oil warning light

[av winans]

when my 73 2.0l warms up in the summer heat the oil warning light will illuminate at idle. this does not occur when the car is cold. the light does not come on during hard cornering or at highway speeds, only at idle after it has warmed up. is this a sign of a failing oil pump, or is the oil breaking down? i'm run 20w50 castrol. could the oil pressure relief valves be getting weak, or maybe just a bad sending unit. i did have an oil pressure gauge in the car that worked, but now when the car is started the needle just flops to the opposite extreme of zero and when shut off it quickly drops back down.

[Dave at Pelican Parts]

It's likely that your engine is simply odl and a little bit tired. Under those circumstances, the oil pressure can and does drop low enough for the light to flicker on at idle with the engine warmed up. The owner's manual mentions this, and claims it's nothing to worry about.

There is a rule of thumb that you want 10 PSI oil pressure per 1000 RPM, up to wherever the pressure limits. If you have 200 PSI or more at 2000 RPM and 30 PSI at 3000, you're OK for now.

It is likely that wire from the gauge to the sender has become damaged. I believe that a short to ground will make the needle "peg" at the top end. A complete disconnection will make it "peg" at the bottom. (I could have those backwards...) Check the condition of the wires. You can unplug the one end from the gauge, and the other end from the sender. Check for continuity, and check for a short to ground. Replace the wire if necessary.

It is also possible that the sender has failed, but I think it less likely than the wiring. Especially since your warning light seems to work.

--DD

[av winans]

Thank you for the advice. The rebuilt motor in my 73 only has 10k on it and still runs very strong. I personally did not rebuild the motor the previous owner of 23 years did. The car was very well maintained, however, I don't know what was done to the oil pump and so forth. The oil sending unit is a dual one but most likely it has a pinched wire. I have not been able to get a hold of a voltage meter to check the resistance yet. Sometimes it has a searching idle when the oil light comes on. Could the low oil pressure cause an idle surge or vice versa? Any more advice please.

[John Rogers]

On our 1973 race car there are two senders, one is the stock one going to the stock light and a big red light on the dash and a second sender for a gauge on an accessory panel. I really like the idea of redundency since these engines are really expensive to repair if the oil pressure fails. I would run new wires anyway since the original ones are getting pretty old.

[Dave at Pelican Parts]

If the car is close to stalling (the idle sags down real low), the light can come on even in a healthy motor. Remember, oil pressure is roughly proportional to engine RPMs.

The up-and-down idle sounds like the classic "lean hunt". Richen the idle mixture by turning the knob on the ECU a couple of clicks clockwise. See if that helps.

--DD

[av winans]

Thank You DD, it really helps when I ask the right question, if I expect to recieve an answer that corrects the problem. I adjusted the ECU knob 3 clicks clockwise and have had a nice even idle and no sign of an oil light. Tomorrow will be the real test "hot nasty humid southern afternoon driving". Again, Thanks for the fix.

Aaron