I have a 1997 F150 with a 4.6 and 350,000 miles on it. This past week I've been having oil pressure problems from out of no where, I got in my truck started driving down the road and lost all oil pressure. So I stoped and called my dad we took it up to his shop the oil was a dirty so we changed it. I took it for a 4 to 5 mile test drive and it drove great! Oil pressure was just fine. I got in it an hour later to take a friend home made it half way there and stoped at a stop sign and lost oil pressure again. As long as my foot is on the gas my oil pressure is great and I have no lights on but the second I take my foot off the gas and either slow down or stop I loose oil pressure and my oil pressure light comes on. Any Ideas on what I need to do??

 

Wow what high mileage service you have from the motor.
What's happening is the oil is thinning when it heats causing the viscosity to drop
With the following, i'm not trying to scare you but immediate action has to be taken before using the truck very much longer.
First, have the oil pressure checked with a gage at the sender port to see what it is hot.
It should be at least 15 psi or more to be good for a longer term.
If yes, the sender may be faulty, a cheap fix.
If it is below 10 psi, the oil pump may be worn, bearing clearences may be getting too wide or both.
A judgement has to be made to replace the oil pump first if the pressure is too low.
Then if the improvement is not good enough the bearing clearence is all that is left.
One indication of this is hot starting causes the main and/or rod bearings to rattle/knock for a few seconds.
If yes, you either run it until it gives up or replace/repair the motor depending on what value you place on the truck and the cost.
The oil pump is right afront of the motor driven by the crank.
Front hardware has to come off, the front cover off to get at it.
Don't go to heavier oil than 5w30 trying to boost oil pressure and getting more life, for other reasons. The end result will be the same later on.
Good luck.

 

Thank you and it doesnt scare me Ive been glad to make it this far with the truck with the way its been drivin hauling things and all. And I only drove the truck back to the shop and at that time there was no noise being made. I will be spending tomorrow doing what you suggested at my shop If i have more questions we will be in touch thank you again!!

 

the oil sender and light are just an on-off switch set at 8 psi. If you have 7 psi, the gauge will say zero. If you have 10-20-30 psi the gauge reads half way up. Simple test is to change the sending unit and see what happens. It might have just got out of calibration. Better solution is to put a gauge on it and see if you have 7 psi, or actually zero.

 

The truck reads that the oil pressure is good through the gauges and when starting it cold the truck runs fine till its warm then there is no oil pressure reading at all either on the gauge or the pressure tester.

 

Since O/P builds as soon as you increase engine speed, I might check the oil pressure reliefe valve. If there is some trash holding it partially open, you would loose O/P at low RPM. I don't know where it is located exactly on your truck but it's usually mounted on or close to the oil filter. Good luck

 

Your at the point I mentioned earlier.
Pump has to be replaced if for no other reason than the mileage.
All pumps have pressure relief valves but if the pump is worn to excessive internal clearance, your only seeing the effects of oil viscosity vs temperature.
Good luck.

 

Thank you and I wasnt sure if it had an oil pressure relief valve or not but I will find it and look at the condition its in and the oil pump is the first thing thats going to be replaced because yes its a lot of miles but its been one heck of a truck
Thank you all for the help!!