My 1998 c1500 the 5.7 liter appears to be having low oil pressure problems. When I start it up the gauge its at 40PSI at idle, and as the engine warms up the oil pressure slowly goes down to about 34 when driving. Once its warm when i come to a stop it will idle just above the 0 Psi on the gauge. Im gonna hook up a mechanical gauge tomorrow to confrim if the gauge is correct what else should I look for when taking the pressure? Ive been driving for a while now and i changed my oil and only saw few shavings, nothing out of the ordinary.

 

Pay attention to the needle movement. If it’s erratic/bouncy, it’s an electrical problem - pump failure can’t produce that type of pressure fluctuation.

The most common causes of gauge problems on these trucks are the sending unit and the wiring due to corrosion and chafing damage. Inspect the harness for obvious faults, but if it’s in decent condition, replace the sending unit before further circuit diagnosis.

Pump failure is indicated when the pressure initially builds at cold start and then SLOWLY and STEADILY decreases as the engine warms up.

You’re on the right path if you’re going to verify the pressure with a mechanical gauge.

 

How many miles on the motor? What oil viscosity are you using?

 

I just put fresh 5w-30 in it. The dash says 120000. But I’m not sure how accurate that is.

 

This definitely describes what I’m seeing. Luckily I’ve already got a pump and oil pan gasket in advance. Anything else I should consider replacing/inspecting if I end up removing the oil pan?

 

Besides the 2 parts you mentioned, I would also change out the oil sending unit, especially if its never been changed before.

 

Inspect the connecting rods for excessive play on the crankshaft - rock them up and down and side to side.

Thoroughly clean the bolts and block threads before installing the new pump and torque to spec.

If you live in a rust prone area and the exterior of the pan has rust, then repainting the pan is recommended. If you’re not sure of its condition, replace it. I’ve seen these oil pans start seeping through the sides due to porosity created from rust/corrosion.

 

This was after changing out the sensor just sitting at idle from what I’ve seen this is pretty good but, it will still gets pretty low every so often but it’s usually sitting there.

 

Try a heavier weight oil. You can also add some STP oil additive to see if it makes any difference.

 

Did you verify the actual pressure with a mechanical gauge?

The rule of thumb for oil pressure is 10 PSI per 1,000 RPM once the engine is at operating temperature - if it’s still slowly dropping off, verify the issue with a mechanical gauge. If the mechanical gauge isn’t reading the same as the gauge, it’s still an electrical fault.

The clusters in these trucks are known for having problems with the stepper motors - that will be the next step if the mechanical gauge doesn’t support an actual loss of oil pressure.