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  #1  
Old 07-03-2014
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low oil pressure at idle only

Alright guys so my 3.0 ranger has been having a problem with low oil pressure when it idles or you are coasting at anything less than 1200 rpm. I have replaced the oil sending unit sensor, just did a oil filter and oil change this morning and still didn't fix it. It has 146500 miles on it and I replaced the heads at 140000 when they went out.
My next attempt to fix it will be to replace the oil pump but I have been reading and some people say their cam bearings went out or related problems that is causing the same problem.
What do i do to fix it?
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Old 07-04-2014
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What oil are you running now?

Try a thicker oil, if you are running 10w-30, switch to 20w-40.


Oil pressure is back pressure.

The oil pump provides the flow of oil to the filter, that filtered oil flow goes to the main oil passage, where the oil pressure sender/switch is located, there is 0 pressure there at this point.
When the flow breaks off into smaller passages to the bearings and is then forced out thru the narrow bearing openings there is too much oil, so a back pressure builds up in the system, this back pressure is what the oil pressure sender/switch is detecting.

Depending on the size of the engine there are 30 or 40 oil outlets and any one of them can effect overall oil pressure.
If for example a Cam Bearing is getting worn then it passes more oil than the other bearings, so overall pressure drops.

Ford oil pressure switch is set at 6psi, so at 6psi or higher oil pressure will show about 1/2 on the gauge(and oil light will be off).
At 5psi gauge will drop to 0(oil light will come on).
So if light is going on and off your pressure is hovering between 4 and 7psi at 700-1,200rpm

In OHV engines the rockers will start "tapping" when oil pressure is at about 2psi, gravity is not allowing as much oil to be lifted up the push rod and upper passages.

A thicker oil will bring up the overall oil pressure even with a worn bearing, so upper engine gets lubed and other bearing still get good flow.

Lucas Oil makes some good additives to raise oil pressure since you have just changed the oil, then next time use a thicker oil with no additives.


The oil pump is the most lubed part of the engine, lol, so these generally last the life of the engine.
But you could have a loose or broken oil pump relief valve which is leaking oil back into the oil pan so the flow to the oil filter is less than it should be, this also lowers oil pressure at the bearings.
The relief valve is part of the oil pump housing.
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Old 07-04-2014
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Quote:
Originally Posted by RonD View Post
What oil are you running now?

Try a thicker oil, if you are running 10w-30, switch to 20w-40.


Oil pressure is back pressure.

The oil pump provides the flow of oil to the filter, that filtered oil flow goes to the main oil passage, where the oil pressure sender/switch is located, there is 0 pressure there at this point.
When the flow breaks off into smaller passages to the bearings and is then forced out thru the narrow bearing openings there is too much oil, so a back pressure builds up in the system, this back pressure is what the oil pressure sender/switch is detecting.

Depending on the size of the engine there are 30 or 40 oil outlets and any one of them can effect overall oil pressure.
If for example a Cam Bearing is getting worn then it passes more oil than the other bearings, so overall pressure drops.

Ford oil pressure switch is set at 6psi, so at 6psi or higher oil pressure will show about 1/2 on the gauge(and oil light will be off).
At 5psi gauge will drop to 0(oil light will come on).
So if light is going on and off your pressure is hovering between 4 and 7psi at 700-1,200rpm

In OHV engines the rockers will start "tapping" when oil pressure is at about 2psi, gravity is not allowing as much oil to be lifted up the push rod and upper passages.

A thicker oil will bring up the overall oil pressure even with a worn bearing, so upper engine gets lubed and other bearing still get good flow.

Lucas Oil makes some good additives to raise oil pressure since you have just changed the oil, then next time use a thicker oil with no additives.


The oil pump is the most lubed part of the engine, lol, so these generally last the life of the engine.
But you could have a loose or broken oil pump relief valve which is leaking oil back into the oil pan so the flow to the oil filter is less than it should be, this also lowers oil pressure at the bearings.
The relief valve is part of the oil pump housing.
I just did an oil change on it and I was reading up and they said to use the recommended 5w-20 so I put that in there. But now you're telling me to put thicker oil. Lol But what you're saying does make sense
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Old 07-04-2014
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Yes, 5w-20 will make the oil pressure very low on higher mile engines.

The oil sold specifically for higher mileage engines has a higher viscosity(thicker) when warmed up.
This keeps pressure up and also helps seal rings and valve guide seals so lowers "oil burning" potential.

Too much oil pressure is bad as well, it will cause a squirting effect at the bearings, this will cause uneven cooling and wear patterns.
This is why there is a relief valve on the oil pump, at high RPMs the oil flow gets too high so the relief valve(ball bearing with a spring, lol) is pushed open and some of the flow is diverted back to the oil pan.

Since the oil being sent passed the relief valve isn't filtered, at high RPMs when relief valve opens a bit a piece of gasket or silicone or ???, can be sucked in to the valve and lodge next to the ball bearing so it doesn't close all the way when RPMs are lower.
This can lower oil pressure as well, since low RPM flow is less than spec for the pump.
It is a long shot but not a 0 shot

Last edited by RonD; 07-04-2014 at 12:47 PM.
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Old 07-04-2014
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Quote:
Originally Posted by RonD View Post
Yes, 5w-20 will make the oil pressure very low on higher mile engines.

The oil sold specifically for higher mileage engines has a higher viscosity(thicker) when warmed up.
This keeps pressure up and also helps seal rings and valve guide seals so lowers "oil burning" potential.
Okay well I'm going to go and drain the oil in it and going to go add thicker oil and some Lucas oil additive and I will see what that does. Thanks
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  #6  
Old 07-25-2014
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I bought my used 2000 3.0 Ranger from a Ford dealership. It had 212K on it's odometer when I purchased it. On the service order record that the dealer presented me with, it showed their usual "50 point inspection passed" along with an oil change. The dealer used 5w-20, which I thought was a bit thin for aan engine with this high of milage, but I did not question it.

After about a week, I discovered a small dime size puddle of oil on the driveway. After some inspections done by me, I determined it was coming from the rear main seal (seperate post on this here). I switched the oil over to Valvoline high mileage 10w-30, along with a quart of bars leak rear main seal conditioner. No more leak.

The truck is running fine with the higher viscosity oil, so no worries there. I believe Fords stand on the recommendation of 5w-20 is more because they want to meet the fuel economy ratings pressed onto them by the government requirements; a lower viscosity fuel can provide a minute gain in fuel economy; not seen by you and me, but with thousands of cars on the road, it adds up.

One note I discovered; 5w-20 is not available in some European countries, so Ford has to recommend 10w-30 for use. So if anyone tells you its unsafe to use anything other than 5w-20, that would be an untruth.
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