[horseman68]

Hello everyone,

I am new to this forum, but glad to be here. I have recently been noticing that my oil pressure drops down very low when idling. But when moving at high speeds, particularly above 40mph the oil pressure gauge needle goes back to the middle. I am currently running Valvoline 5w-30 btw. This is my daily driver so I don't want to be running the engine like this for too long. Thanks in advance for the help.

1968 Mustang coupe w/ 289

 

[Killer95Stang]

Go to Autozone or another local auto parts store and buy a cheap oil pressure gauge so you can verify things before you drive around like that. You'll be out $20 -$ 25, but it can confirm either a bad gauge in the car or a more serious problem with the engine.... 3...2...1.... GO!!

On a sidenote, if everything is good to go and it wasn't major engine trouble, take that gauge out and put it on a shelf somewhere. I used to enjoy driving my cars without accurate aftermarket gauges. Then one day I installed a water temp and oil pressure gauge from Autometer. I haven't stopped looking at them ever since and I never enjoyed driving my weekend cars ever again. Ignorance is bliss!!!

 

[myfirstcar66]

There are so many things that can affect oil pressure...and these gauges aren't very consistent across the board, so the first thing I would do is use a quality mechanical gauge with numbered increments to tee into the same port as where your gauge sending unit is and see what you truthfully have at idle and at varying RPMs. Some say use a different weight of oil for summer vs winter depending on where you live, low o.p. can be an indicator of worn bearings or a weak pump. Measure well.

 

[1ofAMillion+]

Your oil pressure has recently dropped or you just recently noticed?
Change the oil, send some of the old oil to be analyzed.

 

[66conv6]

Also, what part of the country are you in? Climate...

 

[Killer95Stang]

We know zero about his engine or the gauge he is using. We can assume this is a stock engine (maybe a rebuild) and 52 year old stock dash oil pressure indicator gauge. And to top it off, this isn't a weekend car, so putting it up on stands until a Lab result shows up probably won't work for him. But, $20 bucks and 30 minutes of work to accurately know what the pressure seems reasonable to me.

 

[horseman68]

Also, what part of the country are you in? Climate...

I live in southern California, so good weather year-round.

 

[j persons]

I remember reading in a shop manual that a SBF should have a minimum of around 20PSI at hot idle. I could be wrong on the PSI but I remember that it was fairly low.

 

[66coupe289]

I have owned my mustang for 37+ years. I bought it as a poor college student and used it as a daily driver at that time. What I would (could) do then is different than now. Going back in time when I had little money and needed to band-aide my car to keep it running this is the course of action I would take in this particular order:

1.) 5-w30 is thin for an old engine, let alone a worn engine. Try a thicker oil. 10w-40 is harder to come by these days. You might try a 15w-40 or even 20w50. If no improvement, then...
2.) A high volume pump will help compensate for larger bearing clearances and reduce the chance that the bearing on the far end of the oiling system gets starved.
3.) While having the pan off to install the pump I would inspect the bearings. Main and rod bearings can be replaced with the engine in the car and the car on jack stands. Without turning the crank clearances are not likely to be ideal, but if bearings are worn it should help some.

Again, those are band-aides, not fixes. However, they may help you get by for a year or two for less than $200 and can be done on a Saturday.

 

[22GT]

Hello everyone,

I am new to this forum, but glad to be here. I have recently been noticing that my oil pressure drops down very low when idling. But when moving at high speeds, particularly above 40mph the oil pressure gauge needle goes back to the middle. I am currently running Valvoline 5w-30 btw. This is my daily driver so I don't want to be running the engine like this for too long. Thanks in advance for the help.

1968 Mustang coupe w/ 289

Define "low". Ford specifies a minimum of 35 psi hot at 2000 rpm.

 

[awhtx]

I have recently been noticing that my oil pressure drops down very low when idling.

What do you consider to be low? 10 psi per 1000 rpm is considered adequate. Your engine should idle at around 600-700 rpm so 6 or 7 psi is adequate at idle.
I assume you still have the factory gauge. If the needle moves off of the low end mark you have enough pressure. Many older cars only had a warning light that came on when the pressure was low and it only takes about 5 psi to turn that warning light off.
Put 10w-40 oil in it if you're concerned.

 

[Woodchuck]

What do you consider to be low? 10 psi per 1000 rpm is considered adequate. Your engine should idle at around 600-700 rpm so 6 or 7 psi is adequate at idle.
I assume you still have the factory gauge. If the needle moves off of the low end mark you have enough pressure. Many older cars only had a warning light that came on when the pressure was low and it only takes about 5 psi to turn that warning light off.
Put 10w-40 oil in it if you're concerned.

Ford specifies a hot oil pressure of 35-55 psi at 2,000 rpm. That is considerably higher than the "10 psi per 1,000 rpm" urban legend. IIRC, way back when, somebody tested oil pressure senders for the warning light application and found they opened at around 8-10 psi. I would not want to see a hot oil pressure, at idle, of less than 15 psi in order to provide the crankshaft with an adequate "cushion" of oil between the bearings and journals.

I've seen a number of old Fords where the oil pressure warning lamp would "flicker" at idle and, upon dropping the oil pan for inspection, would many times find the oil sump screen (and pan) loaded with sludge, inhibiting oil flow to the pump.

 

[DKGRN68]

5W-30 is the wrong oil weight. Switch to 10W-30 or 40 and you should see a difference.

 

[66coupe289]

What do you consider to be low? 10 psi per 1000 rpm is considered adequate. Your engine should idle at around 600-700 rpm so 6 or 7 psi is adequate at idle.
I assume you still have the factory gauge. If the needle moves off of the low end mark you have enough pressure. Many older cars only had a warning light that came on when the pressure was low and it only takes about 5 psi to turn that warning light off.
Put 10w-40 oil in it if you're concerned.

That "10psi per 1,000" was more so an upper end guide line and not gospel. The point of it was, a stock engine that rarely sees over 4,500 rpm is good with 45psi. Start spinning that engine to 6,000, then you really want more pressure.

6-7 psi at idle? I would not run the engine. Pressure is measured near the pump/filter which is at the start of the journey for the oil. As the oil moves through the system it loses pressure due to friction losses.

 

[awhtx]

If the recommended hot oil pressure is a minimum of 35 psi at 2000 rpm why did Ford install a sensor for the warning light that turns the light Off at somewhere between 5 and 10 psi? Why didn't they install a 35 psi sensor?

 

[stephen_wilson]

At idle the oil pressure is obviously going to be quite a bit less than 35, so the light has to be set low enough that it will never go off during normal operation. The problem is idle pressure can vary a lot, for a variety of reasons, with is why they use 2000 RPM as a "check point". On my race car the light is around 15 PSI, and it's considered safe and normal for it to come on at idle after a hot track session. Bearings just don't need much oil pressure at idle, with no load on the engine.

 

[GT350HR]

10psi per 1,000 was coined by Smokey Yunick and related to CHEVROLETS which use a different style of oil pump. Automotive journalists dog piled on the statement using it as a blanket statement for "every" engine and it is NOT true. My 8,000rpm 331 sees 55-60 at that rpm and has been together with the same bearings for TWENTY years. Hot oil idle pressure is 20 @ 1,000 rpms. BTW "standard" volume oil pump. If the pressure increases with rpm , you are doing good. If you want more "pressure" on the gauge , go to a higher viscosity oil as has already been suggested. Here in So Cal you could easily run a striaght 30 , 10-30,10-40, or a 20-50 like I run. Just don't do that in a 2,000 up engine designed to run on thin oils.
Randy

 

[22GT]

If the recommended hot oil pressure is a minimum of 35 psi at 2000 rpm why did Ford install a sensor for the warning light that turns the light Off at somewhere between 5 and 10 psi? Why didn't they install a 35 psi sensor?

Because the light would be coming on all the time, every time you stopped at an intersection.

A friend of mine has a pretty worn Ford GAA V8 engine. We have the idle set about 100 rpm higher than stock just to keep the oil light from coming on.

To keep a 35 psi light off, you'd have to idle at 2000 rpm.

 

[awhtx]

If the recommended hot oil pressure is a minimum of 35 psi at 2000 rpm why did Ford install a sensor for the warning light that turns the light Off at somewhere between 5 and 10 psi? Why didn't they install a 35 psi sensor?

I asked this question sarcastically. Obviously the driver has no indication of his actual oil pressure at 2000 rpm so it is merely a guideline for a mechanic diagnosing the engine. As far as Ford is concerned the driver only needs to know that if the red warning light is not illuminated there is sufficient oil pressure in the engine.