Oil Pressure Question

[Richard B.]

Hi, All. Have a question on oil pressure variation. Here's the conditions: 1982, 42,000 miles. Shell Rotella, 10-40?. OAT 103 degrees, speed 70-75 mph, rpm approx 1900. Oil temp est 205, water temp approx the same. As I read the gage, it usually looks like 41psi, and is steady as a rock. The other day, after driving about an hour, it slowly rose to approx 50psi, held that for a while, 20-30 minutes, and then returned to normal. It did this on 3 different days, same conditions. Yesterday, same speed, OAT approx 80 degrees, there was no variation from 41psi.

As I understand the pump, there is an over pressure ball/spring that should limit pressure. Any ideas of what this could be? Is the answer to just stay in cooler areas?? Or is this just a quirk that I can ignore?

Thanks in advance for any ideas.

Rich
#36956

[Joe L.]

Re: Oil Pressure Question

Rich-----

Higher ambient temperatures and associated higher engine operating temperatures should have the effect of LOWERING engine oil pressure, not increasing it. However, the 10W-40 oil you are using might have some bearing on what you saw as far as oil pressure. All multigrade oils are actually oil of the LOWER weight with viscosity modifiers to make them ACT like the higher weights at higher oil temperatures. So, for whatever reason, perhaps they were "acting like" a 40 weigth oil before the engine temperature necessitated it.

In any event, I would say that what you described is absolutely nothing to be concerned about. However, it's always a good idea to keep a check on the oil pressure gauge.

[Duke W.]

Re: Oil Pressure Question

What Joe said, plus most modern oil pressure gages are electric, and their calibration can be affected by system voltage, which varies with regulator temperature, or it could be a case where the regulator is going south.
Regulated voltage usually increases with increasing regulator temperature, so on a hot summer day system voltage could be several tenths of a volt higher than cool weather or just after cold start when the regulator is still at ambient air temperature. The Delco system voltage spec is 13.5-14.5 volts @ 80F as long as the alternator load is less than alternator capacity at that speed. If alternator load exceeds capacity, voltage drops below spec and you are drawing energy off the battery.

The next time you observe unexplained oil pressure changes, check to see if system voltage is also at a different than typical level, and if the oil pressure behavior continues to vary in an unexplained way, see if you can find a correlation between oil pressure and system voltage readings. If you want accurate data hook up a digital voltmeter to the system while you drive as the in-dash voltemeters are not very sensitive.

I had problems with the side terminal Deltrotron on my Cosworth Vega due to the high frequency unbalanced second order verticle shaking force. It actually wore away one of the regulator spades to the point where the female connector on the harness was not making good contact, which caused an intermittent overvoltage situation.

V-8s don't create anywhere near as severe a vibration environment as a low mass high revving inline four, but after 24 years, checking that the Delcotron side connectors still fit snug on both regulator spades is a good idea.

Duke

[Richard B.]

Oil Pressure Question

Thank you both for the excellent information! I'll have a look at the voltage regulator connections. The in-dash gage never moved from it's normal reading, but I don't have a lot of faith in it's sensitivity. I suspect ambient temperature was a major factor, but 3000 miles round trip to the National was a longer than usual drive, and maybe I had more time to note small changes.

Best of all, I passed every one of the Chevy Dealers, down and back, without stopping!!!!! Thanks again for the expert help.

Rich.....