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Normal 5.9 Cummins Oil Pressure at Idle? – How to Diagnose Low Oil Pressure

Photo by Vetatur Fumare | (CC BY-SA 2.0)

Even 15 years after the 5.9 Cummins was discontinued, both the 12v and the 24v engines are highly sought after by diesel truck enthusiasts. They’re reliable and you can quite easily triple their power output with the P-Pump.

But even the most reliable diesel engines show problems once in a while, and the 5.9 is not an exception. 

One of the hottest topics around the 5.9 Cummins is oil pressure at idle. It’s not uncommon for the factory oil pressure gauge to show low oil pressure or erratic readings and a lot of people are rightfully concerned about it.

But is the factory oil pressure gauge actually accurate and can it be trusted? What is a healthy oil pressure for the 5.9 Cummins at idle?

Related: The Best Oil for 5.9 Cummins

5.9 Cummins Oil Pressure at Idle

According to the Cummins 5.9 service manual, the minimum acceptable oil pressure at idle is 10 PSI. In ideal circumstances, you would want to hover around 20 PSI of oil pressure at idle. That’s what running an aftermarket oil pressure gauge on a healthy 5.9 shows. 

So, if your 5.9 Cummins has above 10 PSI of oil pressure at idle, then your motor is okay, and if it’s 20 PSI then it’s practically brand new. 

At 2000 RPM, the minimum acceptable oil pressure is 30 PSI according to the service manual. Most 5.9 owners report 50-65 PSI at 2000 RPM using aftermarket oil pressure gauges. 

At this point, you’re probably wondering why I’m saying aftermarket oil pressure gauge and not the factory oil pressure gauge. That is because the factory oil pressure gauge on the 24v is not an actual oil pressure gauge and should not be used for accurate oil pressure readings. 

Why the Factory 03-07 Dodge Oil Pressure Gauge Should Not Be Trusted

While it seems like the factory oil pressure gauge is real and moves similar to the real thing, it’s actually just a glorified oil light.

Seriously. The oil pressure sender is just a two-wire on/off switch and it does not have the ability to sense if the oil pressure is increasing or decreasing. According to the 5.9 Cummins service manual, it only checks if the oil pressure is above 6 PSI. That’s it. 

So, how does the oil pressure gauge move its needle and show different readings? 

First, once the oil pressure goes above 6 PSI, the oil pressure sender turns on and tells your truck’s ECM that there is oil pressure. Next, the ECM references the RPMs and the coolant temperature to estimate oil pressure, and then sends the estimation to the factory oil pressure gauge.

That’s right, the factory oil pressure gauge does not measure actual oil pressure, it just servers you an estimation from a table of pre-determined values. 

Don’t believe me? Here’s the table of pre-determined oil pressure values, thanks to Diesel Camper:

But why did Dodge do this? Well, in the service manual they said this – “The instrument cluster circuitry restricts the oil pressure gauge needle operation in order to provide readings that are consistent with customer expectations.” What they (probably) actually meant to say is that customers did not like the previous oil pressure gauge and how it moved quickly, thus Dodge had to do something about it, and this is the result.  

How to Diagnose Low Oil Pressure in 5.9 Cummins

Before you do anything to your 5.9 Cummins, you must verify if the oil pressure really is low or if it’s just the oil pressure sender that’s showing incorrect readings. Nine times out of ten, it’s the oil pressure sender that’s gone bad and the oil pressure is fine. You can verify if the oil pressure really is low by installing an aftermarket mechanical oil pressure gauge. 

Once you’ve verified if your 5.9 Cummins really does have low oil pressure, it’s time to look at the reasons why would that happen.

5.9 Cummins oil pressure relief/bypass valve
5.9 Cummins oil pressure relief/bypass valve

Now, the first thing to check when diagnosing low oil pressure on the 5.9 Cummins is the oil pressure relief valve. It lives inside the oil filter housing and acts as a bypass if the oil filter gets fully clogged up. However, these relief valves are known to leak oil if the spring inside them gets old, which does reduce oil pressure readings. 

If that’s not the problem, then you’ll have to go through the low oil pressure troubleshooting guide found in the 5.9 Cummins service manual. Don’t have the service manual? Here’s the low oil pressure troubleshooting guide:

Possible CausesCorrection
1. Low oil level.Check and fill with clean engine oil if needed. Check for a severe oil leak that could reduce oil pressure.
2. Oil viscosity thin, diluted, or wrong specification.Verify that correct viscosity engine oil is being used. Check for reduced viscosity due to fuel dilution.
3. Improperly operating pressure switch/gauge.Verify that the pressure switch/gauge is working correctly. Replace if not.
4. Relief valve stuck open.Check/replace valve.
5. If cooler was replaced, shipping plugs may have been left in the cooler.Check/remove shipping plugs.
6. Worn oil pump.Check and replace oil pump.
7. Suction tube loose or seal leaking.Check and replace seal.
8. Loose main bearing cap.Check and replace with new bearing. Tighten cap to proper torque.
9. Worn bearings or wrong bearings installed.Check and replace connecting rod or main bearings. Check and replace directed piston cooling nozzles.
10. Directed piston cooling nozzles under piston, bad fit into main carrier.Check directed piston cooling nozzles position.
11. Loose oil rifle plug with saddle-jet style nozzles.Tighten oil rifle plug.
12. Loose directed piston cooling nozzle.Tighten directed piston cooling nozzle.
13. Both J-jet and saddle jet style cooling nozzle installed.Install correct style jet.
5.9 Cummins service manual low oil pressure troubleshooting guide

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