 3 XJ's require 8.5 quarts of oil.

 

I've owned 2 XJ6's and both allways had low oil pressure at Idle, especially when it was hot outside. Even after changing the sending unit still the same. I was told not to worry until the light comes on. Maybe that's why most of the new ones do not have an oil pressure guage.

 

That's funny! "We can't make the guage read correctly, so let's just remove it!" That reminds me of a friend I had in high school who's S10 constantly threw a check engine light... Rather than fixing it, he pulled the bulb!

 

I guess that he was out of black electrical tape to cover it up!

 

I would still call the guy and have him check the oil level... Maybe the filter wasn't seated 100% and the oil leaked out, or the plug... What you are describing is exactly what happens when the car is really low on oil. Other than that, I'm stumped without actually being able to look at the car. I guess that I would start checking gaskets for leaks at that point. You did say the car was sitting for a while... Maybe one of the gaskets dry-rotted and he can't keep pressure because of it.

 

Could be the sensor, but I doubt it if the light triggered. Those are two different sensors, one for light and one for gauge. I does sound exactly like it's low on oil. It's a head scratcher!

 

Maybe it's the blinker fluid???

 

When I took My '95 XJ6 to the dealer complaining about low oil pressure, they replaced the oil pressure sensor with a oil pressure sender. the gauge just jumps to 40+ pounds as soon as the engine starts.

Well I raised a fuss about this and the service writer said the sensor is no longer available and Jaguars have never had an oil pressure problem. Must be one of Ford's better ideals. That's the same kind of sender that was in my Ford pickup.....Doug

 

Don't give up so easily! You can solve it... We just have to move to the next steps since the easy answer wasn't the solution. Unfortunately, depending on what the problem is... this could be a pretty involved process.

First, let's check the other easy items... Does the car idle low by chance? If it does, that is your problem right there. If the car was meant to idle at 750rpm, but is idling at 400rpms, the pressure wont be to spec at idle. If this is the problem, try cleaning out the throttle body to see if that restores the idle. Once the idle is back to normal, your pressure will most likely be fine.

One other easy thing... You said that this problem happened on both yours and another car immediately after you changed the oil. If you used too light of a viscosity oil, that will cause low pressure too. Check that out in reference to what is recommended for the car. This isn't a problem in newer motors, but ones that have a few miles on them have larger clearances due to normal wear and tear... The lighter oil may slip through and cause low pressure.

We do have one thing solved... Your oil pump is fine. The pressure builds once you give it gas, so that means the pump is ok. At least you have that going for you!

That's it for the really easy stuff... Now to the others. It could be various items, as this is one of the toughest problems to troubleshoot - especially over the internet. I'm not going to go into severe detail with everything because I could be here all night... but this should at least get you started.

Next, move to the pressure sending unit. It's a pretty common cause for this problem. You can do the cheap test and pull the unit and check the guage. If the reading is still the same, then the problem is electrical. Check for a ground loop. It may also be possible that the unit itself is faulty. If the guage goes dead after pulling the unit, then the unit is most likely fine. It seems like these either work or don't work... I have never seen one "half way" work. If this is your problem, the new unit is roughly $100. If your "cheap test" told you that the unit is fine, do the actual test to be sure before moving on to the next steps. The right way to do it is to have an actual pressure guage hooked up to it. I would imagine that a shop would do this for you for not too much money. This will tell you the exact pressure of the motor, and basically whether your problem lies inside the motor our outside the motor (a sensor, etc). Assuming that the shop testwith the mechanical pressure guage (or you can do it yourself if you have the equipment) reads a low pressure, now move on...

...continued in a minute.

 

A picture of the pressure sending unit for reference... This is nt the actual unit from your car, but it should look similar. I haven't pulled one on a jag so I don't know the exact location, but I would check in the area near the oil filter for it.

[img=http://img84.imageshack.us/img84/9100/b52tm9.th.jpg]

 

So now we know that the electrical side of things is ok, the idle isn't causing the problem, the pump is fine, and the oil level and type are ok... This means that something in the motor is causing the loss of pressure. Normal wear and tear will cause larger clearances after some mileage, and this is likely your issue. Where the problem lies though, is tough to tell on your own.

An easy test would be to change the oil with a heavier viscosity oil. If this causes the pressure to rise (using the mechanical pressure guage, not the guage on the dash as that one isn't pinpoint accurate), you have the exact problem described above. Using a thicker oil may solve things for you, however be sure that you don't go too heavy as this may cause hard starting and lubrication problems in cold weather. You could also try an oil additive at this point to see if that helps, but I have personally never been a big fan of the fix-it-in-a-bottles...

The best way to have things figured out at this point would be to take it in and have a test done called a leak down test. Having this done will tell you exactly where the problem lies. Unfortunately, no matter what the results are, fixing it involves tearing down the motor.

There are so many variables at this point that I'm not going to continue at this time. If you get to the point of having the leak down done, post the results and I can try to help you further.

I hope that one of the first items posted is what you problem is! Hopefully it is a nice and easy fix like the incorrect oil. Keep us posted!

 

One more thing... I am not a mechanic - just a hobbyist that toys with cars for fun. If I may have missed something, please don't hold me accountable! I hate to steer you in the direction of tearing the motor apart when it could be something easy that I simply haven't thought of. Be sure to have the mechanical pressure test done to confirm the low pressure and to confirm whether the problem is inside or outside the motor, and eventually the leak down test done prior to tearing into things. Having these two tests done will tell you what the problem is - so at that point you can continue on with the repairs in confidence that you are tackling the correct area. Good luck!

 

Did you have any luck on finding the problem?

 

If after the oil stabilizer was added the pressure went up a bit, it sounds like you have some wear in the motor and the clearances are causing the pressure loss. Personally, I would replace the sending unit to see if that helps, and if not then have the leak down test done. Good luck.

Your friend's problem sounds like a ground loop. Check for a wire that is making contact with metal.