I have a 2003 Ford Expedition with the 5.4 V8. It starts and runs normal but, while driving the Low Oil Pressure light will flash and the oil pressure gauge will bottom out. This may last 1 to 2 seconds and everything will go back to normal. What could be the problem?

 

If you're not hearing any knocking or tapping sounds from the motor, then it could just be that the oil sender is no good.

 

Could it be a sensor? What is involved in changing the sending unit? Is there a way to test the sending unit?

 

Its an eight year old $8 part. Do you really want to waste the time trying to test it?

 

Ok Thanks. Im new to this forum but I must say I love it so far. It's nice to have all this knowledge at my finger tips to be used.

 

Mine does this only when it's below 50 degrees and only at cold start up. As soon as it warms up it's fine. It's done it for years now. The gauge really isn't a gauge, it's just a switch that keeps the oil pressure gauge around 1/2 on the gauge, when the pressure drops below a pre determined pressure it will bottom out & the light will illuminate.

I've never seen an engine build pressure as it warms up, I have seen 60 psi at cold start & 5 psi at normal operating temps due to bad main bearings. So I'm not worried about the cold start up gauge problem.

 

The most common cause of your issue is spun rod bearings. The oil pressure sender does nothing more than say "yes, you have oil pressure" or "no oil pressure at all." When the needle jumps on the gage it is showing you that there is no oil pressure at all at that moment as the bearing(s) has moved and left a gap for the oil to flow unrestricted back to the sump thereby losing pressure. You would get similar gage action if the oil were aerated and pumping bubbles. I have only seen the second example when there is no oil in the sump or if the oil pick-up tube is cracked, broken or has come loose from the oil pump. There are several places in the modular engine that could cause it to lose oil pressure but they occure much more rarely.
If the gage fluctuation really bothers you, you should get and install a real mechanical gage and use the OEM gage only as decoration.