1966 Mustang 302 oil pressure question

1) You probably already have but...make sure your oil level is correct and you aren't losing oil anywhere.

2) Check/replace your sending unit. It's the cheapest to replace and it never hurts to have a new one.

3) Check your gauge and the wiring feeding it.

If you really want to make sure...buy a cheap gauge and a new sending unit and "hot wire" the gauge in. Install the new sending unit and hook it to the "temporary" gauge. If it acts the same way you may need to think about your oil pump.

The rule of thumb is at least 10 psi per 1000 RPM. You need a mechanical gauge temporarily installed into the location of the original sending unit. This can be tapped in with a "T" fitting. Run the gauge tubing into the car so that you can get a true pressure reading in PSI while driving.

If the pressure reading is lower than the 10 psi per 1000 RPM, you either have an engine with too much bearing clearance, an oil pump problem, (weak relief valve spring, pump clearances too great, pump pick up too close to the bottom of the pan- it needs to be 3/8-1/2 inch from bottom of oil pan, pick up not sealed to pump or pump not sealed to block, or a clogged pick up screen, due to excessive use of silicone gasket sealer that has broken loose and settled into bottom of pan).

Without a true reading in PSI, you really don't have much to go on. I don't trust an electrical oil pressure gauge for a true reading. It's OK for use as a reference after verifying the needle position with the true amount of pressure. An engine with 500 miles on it shouldn't be showing too much variation in pressure after warm up unless the bearing clearances were set on the loose side. Was this engine's bearings and bearing surfaces renewed when rebuilt?

All engines will show a slight reduction in oil pressure after warm up due to the expansion of the parts and the slight loss of viscosity of the oil, but if you find more than about a 10-15 psi drop across the pressure range, something isn't right. You really need to know what the true pressure is.

I had the same sort of problem, but mine was a little worse....actually a lot, so i doubt it is your problem, just thought id share it though. I had too much bearing clearance around the crank. when the motor heated the oil up, it became less viscous. instead of seeping out of my bearings, it was like niagra falls. i do agree w/ greek on the releif valve. i put in a high volume oil pump, and run about 70-80 psi on start up and about 60 when engine is at 190, the the 10psi/1000rpm is a rule to live by. you may try an oil pressure gage w/ a manual sending unit. i think they are only about $20 from autozone or oreilleys