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Oil pressure gauge reading low

Scramblin_Jim

Scramblin_Jim

Legacy Registered User
City
Goodyear
State
AZ
I noticed that my oil pressure gauge never gets very high on its scale. Stay about 20 psi most of the time even when driving down the road at 50mph in 4th gear. Of course the Tack is worthless as it is also reading either 2500 or 3000 or 0. Seems to bounch around while engine is at high rpms.

I was told that during a rebuild of an engine if the rebuilder was not careful in trimming the gasket material down to the proper fit, i.e installing a too fat gasket, there could be a restriction of the oil flow within the engine. Maybe this is what is causing my oil pressure gauge to read low. Please explain this one to me as I am a little concerned about my engine with its now 6K miles of use after a rebuild. (no I don't know who did the rebuild or what was done to the engine) The compression check today had pressures at 160 +/- 2 for all 6 cylinders.
 
Welcome to the world of AMC 258s. That's perfectly normal. If you want to see more oil pressure you'll need to swap in a 4.0 ;)

Seriously, that's all you'll ever see. Just get used to it. Simply because there's more numbers on the gage doesn't mean you'll ever need them :D It's like a Hyundai having an 80 MPH speedo :cool:
 
Not entirely to blame on the 258...when I temporarily swapped in a different gauge, I got higher more accurate readings. Now I would just like to figure out how to make the stock set work better...
 
I think Eric meant "in a stock original CJ".

In any case, the hot ticket is a quality "mechanical" oil pressure gauge. Involves running an oil line into the cab, but then you have no doubts about where you stand.

Paul
 
Yup, doesn't matter what the actual reading is, just that the stock gage reads around 20 and that's it and it's normal.
 
going to try testing the gauge

I think I'll try testing the gauge to make sure its reading properly and then put another mechanical gauge into the sensor hole to see what pressures are read there. Will that work?
 
finally got around to testing

I took out the oil sending unit and with parts from the locale hardware store ran a 1/4" tube from where the oil sending unit was into my cab and attached it to a digital pressure gauge. This is a temp setup just to see what readings I would get driving around.

The results:
The digital pressure gauge limits out at 33 PSI and then flashes a "F" on the display as I rev'ed up the engine. At idle I was seeing 10 to 15 PSI bouncing on the display. The gauge has a "zero" function to give a relative measurement. So I "zero'ed" the gauge at idle and then rev'ed up the rpms to real high (tach does not work) and got a reading of 30 on the gauge. As I brought down the engine rpms to idle again the gauge when into flashing "L" on the display. I assume that means a reading below zero.

Based on this digital gauge I think my oil pressure is running between 10-15 psi at idle, and as high as 40 psi at very high engine rpms.

Would you agree with my assessment?
 
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my experiences

When I start my engine up (258 with 4.0 Head and lots of other goodies) I get an initial reading around 60-70psi on the stock gauge. Once it warms up, i get about 20 at idle. When I drive it it goes between 45-60 depending on the rpms of the engine.

Seems I have the only stock unit that still functions as it should. Now if I could only get my speedo to stop bobbing all over the place...
 
jmarkel said:
When I start my engine up (258 with 4.0 Head and lots of other goodies) I get an initial reading around 60-70psi on the stock gauge. Once it warms up, i get about 20 at idle. When I drive it it goes between 45-60 depending on the rpms of the engine.

Seems I have the only stock unit that still functions as it should. Now if I could only get my speedo to stop bobbing all over the place...
Click to expand...

Bobbing speedos are almost always due to a binding or kinked speedo cable.
 
jscherb said:
Bobbing speedos are almost always due to a binding or kinked speedo cable.
Click to expand...

I installed a brand new one on my rig and it still bobs. I heard that the speedos have a clump of grease that needs to be redabbed at some point. perhaps there is a fix to the speedo itself?
 
The general rule of thumb is 10psi for every 1000rpm. So if you run the motor to 5000rpm, then you'll need about 50psi of oil pressure to be safe. The oil pressure in a motor is dependant on a lot of things. The main reason for most motors having low oil pressure is too much clearance at the main bearings. Everybody likes to use Plasticgauge instead of a micrometer. The Plasticgauge tends to give inaccurate clearance readings most of the time. When the mains are too loose, ot hemmorages oil out and kills the pressure for the rest of the system.
 
I fully restored my 8 to stock and it sits from 18-22 at idle. When cold upon startup it's at 40. When running down the highway its between 25 and 30 depending on the outside temperature... Plenty of pressure for that stock 6
 
After I rebuilt the motor.. it sat at about 30 at start up and about 50 driving but its went back to just sticking at 20 psi...
 
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