Under Pressure: Oil Gauge Reading

When I installed a rebuilt engine in my 1936 a little more than a year ago, I was amazed and very happy when it fired up with little difficulty, ran smoothly and showed good oil pressure on the gauge.

On the road, the engine pulled the car along about as rapidly as could be expected with 1936 technology, and all seemed to be good with the world. However, after the engine warmed up, I was startled to see the needle on the gauge drop off to zero at idle.

After investing a lot of time, money and energy into having the engine done by a friend who is an automotive technology instructor, I wondered if I had made a big mistake.

The car seemed to be doing OK, despite the gauge reading, so I drove home very slowly and immediately dug out my 1936 Service Manual, User's Guide and Owner's Handbook. The first two books touched on the topic of oil pressure in somewhat general terms, but the Pontiac Owner's Handbook for 1936 had some specific and reassuring information. Here is what it says:

Oil Pressure Gauge

"The oil pressure gauge on the instrument panel indicates the pressure of the oil in the engine lubricating system. It does not indicate the amount of oil in the crankcase."

“Oil pressure changes with oil temperature, that is, hot oil will give less pressure than cold oil. of pressure gauge should be at least 30 lbs. at normal driving speeds."

“CAUTION - The oil pressure indicator hand may read Zero with the engine idling, but should show pressure when throttle is opened slightly. If the oil indicator hand should fall to Zero while the engine is running at normal driving speeds, the engine should be immediately stopped and the reason for drop in pressure determined.”

Even as my engine wore in, the oil pressure reading at hot idle stayed at zero, though the oil pressure when the engine is hot and running is excellent.

As you can see, the above information, which applies to both 6- and 8-cylinder L-head engines, varies from that printed in many books and articles, which usually say that oil pressure at hot idle should be a minimum of 5-15 lbs.

I would assume that the 1936 information would be applicable to at least all inline engines back to 1933. However, it might not apply to 1937-1954 Pontiac L-heads or all L-head engines made by other automakers. Chances are good, however, that it applies to other makes of cars with similar engine technology.

So, if you’re out driving, have your engine nicely warmed up and see zero oil pressure, don’t panic. Just make sure to read your owner’s manual and dig through the literature pertaining to your car. You may find, as I did, that the situation is normal.

John "Gunner" Gunnell is the automotive books editor at Krause Publications in Iola, Wis., and former editor of Old Cars Weekly and Old Cars Price Guide.