Misc. Oil Pressure Gauge Reads 0 until Warmed Up

[H1LZ]

My 1983 CJ7 IL6 shows an oil pressure of 0 when it's cold, and then will jump up to about 40 once warm. I took the oil pressure sending unit off and tested the pressure with a gauge and it reads between 60-80 psi. I read that the sending unit might be bad, so I replaced it, but it still doesn't show any pressure. I had read it's a bad idea to run it without the pressure gauge working, but I can't seem to figure out why it only works when it's warm. Any thoughts on this?

 

[Manic Mechanic]

Hook up a mechanical gauge and check it to make sure it's not an issue in the motor

Sent from my E7110 using Tapatalk

 

[TSB8C]

You say you tested it with a gauge after removing the sending unit. Can you share more of how that test was done? Are you saying you connected a mechanical gauge to the port the sending unit is/was mounted in? If so, then the engine is fine in terms of oil pressure and you have a gauge issue to deal with (since you already replaced the sending unit).

 

[H1LZ]

You say you tested it with a gauge after removing the sending unit. Can you share more of how that test was done? Are you saying you connected a mechanical gauge to the port the sending unit is/was mounted in? If so, then the engine is fine in terms of oil pressure and you have a gauge issue to deal with (since you already replaced the sending unit).

Thanks for the post. I hooked up a mechanical gauge and it read 60-80 psi. I wasn't sure if it was the gauge in the dash since it seemed to work once it got warmed up. I also wondered if changing from a 10w-30 to 5w-30 might help. I'd read a post that the cold creates thicker oil and might be causing the problem.

 

[TSB8C]

It is true that thicker oil can cause higher pressure when cold, then get a bit thinner once warm. I do know folks that run 5w-30 or 10w-20 in the winter and switch back to 10w-30 in the summer. Basically opposite of what you proposed. They run thicker oil in the summer when it's hotter and the oil will thin a bit and they run thinner oil in the winter when cold makes it thicker. But I don't subscribe to this theory. With a proper working cooling system and thermostat, your engine temp will run the same regardless of outside air temp. It's only when first starting and warming up you might have any issue, and that would be opposite your symptom. You should have high pressure when the engine first starts and the oil is thicker, and then the pressure should drop lower when the engine is warm and the oil is a bit thinner. You're seeing zero pressure when cold, not higher when the oil is thicker and not warmed up yet.

But let's review oil viscosities and when you might want to change. Oil viscosity refers to how thick or thin the oil is and how well it flows at different temperatures. This is represented by the numbers you see on oil bottles, such as 5W30 or 10W30, and it’s measured by the Society of Automotive Engineers (SAE). The first number, such as the “5” in 5W30, indicates how the oil flows in cold temperatures (the “W” stands for winter). A lower number means thinner oil, which circulates faster during cold starts, making 5W30 better for cold climates compared to 10W30, which is thicker and slower to flow in the cold.

The second number, like the “30” in both 5W30 and 10W30, represents the oil’s viscosity at high temperatures (tested at 212 F). At this temperature, thicker oil, like 30-weight oil, clings to and protects engine parts better when the engine is hot and operating under stress.

In practical terms, oil viscosity affects engine start-up by determining how quickly oil reaches vital parts in colder conditions. Thinner oil, like 5W20, flows faster, reducing friction and wear during start-up and also improving fuel efficiency by reducing engine drag. However, thicker oil, such as 5W30, provides better protection at higher temperatures by maintaining a stronger film over engine parts, crucial in hotter climates or under heavy engine loads. Here's some good recommendations:

1. Always start with the oil viscosity recommended in your owner’s manual. Manufacturers design engines to run with specific oil thicknesses to maximize performance and protection. Using the wrong viscosity could lead to poor lubrication, increased wear, or even engine damage.
2. If your owner’s manual provides a range of oils (e.g., 5W20 or 5W30), consider the climate where you live.

• If you live in a colder climate, opt for thinner oils like 5W20, which flow better during cold starts and help your engine warm up faster.
• Choose 5W30 or 10W30 if you drive in warmer areas. These oils offer better protection at higher temperatures and are more suited for sustained engine loads.

1. Driving conditions matter. For city driving with frequent stops and starts, a thinner oil like 5W20 may improve fuel efficiency. For towing, heavy loads, or off-road driving, consider a thicker oil like 5W30 or 10W30 to provide better protection at high engine stress.
2. Vehicle type is important. Modern cars and smaller vehicles often benefit from thinner oils like 5W20 for better fuel efficiency. Trucks, SUVs, and vehicles under higher stress or load may perform better with 5W30 or 10W30, which offer extra protection in demanding conditions.

 

[TSB8C]

That was getting lengthy, so another post. You could certainly put some thinner oil in there (lower first number) as a test to see if you get a pressure reading when the engine is first started and not warmed up yet. Perhaps you need a thinner oil to start flowing faster during a cold start. But to me that would indicate another engine issue. Oil of almost any viscosity should flow enough to register on the gauge even when cold unless something in the engine is preventing that (oil pump, clogged pick-up tube/screen, blockage in passage way leading to the port where the sending unit mounts, etc). If you have some "crud" in there somewhere that is blocking oil flow when cold but then thins a bit when it gets hot, that needs to be addressed.

If you have an electrical meter, you could test the gauge and the signal from the sending unit (which you have already replaced) to determine a gauge/wiring issue or not. The oil pressure gauge has three connections, red wire with a +12V signal from the ignition (key on), the middle terminal is ground (black wire), and the "S" terminal with a purple signal wire from the sending unit. The sending unit provides a variable resistance to ground. At low oil pressure, the resistance is very high, causing the gauge to move none or slightly. At high oil pressure, the resistance is very low (about 30 ohms at max pressure) which causes the gauge to move more.

Some quick tests are (a) measure the +12V on the red wire when the key is on; (b) confirm the ground connection by connecting a jumper lead to a known good ground and the center terminal on the gauge; (c) with the key on and engine not running short the purple wire on the sending unit (out on the engine) to engine ground to confirm the gauge maxes out at 80 psi. NOTE: Do not hold this short for too long as you are flowing max current through the gauge and may damage it.

Possible test results and fixes:

If not seeing +12V on the red wire, you have a wiring issue under the dash (wire splices in the harness are a common CJ issue).

If the gauge reads properly when you connect a jumper lead to the ground connection, your gauge is not properly grounded. Check that the black wire is indeed connected (some people have left it off and rely on the body of the gauge itself to be grounded via its mounting).

If the gauge does not peg to 80 psi when shorting the purple wire on the sending unit, you have a wiring issue either through the bulkhead connector on the firewall or under the dash, though no splice connectors are in this circuit.

If all of these tests show no problem, your gauge is bad. I've never seen a bad CJ oil gauge fail when cold and then work when warm (others using the "regulator" behind the speedo cluster, yes) but I suppose anything's possible.