Low oil pressure at idle

[imahistorylover]

Ok I have a 1983 Ford Ranger 2.0L, it's a 4 speed manual no powersteering on power brakes. It has a 1 barrel carb natually aspirated. The engine and clutch both were rebuilt in the last 2 years by the guy I bought it from who did have a shop do it. My issue at idle and only after it has been allowed to warm up my oil pressure drops to nill going down the road its fine cold its fine all the time. No noises out of the ordinary and it runs like a champ. The oil is clean but could use a change also I check the level cold its over full I check it hot and its right on the mark. This truck is my baby what do I do am I killing my truck?

 

[Mark_88]

Howdy! I had a similar setup until I change the motor and I've always had the oil pressure too high...too much blow-by...which shouldn't be a problem with yours.

What you might want to check is the actual type of oil pressure sender is on the truck. It should be just behind the breather near the firewall. They screw into the hole in the head and it will either be a round can about the size of a hardball...maybe a lacrosse ball, or it will be like most other sensors with a plug type and wires running out of it.

If it is the latter of the two then it is pretty much a very basic sending unit and not very reliable. Many people switch it out for the other type that are a bit more accurate...

I think if you pull the cap on the valve cover and see oil in the cap or it looks like it is getting oil up to the head then you are probably OK...but I'd suggest getting the best sending unit you can find/afford...

If the oil situation is OK the other thing to keep an eye on is the cooling system. The heads can crack very easily if they over heat...so make sure you keep an eye on the temp gauge, regularly service and maintain the coolant, and if the water pump wasn't swapped out when the engine was rebuilt then consider doing that as well as the thermostat in a few years...it will pay off over time...

 

[imahistorylover]

Cooling system is top notch new pump and radiator could it be just the sending unit because the truck does not act like its starving for oil, oil isnt burnt or anything like that other than being a little cold natured damn thing will run down the intersate at 80 mph like it singing a song. If its nothing but a sending unit I wont worry about too much until next payday but if I need to park it however.....

 

[Lil-Pony]

Or you could get a cheap aux oil pressure gauge and truly know the oil pressure

 

[RonD]

If you don't hear the "tick, tick, tick" of the rockers at idle then you should be fine driving the truck.

The top of the engine has the lowest oil pressure, so if pressure gets down to 0 there, you will hear it.

Normal idle oil pressure when warm is about 6psi, but can drop as low as 2psi.

Oil pressure is based on resistance in the oil outlets in the engine, so mostly the bearing clearances, the gaps between the surfaces where the oil pressure pushes it out.
As an engine gets more miles these gaps increase so oil pressure drops accordingly.
Since oil pressure is resistance pressure one bearing wearing faster will lower the oil pressure for the whole system.
Like a soaker hose, if you make a larger hole in the hose pressure drops at all the smaller holes.

Since your engine was reported as "rebuilt" the bearing clearances should be tight so higher resistance with resulting higher oil pressure.
The '83's still had the "real" oil pressure gauge and sender, later years are using switches.
I would get a new sender, $18, to see if that's the issue.
If not then maybe the "rebuild" was just the top end and timing belt, so crank and rod bearings still have the high miles on them.
If that's the case just run a thicker oil, 40w at operating temp, that will raise the resistance at the bearing gaps and so the oil pressure, this will make sure all passages are getting enough oil.

 

[Lil-Pony]

If you don't hear the "tick, tick, tick" of the rockers at idle then you should be fine driving the truck.

The top of the engine has the lowest oil pressure, so if pressure gets down to 0 there, you will hear it.

Normal idle oil pressure when warm is about 6psi, but can drop as low as 2psi.

Oil pressure is based on resistance in the oil outlets in the engine, so mostly the bearing clearances, the gaps between the surfaces where the oil pressure pushes it out.
As an engine gets more miles these gaps increase so oil pressure drops accordingly.
Since oil pressure is resistance pressure one bearing wearing faster will lower the oil pressure for the whole system.
Like a soaker hose, if you make a larger hole in the hose pressure drops at all the smaller holes.

Since your engine was reported as "rebuilt" the bearing clearances should be tight so higher resistance with resulting higher oil pressure.
The '83's still had the "real" oil pressure gauge and sender, later years are using switches.
I would get a new sender, $18, to see if that's the issue.
If not then maybe the "rebuild" was just the top end and timing belt, so crank and rod bearings still have the high miles on them.
If that's the case just run a thicker oil, 40w at operating temp, that will raise the resistance at the bearing gaps and so the oil pressure, this will make sure all passages are getting enough oil.

this is one concept ive never really understood, if for instance we know there are no oil leaks, and the oil pressure is low wouldnt that mean that the flow of oil through the engine is high which is what we want, while if the pressure is really high that means the flow is getting blocked by bearings and what not which would lead to a slower flow of oil throught the block which could lead to failure?

 

[RonD]

this is one concept ive never really understood, if for instance we know there are no oil leaks, and the oil pressure is low wouldnt that mean that the flow of oil through the engine is high which is what we want, while if the pressure is really high that means the flow is getting blocked by bearings and what not which would lead to a slower flow of oil throught the block which could lead to failure?

The oil pump provides the flow but not the pressure, well not directly, because without flow there is no pressure.
The oil pumps flow/volume does increase with RPM so pressure will also increase, but the pressure can't exists without resistance.

The oil pressure sender is located on the main oil passage after the oil filter, so there is:
Oil pump->>>----high pressure bypass--oil filter-----sender------bearings.

If you were to cut the main passage after the sender, so there was just flow going past the sender, sender would read 0psi, because there is no resistance/back pressure, oil is flowing out at the same volume it is coming in, so 0 psi.
The pressure is created by the resistance against flow out of the bearing gaps.

When engine is at higher RPMs, so oil pump is providing more flow, the actual oil coming out at the bearings doesn't increase as much as you would think, pressure goes up because incoming flow has gone up but outgoing flow is almost the same, so sender is seeing more back pressure.

If one bearing gets worn the back pressure on the whole systems goes down.
The 2.9l had this issue, cam bearings would wear too much and lower pressure.

If oil pressure is low then gravity prevents it from getting to the top of the engine, so rockers "tick"