My car has been reading 0PSI after warmup lately. After revving and driving at a higher speed the oil pump i guess picks up and it runs between 40-50psi. At cold start it shows about the same or just over 50psi. currently running 15w-50

I know for a fact the car isn't running at 0 psi because it would be broken.

Just wanted some opinions. Is my engine worn out and the gauge isn't capable of accurately displaying the low psi caused by the larger clearances in my bearings or is my gauge/sending unit messed up?


Generally I don't worry but was just thinking today, what if its actually really low and causing damage?

 

Sorry for not reading through some other posts first. someone just posted about this and Doug posted the what I wanted to hear about his XJS running 5-7Psi which wasn't detectable on the gauge.

One of these days I will find the time to hook up a pressure gauge and give a reading and Ill post it, along with my specs and mileage to give everyone an idea of what they should expect.


Its just disconcerting when 3 of the 4 gauges are in the center of the dial and you have one that sits at the bottom half the time you are in traffic.

 

A few things to consider about oil pressure -
1 – it will vary with oil viscosity the thinner the oil the lower the pressure.
2 – it’s only an indication if you had a restriction after the oil pump this would increase pressure but decrease flow, not a good thing. Like putting your finger over the end of the garden hose
3 – so flow is way more important than outright pressure.

 

"0" is pretty normal at idle in the 5.3 V-12 once warmed up.

 

in an oiling system pressure its just the resistance to flow. Low pressure indicates low resistance, which means larger clearances. This just happens on older cars, and like you said does not necessarily mean the engine is being more worn, just an indicator that it already is. My concern would just be if it were something else. Like if there weren't people coming in here telling "0" is normal and making me worry there is something wrong with the pump or pickup screen.

 

thanks man, if everyone else is reading "0" on the gauge it makes me much more comfortable.

Its just hard because I don't want anything to happen, but when my oil gauge reads 0 and my temp sensor is between the N and H even when the stat housings are 175 degrees I just have to try and pretend to not see them.

 

We had a club member speed off from a caravan last summer in her newly aquired XJS and once she showed back up I asked where she went...she said her oil pressure was at zero so she went to get oil and add it...I asked her how it was now...yup, she said ZERO! :0

 

My experience is pretty close to what everyone else has said - a bit over the 50 mark at idle after a cold start but drops >quickly< as it warms up. It doesn't go to zero on the gauge but is between zero and the next mark at full temp and idle. At highway speed the gauge is about at the middle mark between 50 and zero when warm. It's a 91 V12 with almost 110,000 miles.

I haven't worried about it too much (a little though) because a) the dummy light never comes on, b)experience with other old engines, c) previous owner told me it would do that before I bought it, and d) read about it here.

I do wonder though if the pressure is really dropping that fast or if the sending unit is affected by the engine warming up, or maybe some other variable that I haven't thought of - this engine holds a LOT of oil for it's size and to me it doesn't seem like the oil would warm up and thin out as fast as the gauge pressure drop would indicate - if that's what's going on.

I'd be interested to hear if anyone has accurately measured the oil pressure compared to the dash gauge as their engine warms up and can fill in some of the blanks here.

 

Now you know why Jaguar and others went to the "idiot gauges" years ago


Cheers
DD

 

I have two gauges that are always at the bottom, fuel and oil
I just got my XJ12 on the road and also noticed the same thing for the oil pressure.

 

I'm the previous thread about oil pressure. It seems that this is a very common occurence. I thought I would ask one question that may for all of us. I read in one publication that loose oil pump bolts could cause erratic PSI readings. Anyone heard of this? Sorry about posting questions on your thread. Just very interested.

 

I can't take the stress. My old Daimler V8 used to show near zero running 20W50 in a rebuilt engine with only 9000 miles on it. Everyone said "don't worry" but I did anyway.

Then again, my MGB seized a main bearing at about 50mph showing good pressure.

 

What is your experience with this though.

Mine is NOT erratic. My car has high pressure when cold and drops to 0 at idle after it is at temp. It does this CONSISTENTLY.

Thats a pretty easy fix if you are interested in finding out or if you want to double check, but I don't think thats it.

On the other hand our cars are 30 years old. May as well replace that old oil pan gasket if you have the chance, and check the torque of the pump while you are there!

 

My car is doing exactly what yours is doing. Exactly. Just read that loose oil pump bolts could be the culprit. I belive it was in Kirby's Book. Wanted to know if that is possible. Thanks.

 

I really think this is a function of the sender. It is using a combination of a diaphragm an a bimetallic strip that is either off or on. At some point below a certain point the pressure does not register. There is a separate switch and associated light. If the idiot light is not on then the pressure is not really zero. You could remove the sensor and use an instrumentation pressure transducer to test, or unscrew and see how much oil does come out when idling. I expect still significant. Just joking about removing while running, but I expect there would still be quite a few psi there so then oil is being moved.

 

Yep, mine does the same thing. Good pressure at start up, and reads virtually zero at idle once it warms up.

If you actually had zero oil pressure the engine would be making some SERIOUS noise.

 

if you're really concerned go pick up a cheap AutoZone oil pressure gauge and use it to check directly from the tower.

I did this on mine and it worked great because my oil pressure senders didn't work.

 

My 4.0L does the same thing with one more caveat. After an oil change, the oil pressure gauge does not register anything for a few days and then it starts working as before. The 95my had a service change to make the gauge an idiot light by replacing the transducer with a pressure switch and an in-line resister to have the gauge show the pressure at mid-range. I kept my transducer because the car already has an idiot light. I like the gauge function even if it does make me wonder at times…