Oil light turns on intermittantly

In this thread, http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=224374, I detailed some troubles I was having starting my car. It turned out the solenoid was bad, and it was replaced with a rebuilt starter/solenoid.

Now, only 2 weeks later (grrrr…), my oil light turned on. I, of course, immediately pulled over and checked my oil. It’s full to the brim, which makes sense, since I only changed it on Friday. I jiggled some stuff in my engine in the vicinity of the oil sensor, and it turned off. Unfortunately, it turned back on after I drove about 10 miles. I made it home, with absolutely no out of the ordinary engine noises. Rejiggling didn’t cause the light to turn out again. Oil filter and cap are on tight.

I have had recurrent leakage of oil near the sensor, which has been replaced twice in recent years. Leakage occurred even when the sensors were new and the connections were perfectly made (by a mechanic, not me, those times).

This weekend, I’ll be replacing the sensor with an actual pressure gauge so I can keep an eye on the pressure from within the warm, toasty confines of my car. Other than a bad sensor, can anybody in the Teeming Millions think of a reason for the intermittantly tured on oil light with no extra engine noise?

Rick, Gary T, and Q.E.D., since you were so helpful in my last begging-for-car-help thread (Thanks again, BTW), I’m hoping to pick your brains especially.

Please don’t say malfunctioning oil pump. I don’t want to hear it. Lalalalala.

Normaly a dud sensor, but it could be low oil pressure due to a clapped out pump or blocked oilways/filter.

If jiggling something make the light go off, it’s probably a loose wire on the oil pressure switch or perhaps a not-quite broken wire (perhaps it got squashed?) in its wiring harness.

That too, better than a duff pump that you don’t want mentioned anyway.

I’m concerned about that leakage. Sounds like a prior replacement of the oil pressure switch was done incorrectly and either cross-threaded or stripped the threads. Be careful, as further botched replacements could worsen the problem. You don’t want the whole switch getting popped out in the middle of nowhere.

I also vote for a bad switch or switch wiring.

Bad wiring, I can fix! Hooray! I thought that the lack of bad noises from the engine would point to a bad switch/wire.

I was mostly worried about the oil pump because the repair would be costly enough that it would be time to look into buying a new car, which I really can’t afford.

Huh, what? Did somebody call me?
Oh Hi Mithril
Oil light huh?
When exactly does the oil light come on? At idle or when you are humming down the road at speed? If it comes on when driving down the road, is it when your foot is on the gas, or when you are coasting? Does the light stay on or just flicker? When the light comes on, is it full bright or kinda dim?

While I am awaiting your answer let me fill you in on a few things about oil pressure lights (Maybe more than you ever wanted to know, frankly)
Theory 12V is supplied to one side of a light bulb. The a wire from the other side of the light bulb goes out to the oil pressure switch. When there is no oil pressure, the switch is closed the circuit complete and the bulb is lit. Measuring voltage at the oil pressure switch will show at or near 0V. When the oil pressure rises above the set point of the switch, the switch opens, and circuit is broken and the light goes out. Measuring voltage at the switch under these conditions will show approx 12V (could be as high as 14.4)
So in general when a car with high miles gets an oil pressure light, if it comes on at idle, it could be It could be one of the following[ul]
[li]Worn motor[/li][li]Worn pump (Motor still ok)[/li][li]Air leak on the intake side of pump (sucking air)[/li][li]Clogged pickup[/li][li]Leak on pressure side[/li][li]Normal for this engine (yes it happens!)[/li][li]Bad switch[/li][li]No oil pressure problem, but rather a wiring problem [/li][li]Internal fault in cluster (rare)[/li][/ul]

If however it comes on only while driving I tend to think [ul]
[li]Very worn motor, with serious issues[/li][li]No oil pressure problem, but rather a wiring problem[/li][li]Internal fault in cluster (rare)[/li][/ul]
This is because as the engine speed picks up the amount of oil delivered increases and the pressure rises. So most of leakage type problems are eliminated when the engine speed picks up.
Testing Take an ohm meter and check the resistance of the sensor with the lead disconnected and the engine off, should read near zero. Start the engine, should read near infinity. With the key on check the voltage on the wire to the sensor (disconnected) should show near battery voltage. Wiggle said wire. If the voltage fluctuates during the wiggle tests, there is probably a bare spot in the harness that grounds sometimes and turns on the light. If you have much less than battery voltage, you probably have a leak to ground (and the light is probably comes on very dimly when driving)
Oil Pressure gauges are a great accessory. I would like to give you a few warnings before you get yourself in trouble.
Oil pressure gauge kits come with polyethylene tubing. Nice stuff and easy to work with. Only two problems. First it melts so you have to keep it away from the exhaust. (ask me how I know this) The second problem is that it can be cut, so don’t drag it across anything sharp. You would be amazed just how quickly you can pump 4 quarts of oil on to the ground.
The other warning is on the fitting that screws into the block. The gauge will probably come with a NPT thread adaptor. Your Geo may have a metric machine screw thread (straight thread, seals with a copper washer, or a metric tapered pipe thread (BSP?). neither of these are compatible with the NPT thread on the fitting that comes with the gauge. You will need to have an adaptor that goes from what the female is on the engine to what you need. If you can find a pressure gauge that has the adaptor, buy it. Otherwise buy from a parts house that has adaptors in stock.
Warning, the differences can be subtle. IIRC I put a gauge in a Toyota where the difference was 26 vs. 27 threads per inch.
Use Teflon tape to seal the threaded fittings.
Ounce of prevention, pound of cure. Etc.

Reading the other thread, I think that youami might be right

Wow, Rick! Thanks!

I noticed when I was driving that the light seemed to bounce between dim and bright, but I thought I was imagining it. Good to know I wasn’t.

The first time the oil light turned on, I was stuck in traffic, probably averaging about 10 miles an hour. The light stayed on both in neutral and in park. I pulled off, checked my oil, turned it on again, and the light was out. Drove at highway speeds for about 10 miles with no light. It turned on again while I was at a stop sign and stayed on the length of the backroads home, about 5 minutes.

This morning, no light. Hopefully it stays that way, even though I will be putting in a gauge this weekend regardless.

Thanks for the warnings with the threads. I’ll be extra careful about it. Should I invest in some non-polyethylene tubing?

My first-ever brand-new car was a VW Rabbit, bought in 1982. It was in most respects a great car, once I learned how to put it on (I’m 6’5").

However, it developed the most maddening problem with the oil light very occasionally lighting up for no apparent reason. Sometimes it would flicker, and the warning buzzer would stutter. My dealer was never able to figure it out.

Despite the fact that it was a great car (and it cost $5,700!), I unloaded it a year before it was due to be paid off, the only time I have ever done that in the seven cars I’ve owned since.

I never did figure out what the problem was, although the Consumer Reports’ Car Buyer’s Issue did note some years later that that model year was notorious for electrical system problems.

Bright to dim would lead me to look at a wiring problem
The Polyethylene is fine as long as you don’t melt it or cut it. Use lots and make sure that you run it through the firewall in a rubber gromet.
I did not make myself real clear in the last post, teflon tape is only for pipe threads. Machine screw threads use a sealing washer.

First I’ll discuss the worst case, actually having low engine oil pressure:

Oil pressure warning lights typically turn on at about 8 psi, which is low pressure. The light coming on doesn’t necessarily mean there is no pressure. So, even if the light is accurately telling you that the pressure is low, you wouldn’t necessarily hear engine noises right away. It can take some time for the low pressure situation to cause enough damage to result in engine noises. Of course, by the time you hear noise, the damage has been done and an engine overhaul may be needed.

If the pressure is low, most of the time this first occurs at idling speed, eventually getting worse and turning the light on while driving down the road. This scenario is somewhat consistent with your description of no light on the highway, then the light at idle and at lower speeds. It’s also common to have higher oil pressure with a cold engine, before the oil has warmed and thinned.

The certain test for this is to use a mechanical pressure testing gauge, attached to the hole where the oil sending unit* goes. The mechanical gauge system you plan to install would essentially do the same thing, though its accuracy may not be as good as that of a testing gauge. A good manual should have specifications for oil pressure (for a warm engine) at idle and/or at engine cruising speed.

If the pressure is low, the suspects include the oil pump, the pump’s pickup tube, the pump’s pressure relief valve, camshaft bearing wear, and crankshaft bearing wear. On some designs the pressure relief valve is part of the pump, on others it is separate from the pump. If I were going to pursue a low pressure situation, I’d first replace the pressure relief valve if it were separate. Next on the list would be to replace the oil pump, including cleaning or replacing the pickup tube if needed (sometimes a new pump includes a pickup tube, sometimes not). After that, were getting into engine overhaul territory. The pump related repairs would be experimental surgery, so to speak. If doing them doesn’t solve the problem, they would have served the function of diagnosis rather than repair.

Now I’ll talk about the hoped-for case, an electrical problem in the light system:

It’s not unusual for a sending unit to get weak and turn the light on when the pressure is still OK. You mention replacing the sending unit, but not how recently. If the sending unit is just a year of two old, it’s unlikely it’s faulty, but not impossible. Replacing the sending unit is a fairly cheap way to find out. However, if you plan to install the mechanical gauge system anyway, there’s not much point to replacing the sending unit just to see.

The leakage near the sending unit would be irrelevant unless it’s a sizable leak. Small leaks, even to the point of dripping, don’t usually affect the pressure noticeably.

The remaining suspicion would be that the wire to the sending unit is being grounded somewhere somehow. This could be anywhere along its length, part of which is in wiring harnesses, some even inside the cab. The most likely places, however, are under the hood where the wire, or its harness, have rubbed against something that has worn its insulation (exposing a bit of bare metal wire) and provides a grounding point.

One test for this is to wiggle the wire everywhere you can find it, with the engine running and a helper watching the light. If the light goes on and off with the wiggling, that pretty well confirms a wiring problem, and it’s just a matter of locating the bad spot. If the light doesn’t come on doing this, nothing has been proven conclusively.

Another test is to disconnect the wire. If the light is on (engine running) but goes off when the wire is unplugged from the sending unit, that pretty well confirms that the sending unit is registering low pressure. Touch and remove the wire end from the sending unit terminal back and forth several times to verify that the light goes on and off with the wire connected and disconnected.

Another test would be to disconnect the wire and drive. Tape up the end of the wire so that it cannot short to ground. If the light comes on any time while the wire is disconnected, it has to be a wiring problem. If the light doesn’t come on, nothing has been proven.

In cases where the symptom is intermittent to the point of defying reaching a definite conclusion, the thing to do would be install a “T” or “Y” fitting on the engine block’s sending unit hole, then install both the sending unit and a mechanical gauge. At some point the light will come on, and the gauge can be read to see if there is low pressure at that moment. Doing this is a pain, but it’s a way to eliminate all doubt.

Hope you find a wiring problem!


*“Sending unit” is the term commonly used to indicate a device that sends a signal to a dash gauge or warning light. Sometimes “sender” or “switch” will be used for the same thing. “Sensor” is commonly used to indicate a device that sends a signal to the electronic control unit (ECU, “computer”) that manages ignition and fuel functions.

These cars had a unique oil pressure warning system with two sending units, at different places on the engine, and a warning buzzer that only engaged at engine speeds over 2000 RPM. They were very sensitive to the oil viscosity used, and even with the right oil were touchy. The cure (in addition to using the right oil) was to modify the wiring with respect to which wire went to which sending unit. Chances are that procedure would have solved your problem.

I have had recurrent leakage of oil near the sensor, which has been replaced twice in recent years. Leakage occurred even when the sensors were new and the connections were perfectly made (by a mechanic, not me, those times).

I replaced two sending units on my daughters car within 2 years… both were bad units… oil would start leaking from the unit.

If I were you I would first replace the sending unit.

Thanks for the info. Not real useful at this late date, but…

Are you Click or Clack?

Cluck, actually. :wink:

Yes, even new parts can sometimes be faulty. Be aware, though, that on oil sending units, leakage and electrical operation are two separate things. I’ve seen many sending units that leaked while still performing correctly (and of course many that didn’t work but didn’t leak either).

It’s lucky for you guys that you don’t live in Rhode Island - I would be over at your houses getting my car fixed :slight_smile: It amazes me how much some people know about auto repair. Thanks!

If it is not coming on when cold, and is only coming on when at idle or coasting it sounds bad; pump or bearings.

Of course the many repairs to the sensor could have damaged the wire.

What kind of vehicle are we talking about? TSD would not let me see the thread you linked.

5psi will float a crank, but 5psi sucks - good luck and let us know how it works out.

It’s a 93 Geo Prism with 158K miles on it.