Oil pressure warninglight comes on when you stop goes when you drive

You need to find out why you have low oil pressure. It is normal that as engine rpm's increase, the oil pressure will rise- the oil pump is moving oil faster. Even though your light goes out, you could still have critically low oil pressure. There should be sufficient pressure at idle to keep the light off. The light will come on when your oil pressure is almost nil- I believe for some it's as low as 8 psi. Normally, oil pressure at idle is specified anywhere from 15 to 40 psi. You would have to check specifications for your Expedition. To verify oil pressure, hook up a mechanical gauge at the oil pressure sending unit on the engine block. Check with a local shop.

The causes run from easy, simple to budget busting ruin. Low oil level, wrong grade oil, real old oil, oil leaks, internal oil leaks, worn oil pump, worn crankshaft bearings. It was pretty common on older very high mileage cars to have a fluttering oil light at stops and idle. It was time for a rebuild, or time to look for another car.

Keep driving & you won't have to worry
about the motor anymore

Did the mechanic drive the truck with a mechanical gauge to check the pressure ? The engine bearings or oil pump could be worn enough to allow pressure to drop at idle after it warms up.

Check oil level on dipstick. Check for loose connection on oil pressure sender unit. Have the oil pressure checked using a master gauge.

Okay What it means is that the oil in your car is either low in level or the oil pressure at low revolutions is low. I highly recommend you to stop driving the car when this happen because you can internally damage the engine with a low oil or a low pressure of oil. Check the oil level first..make sure level is fine and oil looks good. Black and burned oil will give you a low oil pressure as well. If it happen that your car need a oil change. Make sure you get it done and see if the problem goes away. Very rarely oils pressure sensor goes bad but if oil is new and level is fine you can take the oil pressure sensor and connect a tester for a oil pressure. If this is not under the manufacturer specifications.. oil pump is worn out and you have to replace it for a new one.
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There is less oil distributed to the engine in idle position than when you drive. So when the oil light comes on, you may have to put a little oil in your car. While driving, the oil reading may be enough for your sensors in your car to not send a signal, but when you stop and the oil pressure goes down, it may not be enough and that's when the light comes on. When you drive 20 to 30 minutes, your oil gets thinner because it is warm. That also will send a different reading to the sensors. You may want to try a thicker oil (instead of 10/40 try 5/20 ... ask before you do this!).

the motor needs to be rebuild it is losing oil pressure when the motor warms up

engine oil pressure may be low remove oil pressure sending unit and install a pressure gauge check for pressure while warm if pressure is ok replace oil pressure sending unit oil pressure at idle should be between 5-15 psi at min

the oil pump is worn out the quiker its changed the better

If it's your oil pressure gauge, that is usually normal. The pressure will rise and fall with the RPM's of the engine, since the oil pump runs off of the engine itself.

Do you have a CEL coming on when it does that? If so, got to Autozone or Advanced, and get the codes from it, and myself or someone on here can help you fix it.