I have a 5.9l engine in a 2001 Dodge. I have noticed as the motor warms up the oil pressure drops. It will increase as the rpms increase. I use syntech blend oils and x2 filters. Is this normal? Should I worry or what should I do?

Seems like you are OK. Can you give us some psi numbers for your oil pressure (see my examples below).

Oil pressure will run higher when cold. The oil will take 10 maybe 15 miles to get fully warmed up. Then it will run lower. Here are two examples from my current experience for you to compare with:

F150, 1995, 4.9L inline six, 114,000 miles, with 10w40 oil:
Cold: 55 psi
Hot at 2000 rpm: 42 psi
Hot at idle: 20 psi


Ford 7.5L V8 motorhome at 74000 miles, with 10w30 oil:
Cold: 70 psi
Hot at 2000 rpm: 57 psi
Hot at idle: 30 psi

This sounds fairly normal and is a common question. If you understand the oil pump system it will make sense. Here's a quick summary.

Oil pumps are almost always fixed displacement (gear, gerotor, etc) driven directly by the engine. So the flow output is directly related to engine rpm.

Pressure is a result of pushing fluid against a load, it is NOT produced by the pump. The pump only makes flow.

There is a relief valve limiting the maximum pressure out of the pump. At that setting it sends some oil back to the pan througha spring loaded ball or poppet. Below its setting, it stays closed and has NO effect on the pressure the gauge is reading.

The load, (engine bearings, etc) are essentially various orifices of fixed clearances. Of course the clearances get bigger as the engine wears, but in the short term they are constants.

Oil viscosity drops as temperature increases. Thin oil goes through the orifice with less pressure drop.

So, when the engine starts up cold, the pump is trying to push xxx gpm through small clearances. With thick oil, the pressure drop required to do this is high. Gauge will read higher and higher pressure as the engine speeds up, until the oil goes across relief valve back to the pan.

As the engine warms up, the oil goes through the galleries easier, so less pressure is required.

At low speeds, the pump is turning slower, less output flow, and less pressure drop to push it through the various galleries.

As engine speeds up, the flow is greater, the galleries are the same sized orifice restrictions, and more pressure results from pushing it through.

As long as the pressure is below the relief valve setting, it will vary the reading on the gauge.

Sum total: cold start the gauge reads higher than normal. Gauge increases as engine speed increases, until it reaches the RV setting when it will be fairly constant above that point. (Keep in mind there isn't much lubrication happening if the cold oil isn't reaching the bearings.) As the engine heats up, the pressure reading decreases, especially at idle.

The actual numbers aren't that important, because the auto gauge isn't really too accurate. The real key is getting familiar with the normal ranges of your engine, oil and gauge. Once you know what is 'normal' you can spot something unusual.

pushed for time, later, kcj

When replacing the oil pump we could not get the oil pan out how do we do this with out taking out the engine?

quote:

Originally posted by novakca:
When replacing the oil pump we could not get the oil pan out how do we do this with out taking out the engine?

I have never done a pan and don't know what vehicle you have, but when I had my tranny out a few years ago ('95 F150 with manual tranny) at a shop, the owner knew I wanted to replace the rusty pan. He called me and said it might be a good time to do it since with the tranny out the pan would be a lot easier to do. Not sure that is true of all vehicles. Just a thought.