Oil and Oil Filters

Your oil pressure light comes on after oil gets warmed up but your oil was just changed you were told it was the oil sender but it only does it at idle?

Answer

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05/30/2012

Unfortunately, there are plenty of people running around who give out free advice and often it's worth exactly what you pay for it. MOST of the time when the oil pressure is low there is nothing wrong with the sender OR the oil pump, but many people who don't understand what's going on inside the engine make all kinds of nonsensical assumptions.

I'll give you a brief description to help you understand what's going on inside your engine. The oil pump intake tube has a screen over it and the inlet is down, near the bottom of the oil pan. The screen keeps the big chunks out in the event that something terrible happens and there are big chunks of something floating around. The screen doesn't really protect you from much except those big chunks.

The pump pressurizes the oil and pumps it as fast as it can, depending on how fast the engine is running. When the engine is running fast, lots of oil gets pumped, when it's running slow, only a little oil gets pumped. The pump doesn't have anything more to do with pressure, since it just pumps as much as it can. There IS, however, a spring loaded piston that sits in one of the oil journals (that's a tube through the engine casting). When the oil pressure exceeds a specific amount the piston is pushed up, out of the way, and oil leaks past and back into the crankcase. As the pressure starts to drop, the piston closes back over the hole. The idea is to keep the pressure at a relative constant.

Where, you might ask, does the pressurized oil GO? That's the part that's important to the engine. Pressurized oil is forced into the space between the crankshaft and the bearing surface. In a perfect world, the crankshaft main bearings and the rod bearings never actually touch the crankshaft. In that perfect world, the pressurized oil forms a thin film around the bearing and in that perfect world the bearing would never wear out since it never touches metal.

In the REAL world, things are a little different. Every time you turn off the engine, the oil leaks past the bearings. Every time you change the oil the oil drains from the oil pump pickup tube. When you start the engine without that pressurized oil pushing through the oil pump, the bearings come in contact with the crankshaft. Engineers have made certain that the crankshaft is smooth and if everything is working properly there is still a little oil on the bearing surfaces from last time the engine was run, But having oil THERE is different from having PRESSURIZED oil THERE. While the pressurized oil keeps the bearing surface from touching down on the crankshaft, left-over oil just acts like a nice lubricant and keeps the bearing from wearing out completely next time you start up and before the engine builds oil pressure again.

Over years and many times of starting the engine, the bearing surfaces wear and more oil starts to leak past the bearings. Eventually, oil starts to leak fast enough that the oil pump can't keep up, and the oil pressure starts to drop because of all of the volume of oil that's leaking past. Remember that the oil pump pushes more oil through when the engine is running faster, so you'll get good oil pressure when you're going down the road, but at an idle the pressure will drop.

That's the bad news. The good news is, if you're still able to maintain oil pressure when the engine is running at highway speed, you can probably get the oil pressure to stay up a little higher at an idle too.

That's what higher viscosity oils are for. New engines are recommended to use low viscosity oils. If you look in the manual you'll see that your engine was supposed to have a light weight (probably 5W15 viscosity) oil when the engine was new. Now it's a little older. To bring the oil pressure up to a safe level, change to a 10W30 or even 15W40.

The thicker oil will not be able to leak past the worn bearings as fast. That will allow the oil pump to keep up with the rate that the oil is leaking out.

Understand that if the oil pump is able to create oil pressure at highway speed and while the oil is cool, there is nothing wrong with the oil pump... also, there is nothing wrong with the oil pressure sender unit either. Change the oil to something with a higher number and the problem should be solved.

That was probably more than you wanted to know.