[Terry M. Hunt]

I'm the original owner of my '71 GTO, which now has 110,000 miles on the stock 400. Everything on this car has been meticulously cared for including religious oil and filter changes, and it runs like a watch. Lately, I've noticed some weird behavior from my analog(not idiot light) factory oil pressure gauge. The gauge shows about 20 psi at idle and about 45-50 psi at a steady 70 MPH cruising speed. Sometimes(random) I see the needle vibrating once the car reaches cruising speed instead of just resting motionless at some specific psi reading. The needle is barely moving back and forth, but enough that the tip appears to be vibrating. This seems to happen most often at a steady cruising speed. Don't recall seeing it at idle or when shifting up through the gears(it's a 4 speed). I can't seem to establish any pattern for when it happens( such as a certain speed, going up hill or down, engine cold or warm, etc.), and it doesn't seem to happen every time I drive it; only sometimes. I'm wondering if something could be failing with the sending unit or the gauge after 46 years. Any ideas about what could be causing this?

[pggto]

I was getting unstable oil pressure readings last year. I borrowed a friends oil pressure gauge (engine shop type) and screwed it in where the sending unit mounts. I ran the engine up from cold to hot and watched the gauge. The engine shop gauge was steady. I replaced my sending unit and the factory dash gauge was fine. I would start with the sending unit. Not that expensive and easy to replace.

[Terry M. Hunt]

I'm gonna check the electrical connection at the sending unit first. Probably too good to be true that it is just a poor or corroded connection. I'll probably move on to a new sending unit after that.

[77 Canamman]

This behavior could be either the gauge or the sending unit. The sending unit is the first thing I would replace.

[Peter Serio]

GM car gauges will go bad after many years of use and seasons, winter/summer. Inside of the electromagnet coil there is a disc shaped magnet that has a north and a south pole. This disc rotates a maximum of 120 degrees inside of a sealed environment. (Lint dust and dirt free). There is an air space around that disc so that the gauge can never stick or bind regardless of the outside air temperature, dust, dirt or humidity. Since it is impossible for GM to predict when or how the car could be used and/or abused over time. GM cannot know exactly where in the world the car will be driven or stored. GM designed all their cars & trucks (within reason) to operate in all parts of the globe and the cars could be driven for years and then parked. And later put back into service.

When the gauges are manufactured; inside the air space GM introduced 2 or 3 drops of silicone buffering fluid. In effect the silicone gives the pointer it's "hydraulic cushion" so that the pointer does not over-react when you first start the car. (Key on: 12 volts to the back of the gauge.) The silicon fluid inside a confined space also keeps the pointer at a steady reading over the entire span of the dial. Over many years and seasons, winter-summer, hot+cold, rain or snow; the silicone tries to escape. It will creep thru the nylon former and eventually evaporate out into the air. This can take up to 30 or 40 years. Once this happens the gauge will bounce and vibrate, eventually the pointer will become loose on it's shaft and fall off.

Electrically the gauge will still operate until just before the pointer falls off. As the vibrations get worse the pointer sometimes will skew on the shaft giving you false readings.