Fleetwood Meadow

Oil light comes on after warmed up

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My oil light comes on in my '52 Cadillac when the engine has warmed up. Its more than likely the original oil sensor. The light is off when the car is first started. When the engine warms the light turns on at idle and below 15mph. I am using 5W-20 in the engine. The book says to use 20 weight. The oil pump was rebuilt when the engine was rebuilt, about 50-100 miles ago. Does it sound like a faulty/failing oil sensor or is this an actual situation that others have dealt with?

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It sounds like your main bearings are worn out.  install an actual gauge to see what is really going on. Changing to a thicker oil may help.

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5W-20 seems way too light to me.

The lightest oil I use is 5W-30 in my Model T.

The others get 10W-30.

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The book says 20 weight. I thought it was light too but the manufacturer of the engine says it, not the be that guy that only does things without thinking just because the book says it. It’s a 331 V8. Everything was replaced when the engine was rebuilt. It idles and drives great. I don’t get oil consumption. Is there a way to test that oil pressure switch?

Edited by Meadowbrook Fleetwood (see edit history)

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5W20 is really a lot thinner than straight 20 weight was back in the day. 10W30 or 10W40 would be better.

 

When the car is hot and the light is on, are you getting lifter noise at idle? If not, I would bet that your problem is the sensor.

 

The only way to be sure is to either put a test gauge on the engine or to substitute another sensor.

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Agree with the suggestion that fitting an oil pressure guage is the only sure way to see if the pressure is ok, but wondering if you only have 100 miles on the engine after rebuild, why wouldn't you be taking it back to the rebuilder ?

As to oil, I would be thinking along the lines of 20w40 at least.

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Doesn't sound like a sender to me. In fact it sounds as if the sender is doing its job telling you that the pressure is low.

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That answer is easy, I have brought it to the rebuilder. I brought it back to myself and said hey my light comes one. And my response to myself was, hmm let me look into this. When the idle was very low and the light was on I heard lifter noise. When it’s idling at normal idle I don’t hear it. I’ll try a thicker oil, such as a 10-30 and see what it does. I’ve asked many times if the weights are the same now as they were then and some people say yes, some people say no. So out of this I’m getting that we don’t think it’s a sender issue, it’s more of an oil viscosity issue. 

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It is actually a guessing game unless you install a pressure gauge and see what the cold and hot operating pressures are. If the pressures are below around 5 psi hot idle the light will come on. you can check the sender by measuring resistance with 10 psi pressure applied by an air regulator and gauge readout and then slowly turn the regulator down in pressure until the light goes on. If the pressure is low check the relief valve, if OK then remove the oil pump and pressurize the oil galley and see where the larger leaks are or check each main and rod bearing. Also pull the cover off the oil pump and check the gears and pockets for scoring. Gears should only have about .001" of clearance. Also when pressurizing the oil galley watch for oil leakage in the timing cover  or another area. I have seen many "rebuilds" where someone forgot to plug or correctly reseal an oil galley.

Best of luck to you

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Take out the oil pressure sender and put in a gauge temporarily. You should have good oil pressure in a newly rebuilt engine even with 5W20 and even at idle, hot. Something is wrong somewhere. Maybe the pressure sensor, maybe internally.

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Just a note that the SAE viscosity bands are just that, bands. Not all oils within that band have the same viscosity. If you have an oil from nearer the bottom of the SAE 20 band, is will be lower viscosity than one from near the top.

 

I wonder if the manual for your car is meant for cars sold in the north, in the snow zone. The oil recommended there would be thinner than in the south, warmer zones. Oil technology has moved a bit since 1950! Personally, I would be expecting to use a 5W-30 in a new engine such as yours. Keep the first number (before the W for "winter") low - it describes the oil viscosity behaviour when cold.

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Even in the 50's oils weren't anywhere what they are now........kind of like comparing oils of the teens to 50's oils.

What times of year do you drive this car?

If it's a fair weather vehicle maybe consider moving to synthetic oil:

"It took researchers years to create quality synthetics that outperformed conventional oils, but there's little doubt that synthetics are now superior products in most scenarios. They feature resistance to high temperature oxidation, good film strength, stable viscosity and rarely cause harmful engine deposits."

Pay particular attention to the part about stable viscosity.......something conventional multi-weight oils attempt but aren't very good at.

 

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Look, the most important thing at this stage is to check the actual pressure with a guage worry about viscosity etc after.

 

You will get every man and his dog telling you all they know about oils, do a search here and you will be overwhelmed with information, both good and bad, none of it is relevant if you dont have good oil pressure.

 

 

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