Oil pressure

[jlb]

I have 1990 bronco 351 I recently replaced oil gauge because it would just drop and engine light would come on and then would jump back up the problem I'm having is when I start the truck it reads 40 to 45 psi as I drive it and while it's idling it will drop down to around 20 psi haven't seen it go any lower haven't driven it to far incase I lose pressure completely is this normal or is it a problem if so any advice.

 

[nelbur]

This is a fairly common problem with old motors.   The cause is worn bearings that lose oil fairly rapidly.   The reason you have good pressure when you first start up is that the oil is thick when it is cold and stays in the bearings better than after it is warmed up and thined out.   It is not really a problem, if the oil pressure at idle with the engine warmed up, stays at 8 to 10 PSI.   If you want to improve your hot idle oil pressure you will need to go to a thicker oil like 20W50, which should help the oil pressure and actually reduce further wear on the engine at the expense of slightly reduced gas mileage.

 

[jlb]

The 8 to 10 psi is that while the truck is driving I also did an oil change when I replaced the gauge but didn't go that thick The truck only has 177,000 original miles on it motor still runs strong but pressure drops at a short distance.I only drive it a few miles a day to work because I don't want it to drop to nothing so im thinking I will go take your advice and go with a thicker oil Thanks for your help anything else you can provide would be greatly appreciated.

 

[nelbur]

No, the 8 to 10 psi is when idling like at a stop sign.   When you are at idle the oil pump is turning slowly and the oil leaks out of the bearings nearly as fast as the pump can pump it.   Say you are idling at 600 rpm, and you speed up to 1200 to ease off from a stop.   Your pump is pumping twice as much oil at 1200 than at 600, and your oil pressure should go up some.   When driving your rpm is probably at least 1800 which means three times as much oil is being pumped as when at idle.   You will likely have decent oil pressure when driving.   Use a good filter that will pass oil easily.   Walmart has Motorcraft filters at good prices. 

If 20W50 doesn't give enough oil pressure, it is possible to add an oil thickner like Lucas, STP, Motor Honey, etc. but I would avoid this if possible as it could get too thick for easy starting in winter, depending on where you live.   When my original 302 was on it's last legs I was using 20W50 and adding two bottles of STP.   I ran it like that for a couple of years before I decided to put in a remanufactured long block.   I'm sure it would have gone much farther if necessary.

 

[jlb]

Ok thanks a lot I will go with 20w50 and use the filter you suggested and give that a try

 

[Art5]

My 88 with a 302 did the same thing i thought it was the gauge so i put in a mechanical one...when i started the truck cold i would have like 35psi and by the time it warmed up i had 0 at idle. Pretty sure mine was bearings as someone else said. As long as it's not missing or knocking or anything i would try the heavier oil and see how much life you can get out of it. let us know.