[Tydeorum]

Where should the oil pressure be on a 1967 289? Mine stays at about 1/3 . should it be higher or it that fine?

[Ivy66GT]

The pump relief valve is 55 psi. The stock Mustang gauge has no numbers on it because the senders they used are somewhat, shall we say, variable. Unfortunately, the replacement senders you can buy today look exactly like the original ones but work even worse.

To center the needle on a stock Mustang oil pressure gauge requires the sender to be around 25 Ohms. I have two original senders on two different '66's and one reads about 1/3 on the gauge and the other reads 2/3. A real gauge in place of each sender says both engines have 55 psi. I tested several new senders thinking I would buy a new one to replace the 1/3 reading sender. None of the new ones would have read above 1/3, some of them didn't work at all straight out of the box.

If you want to know the details see: http://www.allfordmustangs.com/forum...at-expect.html

What you have is probably OK but the only way to really know is to put a 'real' gauge on it.

[Tydeorum]

My pressure is dropping the longer I drive mine. I know they put the 289 in my car not so long ago so if they changed oil senders could that be why its not high and then dropping?

[Ivy66GT]

You are using the original dash gauge and the sender looks like the one I cut open in the thread I referenced? Assuming those are both correct...

A 'new' sender could well be the cause of your low reading. You say the pressure indication drops. Is it lower at idle or lower while you are driving? As the oil warms the idle oil pressure may well go down and some gauges, like ONE of my original ones, actually shows this drop although its a very subtle movement of the needle. The other original one works like an On-Off switch and rarely shows any change at all. If the pressure really does drop at highway speeds when the engine oil warms up then either the pump isn't putting out enough oil or you have excessive clearances, like in the main bearings(?), which requires more oil than your pump can supply. At decent RPM the oil pressure should be the 55 psi set by the relief valve on the pump.

Because of the problems I detailed in the other thread, you may need a 'real' gauge to see if this 'drop' is even real. Its easy to screw on a mechanical gauge in place of the stock sender and see what it reads. The thread on the adapter pipe is the same as found on most 2" mechanical gauges; they screw right in. The only problem with that is trying to read it while you drive.

[Tydeorum]

do i need to drop my oil pan for this?

[cindys_sn95]

no. hook a mechanical gauge to the extension where the switch screws in. the only true reading!