Droping oil presure as the car warms up

[StinkyFatPig]

anyway, when its cold the oil seems great, but as it warms up it drops.. and its quite low when its at op temp..

I have no idea what this is.

thanx

[halz]

Barring major oil leaks, I think what you are seeing is the normal behaviour of a warming engine. I have seen this too in cars that I've owned in the past (ones that have an accurate working oil pressure guage that is).

What is happening is this: Cold engine = highly viscous (thick, hard to pump) oil. At startup, your engine will be working hard to move cold oil around and the pressure pickup will show this as high oil pressure. Warm engine = low viscosity (thin, easy to pump) oil. As the engine warms so does the oil. Warm oil flows more easily than cold oil and your engine requires less effort to move it around...Hence lower oil pressure for a warm engine.

Hope this helps (if I've got by definition of viscosity wrong then someone plese tell me!)

[StinkyFatPig]

thats actualy what i was thinking, But just driving around and seing your oil presure at low just is kind of scary...thanx for the advice

[halz]

Yep, it weirded me out the 1st time I saw it too. To reassure yourself, just check the oil level when the car is fully warm (oil pressure guage at minimum) and then enjoy the rest of the drive!

[2ManyZs]

While the oil you are using could be a major factor (what weight are you using?) it could also be a sign of something more serious. I recommend putting a mechanical gauge on it and finding out EXACTLY what kind of oil pressure drop you are experiencing.

I'm not trying to scare you but it could be a sign of an impending problem, you may have a bearing on the verge of going bad. If this happens it could cause major problems.

After you find out what kind of oil pressure drop you are experiencing you could try a little heavier weight of oil to see what difference that does. But in the end I believe you may find you are on the road to a mechanical breakdown. How many miles are on the engine? If you are up in the 70,000 or above I would advise you to check this out carefully. If the oil pressure is dropping 10 to 20 psi I believe you have an internal problem that will require your attention very soon. It could be just one bearing or all of them. Take your time and check it thoroughly.

[jcdozier]

2ManyZs is asking the right question - what kind of oil are you using? halz's explanation of viscosity change applies to single-weight oils only. If you are using a multi-grade oil, you shouldn't see a significant pressure change with temperature change.

A 10W30 oil, for example, will "act like" 10W at cold temperatures and like 30W at operating temperatures. That "constant viscosity" quality is why multi-grade oils are recommended by the manufacturers for four-cycle engines. Single-grade oils are typically used only in two-cycle engines (lawn mowers, etc.) or for special applications.

I agree with 2ManyZs that a 10-20 psi drop indicates an internal problem that should be checked out before something bad happens.

[2ManyZs]

I got to thinking that there is another possibility. It could be a combination of the oil that you are using along with a worn oil pump. Or it is possible that the pressure spring in the pump is to the point that when it gets hot that it is losing tension and therefore letting your pressure drop. It is possible to replace just the spring with a stiffer one for more pressure, however, if your engine is one with high mileage it may be best to go ahead and replace the whole pump to cure your problem. Just another idea you might want to keep in mind and check out. I will tell you this though, changing an oil pump can be a maddening experience. It is not an easy job on a L series. Read your service manual before you decide whether or not you want to tackle this, it can get a little complicated.

[EScanlon]

I'm not an engine mechanic, so I'll post my answer on the basis of what I know of engines. Since my knowledge of these systems is very basic, it might shed light in a different manner.

Having a Full reservoir of Oil is NO indication that the oil is actually being moved around the engine to the different areas that REQUIRE the oil, i.e. TOP of engine. This is where the oil pump comes in, as I understand it, and it's function is to pick up the oil at the BOTTOM of the engine and PRESSURIZE it UP a tube for it to flow down through the various channels BACK through the filter etc. Someone feel free to correct my basics.

That you are registering HIGH pressure on start up and LOW pressure after warming up, has me FIRST question the PRESSURE SENDER. Not the engine. It turns out that the Datsun Z engine was notorious for having bad Oil Pressure Senders. IF after checking the sender and it checks OK, then you need to address the engine.

[2ManyZs]

Actually I touched on that in my first post by telling him to get his pressure checked with a mechanical gauge to find out the exact pressure variation. I never have trusted the electric gauges and probably never will. There are just too many things that can mess up an electric gauge. Sending units, electrical connections, wiring problems, gauge problems etc.

So the best advice is get your pressure verified accurately before you do anything. Once you have established how much if any pressure drop you may be experiencing then move on to some of the other ideas. I would start at the oil pump first and proceed from there.

[nickbean]

I am having the same problems with the oil pressure dropping as the car warms up. But I am having increased valve noise too. What do you guys think?

[sjcurtis]

Temp affecting oil pressure, Hmm I have seen this problem in many forms, what I have read on the previous replys I must totally agree with 2ManyZs the mech guage is a must, you need to know the PSI variation.

The one I have not seen is (Engine warmed up) you stop at the lights oil pressure dropping down indicating low when you leave the lights and off down the road revs build and oil pressure returns to normal.

NickBean

If you have associated mech noise change in the engine you have an oil flow problem stop shut it off before you loose it completely, check quantity & (most probable cooked oil pump) lack of flow. In this situation check with pressure guage immediately to assertain pressure and flow.

I have heard of big power engines flashing out in 10 seconds when that 20c spring in the oil pump fails.

My 2c

Steve :classic:

[wayquick32]

A good rule of thumb is to have10 lbs of oil pressure for every 1000 rpms of operation. She will live happy.