Oil Light on at Low Idle

[Achilles]

I have a 97 Montero Sport XLS. I recently had an oil change done. After picking up the vehicle I noticed the oil light coming on. It comes on at low idle, typically at stop lights, when the RPM is down to just under 500. The engine also tends to run a little rough when the oil light comes on - I'm not sure if the oil light issue is caused by the rough idle or vice versa. As soon as I accelerate the light goes out. This happens more then the engine is warm. The car seems to burn a little more oil than I am used to but it's at 140K miles now so that might not be unusual.

The first time I noticed it, I stopped and checked the oil and it appeared to be overfilled. I let the situation go since I didn't think overfilling would be an issue but now I am concerned as the situation hasn't cleared up. I asked the mechanic to check the situation and he said he couldn't figure out what was causing the problem. I've continued to check the oil level and it's running within the marks on the dipstick. I've put a lot of miles on the car in this condition but now I am getting worried that I might be doing some damage to the engine.

I considered that it might be a defective oil pressure switch but I think other things are more likely since the idle is rough, too. I thought the car might need a tune up and had that done but the problem didn't go away. I suspect that the something is wrong with the oil pump - perhaps there is a shaft seal of some sort that is worn/blown? Or, since the light comes on only at low idle, there is not enough pressure being created by the pump to close the switch at that RPM. Could it be a clearance issue, as in a worn pump not pumping efficiently?

Any help is greatly appreciated!

Thanks,
Larry

[curtis73]

Low oil pressure is caused by one of a few things. A failing pump, oil viscosity too low, oil too hot, or engine bearing tolerances too large.

A failing pump usually goes very gradually. You might start noticing it only when its really hot, or once a week and it would get more and more noticable. I doubt it would be a one day good then one day bad kind of a thing. Failing oil pumps also usually supply enough at idle, but start starving at high rpms which is another indication that its not the oil pump at fault.

Improper viscosity is a possibility since it was associated with the recent oil change.

Engine bearing tolerances are also unlikely since they are a gradual thing.

I highly suggest you take it off the road immediately. Oil pressure requirements are about 10 psi per 1000 rpms. The oil pressure light only tells you when it drops below something like 7 psi. A proper system should offer 40-60 psi on the highway, and if yours is only producing 10 on the highway, its catastrophically low for operation, but the light isn't on to warn you. If you see the light come on at idle, chances are its not providing adequate pressure everywhere else even though the light shuts off. Using some hypothetical numbers, lets say your engine is supposed to operate between 10-50 psi. Your oil pressure light would be calibrated to come on at say, 7psi and below. Unfortunately, it can't tell you anything else. If the light is coming on at idle, that means its below 7 psi, but at highway speed when the light goes out, it may be at an acceptable 47 psi or it may only be at 8 psi which can make really short lifespans of engines.

It might be the pressure sending unit, but don't risk massive failure. Just stop driving it right now and fix it or you may literally end up with an engine that can't be rebuilt or used as a trade-in core for a new one.

One other thing to check... The sputtering/rough running at idle wouldn't be caused by low oil pressure, but it could be the other way around. If its running a low idle, it could be dipping down pressure and tripping the light even though its still pumping enough oil. This sometimes happens on manual cars when you almost stall it from a stop. The RPMs dip down and so does oil pressure. If that's the case, its not dangerous, its just that the pressure switch is just dumb. Are they always correlated? If so, it might be that something unrelated about the engine is causing it to sputter and therefore dropping the idle and oil pressure.

the best way to diagnose the problem is to buy a cheap-ish oil pressure gauge that comes with its own sender. Remove the pressure switch for the dummy light and put the sender in its place for the gauge. It takes a couple wires, too; power and ground. Either you or your mechanic can temporarily wire it up and it will tell you exactly what your oil pressure is and the mystery will be over.

electronic gauges can be had pretty cheap; under $30.

[Achilles]

What problems would overfilling the crankcase cause? Could the be the source of my low oil pressure problems? Would overfilling affect the Oil pump?

[Racincc85]

If the crankcase is overfilled enough the oil could come in contact with the crankshaft which would immediatly whip in into a froth. The oil pump is not capable of pumping this and little if any oil pressure will result. How much was your crankcase overfilled? It would have to be overfilled at least a quart and a half or more to cause this. I think that curtis is right in that the rough idling is allowing the oil pressure to drop low enough to cause the idiot light to come on, but damage from overfilling still is a possiblity.

[Achilles]

Originally Posted by Racincc85

How much was your crankcase overfilled? It would have to be overfilled at least a quart and a half or more to cause this.

I don't think it was that much, but it was clearly over the top notch on the dipstick. I would guess at most a quart. The oil on the stick did not look frothy as I recall, I think I would have noticed that. I think if this situation had happened I would have noticed the engine running rough but I didn't.