Oil pressure gauge drops to zero then back to half normal pressure after engine warms up. Has good pressure when first cranked ? Is oil sending unit on exterior or interior of motor ?
Most common cause for this would be a faulty oil pressure sending unit. It is a fairly in expensive part and would be the first place to look. Check the connections and if any sign of oil is around, replace it.
Oil Pressure Sensor Description
The 3–wire, solid-state engine oil pressure sensor (sending unit) is located in an engine oil pressure gallery.
Operation
The oil pressure sensor uses three circuits. They are:
- A 5–volt power supply from the Powertrain Control Module (PCM)
- A sensor ground through the PCM’s sensor return
- A signal to the PCM relating to engine oil pressure
The oil pressure sensor has a 3–wire electrical function very much like the Manifold Absolute Pressure (MAP) sensor. Meaning different pressures relate to different output voltages.
A 5–volt supply is sent to the sensor from the PCM to power up the sensor. The sensor returns a voltage signal back to the PCM relating to engine oil pressure. This signal is then transferred (bussed) to the instrument panel on either a CCD or PCI bus circuit (depending on vehicle line) to operate the oil pressure gauge and the check gauges lamp. Ground for the sensor is provided by the PCM through a low-noise sensor return.