When she first starts and idles and runs for a short while, the oil pressure is around 40 (the gauge needle twitches a bit).
But after about 20 minutes of driving, at idle the gauge drops pretty low. The needle sits just above the slash after "0".
Not sure if this means it's near zero or does the slash mark indicate 10? Is this normal for a warmed up engine at idle?
Engine temp and oil temp are normal. I've included a pic.

Thanks in advance!



Oil Pressure reading at warm idle

 

What weight of oil are you running?
Is this a new occurrence or something that's getting more obvious?
Do you know when the last engine rebuild might have been?

Too thin, or if you have moderately to heavily worn bearings, and you'll get a very low hot pressure reading.
If you're using a 30 weight, try 40 or even 50. You don't want to run a diminished oil cushion on the bearings and make wear worse.

 

You should be using a 10W30 oil. The accuracy of the gauge has to be determined by temporarily connecting a hand held gauge to the oil pressure sender port.

 

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This is a good question, did this happen all of a sudden or has it been getting worse over time...

You should verify your oil pressure gauge but on an 82 you can't remove the existing sending unit because it is needed to tell the ECM there is pressure or it will turn off the fuel pump, a safety feature. Right above the oil filter is the sending unit for the oil temp gauge, put the manual gauge there and fully warm up the engine to check it...

Without knowing the mileage or history of the engine it's hard to speculate, worn bearings are a likely suspect, change the oil and try 10-40 if you are now using 10-30, just make sure it's High Zinc and get the gauge accuracy checked..

​​​​​​​60

 

The 2000 Jeep Wrangler I just bought has 172,000 miles on the 4.0L inline six.
When I first start it, the oil pressure reads about 40 psi on the gauge, about 30 psi cruising and drops to 5-10 psi at warm idle.
I found this is pretty normal for this engine with this milage and guys on the Jeep forums are running over 300,000 with the same oil pressures.
Valveoline make a 10w-30 “High Mileage” engine oil formulated for hi mileage engines.
I just changed to this oil and so far no issues.

 

I run 10/30 in my 84 which has the same CF engine.My gauge shows @ 45 when hot

 

Hi DF2000,
Stop driving it a do a few rests!
Try a mechanical gauge to get actual pressure.
Try a replacement sensor..

That's the oil I use in my '95 Jeep that has 223,000 miles on it. Give it a try.

 

First thing to do is confirm the actual oil pressure by putting a mechanical gauge on it.
If accurate that is way to low.
And I totally disagree with a earlier statement that says you should be running 10W-30 oil.
I've never run oil that thin in my C3 with original engine and I never will.
That's why we have a forum, you get more than one opinion here. I run 20W-50 Mineral based oil with high zinc.
And if my pressure dropped out like yours when warm I'd be pulling crank and rod bearings.

 

Are there any other symptoms like knocking?

years ago I had a very similar condition, but accompanied with a cold knock. Dropped the pan and my rear main was shot.

Sbc are easy peasy to rebuild though, thankfully.

 

Thanks for all the help. Will a low oil pressure at warm idle send a fault code?

Very confusing as to what a warm idle pressure is normal. I've read on this forum, people saying anywhere
from 40 all the way down to about 10. I'll try to put a mechanical gauge on it to see if its the sending unit,
but just wondering if going from cold idle of 40 to warm idle of about 10 is in fact normal.

Finally, in the picture I sent of my gauges, does anyone know what number the slash mark above the "0" represents?
Hard to tell if it denotes 10 or 0.

Thanks!

 

No knocking. Seems to run fine even at that low warm idle pressure reading

 

I would be suspicious of the oil pressure gauge or even more likely the sending unit. You need to get an external gauge that you can connect and VERIFY the actual oil pressure. I bought one at Harbor Freight for less than $25 and has all the adapters in a neat plastic box made to store the parts. A worthwhile investment to have in your garage.

A heavier oil will make a huge difference in the oil pressure readings I suspect. I use a specific grade of oil because of the clearances in the bearings and it was the builder in my case helped me understand the importance of the weight of the oil.

Be very careful if you change the sending unit as I found many of them are the type that can indicate ON or OFF and are for an "idiot light" on the dashboard. I had to search to find the Oil Pressure gauge sending unit that was designed for a sweep gauge. The wrong sending unit could simply be on and off instead of the sweep, it was a PIA when I had to find a new oil pressure gauge for my Boat's diesel engine.

 

10 PSI on a SBC at idle is not normal. More like 30PSI is normal.
You've had a lot of good advise here suggesting get a mechanical gauge and tap it into the oil port at the back of the block near the intake parting line. We say this because there are many different senders out there that aren't matched to the oil pressure gauge, and will give false readings.....we're trying to see if it's just a false reading by substituting a mechanical gauge.

 

Chevy has a 40psi spec for most SBC, but that is hot and running and at ~3000 rpm.
That means you are basically in good shape and the motor should live a long time.
But how low it drops to at idle is not in the specs. This is just based on experience. And has a lot to do with engine wear.

True some SBC engines 30 psi at idle and some have 20 psi. Either is perfectly acceptable and has more to do if it is a factory build, or a rebuild, and what bearing and rod side clearances the motor was built with.
My LT-1 was blue-printed at the machine shop to the loose side of the specs ~1980. Common practice back then on an engine to be run hard. It had 20 psi at idle. But by 2000rpm was already over 30-40. That one ran 65psi past 30-4000, had the high pressure Z28 oil pump spring, and got raced for 27 years, frequently to 7000rpm with no issues, and no further change in oil pressure. IIRC it was 10-30 wt or 10-40 wt. That was the std volume, but high pressure oil pump. All other SBC use the std pressure pump.

Now 10 psi at idle means you have rod & main bearing wear, or someone did a very sloppy rebuild, and the bearing oil clearances should be checked and replaced. Sometime in the near future, or the oil viscosity increased a notch and see if that gets it back closer to 15-20. If 40 or 50 wt won't get it up to 15-20 psi anymore, now it is bearing time.
Zero at idle means park the car right now.

And yes I know some people who have run their SBC 300,000 miles with 10 psi at idle. But they also drove it very easy.
Some people drive a corvette easy, and some do not....

I have seen engines where the bearing clearances were fine, but the oil pressure at idle is still low. If the crank journal radius was ground badly, or the rods worn, and the rod side clearances opened up to the outside of the spec, you will never have good oil psi at idle. Not without a new crank or rods.

So anywhere from 20-30 at idle is fine. 10 psi is pushing it, and you should try different oil or investigate.

 

Thank you for the reply.

Looking at the pic I provided, can anyone advise if the slash mark just to the right of "0" indicates zero or 10 psi.

Thank you

 

If the gauge is linear the mark halfway between 0 and 40 should be 20psi. So yours is showing ~5 to ~10psi, if it is correct.
10psi is the commonly recommended lowest "safe" idle psi.
What kind of oil do you have in it?
I would go up from 10-30 to 40 or 50 wt.
Each 10 step in oil wt may gain you 5 psi at idle.

But I would seriously recommend comparing your electric gauge to a known good mechanical one.
The senders for the electric ones are notoriously inaccurate, and change with time.

But thicker oil decreases the oil flow. So you want just enough to get 15psi at idle.
SBC oil flow is 3-5gpm.
Changing the oil viscosity can have a 20% effect on that, so you do not want too thin, or too thick either.
https://www.motortrend.com/how-to/cc...evy-oil-pumps/

 

that indicates zero

 

Thanks for the info.

Would love to put a mechanical gauge on it to see if in fact its simply an issue with the sender or the in-dash gauge.
I've read all the posts where people describe where too find the sender on the block, but I just can't find it looking down
from the top (don't have a way too get underneath if that's what's required.
Sorry for the perceived ignorance, but I'm pretty much a newbie.

Does anyone have a picture or illustration showing where the oil pressure sending unit is?

 

Thanks, that's what I was fearing. But engine and oil temps are both good and don't hear any nasty noises
coming from engine. Oil pressure is 40 psi at cold idle and while cruising around at low RPM.
The needle does flicker a lot, so I'm suspecting that its an electrical issue... hopefully!

 

My 81 has the original motor 100000 kms and when cold it's 60psi and hot it's 30psi. It may be time to look a little deeper