What is "good" oil pressure?

[TallPaul]

If you vehicle specs oil pressure to be, say, between 40 and 60 psi at 2000 rpm (hot), then what to you make of it if you are getting 35 psi at 2000 rpm hot? Or 25 psi? Would you be inclined to run a thicker oil? Or would you just fall back on the old rule of thumb that 10 psi per 1000 rpm is good? But that puts idle pressure below 10 psi. Is that good?

What if in the above example you went from a 30 wt multigrade to a 40 wt and got 42 psi? Would that be good enough or should you mix in some 15w50 to get it up a bit more, say 47 psi or more? What is the criteria? Does if matter? Who cares?

And what about idle speed pressure? What is the minimum you should have there? Is a large spread between 2000 rpm pressure and idle pressure good, bad?

What about cold pressure? I was told a drop from about 54 cold to 35 hot at 2000 rpm was a bad sign and the bottom end of the engine should be looked at. I don't buy it.

More or less curious and hoping to learn something from fellow BITOG folks.

 

[XS650]

No answers for you Paul, but be sure you are using an accurate pressure gauge before you spend too much time trying to solve the suspected problem.

What engine? Someone here may have oil system experiance with that particular engine.

[ August 23, 2004, 12:30 AM: Message edited by: XS650 ]

 

[Ugly3]

Wow, you are asking a lot of questions.

I understand that most "idiot lights" are set between 5 and 7 PSI.

If my normal oil pressure was 60 PSI at 2000 RPM I would get concerned about a possible mechanical problem if the pressure dropped to the 35 to 40 PSI range. That is not to say that engine failure is about to happen, just that something is very different now than when the engine was newer.

If my hot idle pressure was normally 30 PSI I would be getting concerned at about 10 to 15 PSI.

My assumption is that the pressure has been dropping slowly over time.

I would start up the viscosity chart slowly to see what the impact is. If my usual oil were 0w30 I would try a 0w40 first, maybe a 5w50 next, however I would not go much beyond 1.5x the original viscosity (or the maximum recommended viscosiy).

If this does not get the oil pressure moving back toward normal ranges then my view is that oil is not going to solve my problem. My concern would be that going to still higher viscosities while possibly improving oil pressure some would do more damage than good.

 

[TallPaul]

Thanks all, Not really trying to solve any particular problem, but just trying to learn more about oil pressure in general. I am part of the example in that my Ford 300 I6 (103,000 miles) was getting 35 psi on 30 wt and now gets 42 on 40 wt. That has been sufficient, yet I am tempted to mix in a quarto of 15w50 just to see what happens. I reallize too high is not good either and that the 35 psi probably was good enough. I have a similar thread in Car and Truck Lube, but wanted to focus more directly on oil pressure here. I had an article from the web on oil pressure and if I can find it will post it.

 

[tpi]

I've had two engines over the years with low oil pressure. One was a 1977 Camaro I bought new with 305 V8 small block Chevy. I ran 20W50 of various brands. After about 30 miles of fwy driving the oil pressure would drop to 10 PSI at idle. If it was pushed up a mountain road in hot weather fairly hard, at idle the oil pressure would drop to 2 PSI. The 2000 RPM oil pressure was at least 20 PSI.

I had a small block Buick powered boat which exhibited the same behavior. After a half hour run on the river I'd get the "tap the gauge is it working" oil pressures at idle. This was with 15W50 Mobil 1. Neither engine had lifter or bearing noise at idle and both lived long hard lives. It would have been interesting to check the oil pump and bearing clearances in those engines.

 

[TallPaul]

Some interesting articles on oil pressure:

http://www.babcox.com/editorial/us/us80022.htm

http://www.aa1car.com/library/us1097.htm

Haven't read them, just skimmed. Looks good though. Maybe have to post in "Interesting Articles" forum.

 

[OffOrWFO]

TallPaul
You said "I was told a drop from about 54 cold to 35 hot at 2000 rpm was a bad sign and the bottom end of the engine should be looked at."

The way to tell if the bottom end needs rebuilt is to:
Get an accurate mechanical OP guage.
Then run the engine full throttle for 20sec (at highway speeds)then let completely off and watch the guage. When you let off the crank will center its self in the main bearing because there is no longer a down load on the crank. When the crank is centered, less pressure escapes from around the main bearing. If the OP increases more than about 2-3 psi then the main bearings are getting loose.
I have personally seen this and have pulled the pan and found worn mains, replaced them and the change in OP went away. Its been my experience that rod bearings wear slower on my V8s and are not a major source of lost OP(may not be the case on a long stroke straight 6). Cam bearings can also cause low OP.

Do you know what kind of oil temp you are running?
I have an engine that runs 90 psi at 3krpm cold and 67psi hot. I think that kind of drop is normal
A auto trans can make idle pressures really low because it pulls the idle speed way down, also normal.

 

[TallPaul]

Excellent info OffOrWFO! I have an Autogage (by Autometer) mechanical gauge. I have never seen the pressure rise upon releasing the throttle, but will try it later, exactly how you describe to do it.

From the articles (one seems to be a condensed version of the other) it appears my lower pressure (not extreme IMO) is likely due to a weak bypass valve spring or a worn pump. It said loose bearings will throw a lot of oil and can cause increased consumption, but this truck consumes next to nothing. I have had 3000 mile OCIs where I did not have to top up.

Very interesting in the article is that the oil pump wears faster since it is the one mechanical component that regularly gets unfiltered oil. Hmmmm, wish I'd of had the pump replaced when the pan was off.

Somewhere (perhaps NORIA.com) I read about oil pressure mapping. You get it hot and then record RPM and pressure at several hundred rpm increments starting at idle to somewhere in the higher rpm range (can't remember exact). Supposed to use good mech gauge too. Bottom line is when you plot the curve, a line is then drawn from the idle pressure to a peak pressure at higher rpm. All pressure points between must be above that line, else there is a problem. Mine checked out good on that test.

I have no idea what my oil temp is. Would love to know, but am not up to the hassle of gauge installation.

 

[dropitby]

I have a Autometer electronic gauge what's the difference wouldn't it work.

It seems to me each engine should have specs showing oil pressure at idle and 2000 RPM hot-cold etc.

[ August 25, 2004, 05:25 AM: Message edited by: dropitby ]

 

[TallPaul]

quote:

---

Originally posted by dropitby:
I have a Autometer electronic gauge what's the difference wouldn't it work.

It seems to me each engine should have specs showing oil pressure at idle and 2000 RPM hot-cold etc.

---

Historically the electronic gauge is not as responsive as the mechanical; however, I have heard that modern, high quality electronic gauges are just as good. Might want to double check with Autometer tech folks, but just give the test a try.

I tried it and get anywhere from 1 to 2 psi jump. Increments are 2 psi, so .... But it is an Autogauge mechanical gauge by Autometer, so I think it looks pretty good for me.

Of course I did that with 10w40. What if I were running the 10w30 that is specifiec for the truck? Maybe would jump too high? So I think this might be a good test to see if you should run a thicker. Would love to see this test done on some vehicles running 5w20!

 

[HerkyJim]

For the "Classic" Chevy small-block V-8, we say that 10 psi per 1,000 rpm is just fine.

My $0.02

 

[427Z06]

Most shop manuals will give some minimum specs on oil pressure.

For example, on one of my vehicles, the shop manual says to get the engine up to operating temperature (fans come on at least twice), oil temp 176F, at idle 10 psi minimum, at 3,000 rpm 71 psi minimum.

If you want to be more accurate on the oil temp without installing a gauge, consider a $100 IR thermometer. But for basic troubleshooting, it's probably not necessary.

 

[k1xv]

Back in the mid 1970s, I had a summer job driving a 28 foot straight job truck. The truck I drove could best be described as one step from the junkyard.

It was an International Harvester (now Navistar) with a v-8. With 30 wt oil, it had 20 psi when cold, and as it ran, the oil pressure kept dropping til it would read under 5 psi, essentially just above zero. The owners did not care.

I was always expecting that truck to leave me stranded somewhere. It never did. However, if I had a personal vehicle, and the oil pressure was less than spec, I would definitely want to know why, and try to correct it.

 

[JohnBrowning]

Engine Rules of Thumb!!

1) 25-35 PSI at idle.
2)5-10 PSI increase per 1000 RPMS above idle until bypass pressure is reached.
3) Alway look for positive pressure within 30 secons of start up and full pressure within 60 seconds.
4) You should always get positive increase in pressure withincrease in RPM's.

If you have a mechanical pressure guage test to see if sender is bad. If you do not have a guage a new sender is cheap compared to a good guage. A sender will run $10-$15 at a parts store. A decent pressure guage will run $65-$250 dallors.

If this engine is over head cam you really do not want less then 20 psi at idle to ensure long and happy valvetrain life. If it is over head valve you really want at least 10 psi at idle. The lifter will start ticking real bad at around 5-8 psi. SO long as your minimum at idle look ok and presure comes up to 40 psi or higher off idle you will fine. These numbers are for normaly asperated engines that are daily drivers not comercial applications or raceing applications!

 

[TallPaul]

Thanks John, Guess my gauge is not that good as it comes in a two gauge set for around $40 or so. Since I have a pushrod ohv engine and get 20 idle hot and 42 or so at speed, I think I am in pretty good shape. Pressure comes up quickly too.

Also, you mentioned an idle pressure with 5-10 psi per 1000 rpm increase. I always thought the "10 psi per 1000" rpm was not correct, but got oversimplified as it passed among gearheads and such to where folks simply thought 10/1000 rpm, so 10 at idle is fine.

 

[JohnBrowning]

Tallpall, everthing is relative. I am sure that you guage set is fine for what you use it for. I am also pretty sure you do not send you guages out to be calibrated on a regular basis! Again all things are relative.

The oil pressures rules I cite were the same rules taught to me by Engineers and Tech.'s that worked for BMW dureing the last great war! It is hard to say when they were developed? The rules of thumb were meant to ensure no oil related failures with what would not be consider large clearance engines with very poor quality oils. You will never have an oilrelated failure due to lubrication system failing to provide adequate oil volume or pressure at these levels. Today on some applications this type of pressure at needed volume levels would leave the sump dry at high rpm limits.

This is why I amended the general rules with what I have obsevred to be the true limits as seen in the field as a Tech.

GM in years past would not warranty an oil pump related repair under warranty until the pressure was below 10 psi at idle like around 7 psi. This is also when the lifter just make an awful noise. I have seen plenty of these vechiles were the oil pressure came up to about 30 psi at speed and droped down to 7 psi at idle. Usualy these engines had bottom ends that had what could be called normaly wear on the lower end but really accelerated wear on the valve train.

Things to check if this has happen all of a sudden:
Sender
Ground
Distributor seal
Anytype of drive seal touching the oil pump or cam.
Bypass valve and spring