What to do when your oil pressure light comes on

There’s a lot of automotive information bombarding us from a car’s dashboard, and to some folks, it’s not clear which warnings require immediate attention. A rising temperature gauge should be treated very seriously, as driving “in the red” can easily result in a cracked head. The alternator warning light is a bit less pressing, as depending on the complexity of the car’s electrical system, you can get dozens or hundreds of miles down the road before the battery drains. Lower in priority are things like brake lining, ABS, and traction control warning lights—yes you should fix the underlying problems, but you don’t need to do so in the next five miles. At the bottom of the list are the “check engine light” issues which, on a post-1996 car, are usually caused by minor emission-control-related codes.

With all this information, it’s easy to miss the most important indicator of all—the oil pressure light. Ignore it, and it can very quickly lead to a very expensive BANG! And when, after the tow, you get an estimate for the cost of replacing the lunched engine, you’ll cry.

Because oil is the engine’s lifeblood, nearly every post-Model-A car has an oil pressure warning system of some sort. On a vintage car, it’s usually just a light that’s prominently displayed on the dashboard in or near the other instruments. Part of the ritual of driving an old car is to crack the key to ignition, be certain that the oil pressure light and the alternator light are both illuminated, then start the engine and verify that, in a few seconds, both lights go out. On newer cars, these lights may have receded into a distraction of other warnings, or—gak!—been subsumed into a diagnostic screen.

Note that the oil pressure warning light is not the same as an oil level warning light. Nearly all cars have the former. Some cars have the latter, which typically uses a gas-tank-style float and illuminates when you’re a quart or two low, which is the rough equivalent of the oil level being at the bottom of the dipstick. If your car has an oil level warning light, by all means pay attention to it, and if it comes on, check the oil level as soon as it’s safe to do so and top it up as necessary. But driving a few miles to the next exit when you’re a quart or two low is highly unlikely to kill the engine.

But your oil pressure light is different. It’s more important than any of these other indicators. If it comes on, and if the sensor is doing its job, it’s because oil is no longer flowing inside the engine, which is really, really bad. No oil flow means no lubrication, and metal parts which should be happily slip-sliding against each other will instead begin catastrophically scraping. If the oil pressure light comes on while you’re driving, stop. Now! Right freaking now. Don’t wait for the first exit. OK, yeah, find somewhere safe, but while you’re cherry-picking yourself a spot in the shade, imagine a $15,000 engine replacement bill. Rotate the key off the ignition setting but not so far back that the steering locks, and coast the car safely into the breakdown lane. Seriously. That’s how important it is.

Let’s visualize for a moment how oil flows through an engine so we can pick apart the various reasons the oil pressure light can come on. Every automotive engine has a reservoir of oil, and nearly every one built in the last 90 years has a pump that sucks the oil out and sends it coursing through the engine. That reservoir is usually the oil pan (the “wet sump”) at the bottom of the engine. Some high-performance cars instead have a “dry sump,” but let’s just deal with conventional wet-sump engines.

Oil pumps are usually driven mechanically off the crankshaft or camshaft, either via direct gear-to-gear engagement or a chain drive. (At some point in the 2000s, some manufacturers began using a “wet belt” or “Belt In Oil” / BOI system where a belt inside the oil pan spins the oil pump, but this is a family publication, so I won’t speak of those abominations.) A pick-up tube reaches from the pump to the bottom of the oil pan. As the engine turns, gears inside the pump create suction, drawing oil out of the pan and up into the pump. Oil is sent first to the oil filter. From there it flows under pressure via oil passageways in the block and head that then feed holes in the crank, cam, and connecting rods to lubricate the main bearings, rod bearings, cylinder walls, and valve train components.

The rule of thumb is that oil pumps deliver oil at a little more than 10 psi per thousand engine rpm, so at idle it should be about 10-20 psi, and at 4000 rpm about 40 to 50 psi. Note that oil pressure is affected by oil viscosity (thickness), which in turn is affected by engine temperature—as oil warms up, it thins, which results in lower oil pressure. In addition, on a car with a tired engine, badly-worn crankshaft and rod bearings can allow oil to escape around the bearings instead of just oozing the oil into place. Thus, just as a leak in a garden hose will reduce the pressure of the water flowing out the nozzle, worn bearings can reduce the oil pressure to the rest of the engine, including the pressure sensor. The combination of these two things can create a situation where, when the engine is cold, the oil warning light takes longer to go out, and when the engine reaches operating temperature, the light flickers on and off at idle, or worse, stays on. As long as the light goes out when you rev the engine, this isn’t an acute problem, but it is an indication that the lower-end bearings are badly worn and should be checked.

Then again, a flickering light at idle could simply be an incorrect oil filter. I once bought an inexpensive 1985 BMW 635CSi with 230K miles on it and a flickering oil pressure light at idle. I was resigned that it was engine wear until I changed the oil and found this clearly incompatible too-large filter that had been crushed-to-fit inside the housing. I replaced it with the correct filter, the problem went away, and I shuddered to imagine the mechanism by which the wrong filter had lowered the oil pressure.

If the oil warning light comes on while you’re driving, and you’ve stopped the car somewhere safe, here’s what you do. While full-on failure of the oil pump is pretty rare, it is possible that you really do have no oil pressure.

The light has come on for one of four reasons:

1. There’s no oil pressure because there’s NO OIL!
2. The light is grounding somewhere other than through the sensor.
3. The sensor is bad.
4. There really is a catastrophic loss of oil pressure.

Like everyone whose car died and who insisted it didn’t run out of gas only to find that, yeah, it ran out of gas, the engine being out or extremely low on oil can happen, so first, check the oil level on the dipstick. You’d have to have run the sump nearly dry in order for this to be the cause of the light coming on at even throttle on a level road, but stranger things have happened. I’ve engaged in enthusiastic cornering on exit ramps, had the light come on briefly, and found that I was negligently low on oil. It’s certainly possible that the oil drain plug loosened up and fell out, or you hit a rock and cracked your oil pan, or a return line for an oil cooler ruptured. If any of these is the cause of low oil pressure, I’d expect the dipstick to come up dry. Maybe you can fix the problem roadside, maybe it’s a tow, but at least you’ve found your problem.

If the pan is full of oil, then the light is coming on for one of the following three reasons. For #2 and #3, you get to plumb the design of the oil pressure warning system, so gloriously simple and effective that many cars still use it nearly a hundred years after it debuted. On a vintage car, one terminal of the warning light is fed 12 volts. The other terminal is connected to a terminal of an oil pressure sensor screwed into an oil channel in the engine. The sensor is just a make-or-break switch for the warning light’s ground path. When the engine isn’t spinning and oil isn’t pressing against the sensor, a pair of contacts inside the sensor—one connected to the terminal, the other to the body of the sensor—touch each other, completing the ground path and causing the light to illuminate. But when the engine is running and oil is pumping through the channels, the pressure of the oil forces the contacts apart, breaking the circuit and causing the light to go out. Simple. Functional. Perfect.

So go find your engine’s oil pressure sensor. It usually looks like a big nut with a domed plastic central housing and a single wire connected to it. It may be on the front or the back of the head, or screwed into the block. Inspect it. It’s common for the plastic housing to crack and leak oil, in which case the contacts inside may be compromised.

Now that you’ve found it and know how it works, you can probably figure out how to test it. Pull the wire off, and turn the key to ignition. The oil pressure light should have no path to ground, so the light should be off. If it’s on, then the light is grounding somewhere else—the insulation could’ve rubbed off the wire and it’s grounding against the body of the car.

If the light does go off, then do the other test—ground the wire to the engine or the chassis. The light should come on. If it doesn’t, either the bulb is burned out or there’s a wiring issue.

If the light goes on and off correctly, then you’re down to either #3 or #4—bad sensor or a real oil pressure problem. You have two choices—replace the sensor, or perform a direct measurement of oil pressure with a screw-in gauge. You don’t have a gauge? Don’t feel inadequate. It’s pretty rare that you actually need one. And oil pressure sensors have a variety of threads, so compatibility is an issue (though these days, a $30 kit on Amazon has 10 different adapters).

By all means, procure another sensor (they’re usually cheap) and try it, but if the oil pressure light still doesn’t go out, shut the engine off immediately.

If the oil pressure light comes on while you’re on the road and you have neither a gauge nor a spare sensor, there is a seat-of-the-pants test you can do, and I’ve done it more than once, but you perform it and judge the results at your peril, because if you’re wrong, you’ll blow up your engine.

It’s the “thar she blows” test. Clean around the base of the sensor, then take a big wrench (or, if absolutely necessary, pliers), and unscrew it. Then remove the fat wire from the center of the ignition coil so the engine won’t start. Have someone crank the engine while you hold a wad of paper towels on the hole where the sensor was. If the oil pump is doing its thing, a non-trivial amount of oil should hit the paper towels. If the sensor screws into the side of the head or block, you can catch the oil in a container instead, but if it screws into the top of a flat surface, the oil will shoot straight up, and it gets very messy very quickly if you don’t squelch it with a wad of paper towels.

If you still don’t see any oil coming out the hole, or have any doubt whether enough oil is being pumped, reconnect the coil plug wire and start the car and run it for just a few seconds. Given that the rule of thumb is about 10 psi of oil pressure for every thousand engine rpm, oil should absolutely gush out at idle. If it does, you probably just have a bad sensor (though my inner lawyer tells me to advise you not to drive the car anyway). However, if this engine was just running (that is, if it’s not a priming problem), the light came on, and now nothing comes out, you’re kidding yourself if you think you don’t have a major oil pressure issue.

So, if there’s really no oil pressure, what then?

Oil pumps themselves tend to be incredibly robust precisely because they have to be. When the DIY cost of engine rebuild parts plus the machine shop bill for one of my 50-year-old BMWs was $1500 and an oil pump cost $60, you’d throw in a new oil pump whether it needed it or not, but now, many of these parts are no longer available from the dealer, and there’s no aftermarket source. I’m not a professional mechanic, but on the vintage BMWs on which I work, I’ve never seen the gears inside an oil pump so worn that they won’t pump oil. There’s a spec for clearance between the gear faces and the cover, and the cover can be wet-sanded to meet it.

Folks I know who repair vintage cars professionally say that when they’ve seen oil pressure issues, it’s rarely the pump itself and has instead been due to:

• The pressure relief valve inside the pump being stuck open, so it returns most of the oil to the pan instead of circulating it through the engine.
• The screen at the bottom of the pickup tube being so clogged with 50 years of gunk that it barely sucks oil up into the pump.
• An engine being so worn that it leaves clouds of oil smoke at idle or has very noisy rod bearings.

So, if your oil pressure light comes on in an otherwise fine-running vintage car, yes the odds are that it’s just a bad sensor, but you can’t afford to be wrong about it. If you can, you have way more money than I do.

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Rob’s latest book, The Best Of The Hack Mechanic™: 35 years of hacks, kluges, and assorted automotive mayhem is available on Amazon here. His other seven books are available here on Amazon, or you can order personally-inscribed copies from Rob’s website, www.robsiegel.com.

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