Overfill = More oil pressure....explanation?

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Phishin

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In my 1990 Chevy K1500 with the 350TBI, I recently overfilled the oil crankcase approximately a 1/2 quart on purpose.

No harm done.

But when I drove it last night and today, I noticed an increase in oil pressure according to my oil pressure gauge on my dash.

I usually get 30-32psi of oil pressure when cold at start up. Once fully warm, it settles around 27psi. Now with a slight over-fill, I'm getting 35-38 psi at start up and it settles to around 32 at idle when warm.

Why am I seeing this sudden increase in oil pressure?

My pressure gauge has always read the same, regardless of what weight oil I put in it. I've even been a quart low in the summer, and put 20w50 in it. Somtimes in back to back top-offs, and the OP has never moved from where it typically reads.

Now all of a sudden, I'm seeing an increase due to a slight over fill....is this telling me something? Maybe the oil pick up tube has a crack or break in it or the o-ring that connects to the metal line is shot and now it's "cover" in oil and the engine is picking up that oil, instead of sucking air? Just a guess.

Anyone ever experience this or got a guess as to what is happening?
 
Sounds like the gauge is inaccurate at best.

If different oil grades yielded no different results, I'd treat it as an idiot light, and its current new behavior with further suspicion.
 
Originally Posted By: wrcsixeight
If different oil grades yielded no different results, I'd treat it as an idiot light, and its current new behavior with further suspicion.


basically what i was going to say....

if you don't notice a difference on the guage comparing 5W-30 with 20W-50, the sender/guage is lying.

EDIT: you normally run 10W-40? well, less of a difference, but should still be noticable...
 
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Question: I am curious why you overfill by half a quart?

Not that it is the problem, but you could try a larger oil filter to accommodate more oil, or use an oil that can take the heat better like Amsoil Signature Series or Redline for example.
 
another thought came to mind.... oil absorbs quite a bit of heat...

by adding more of it, you could be lowering the average temperature of the oil for otherwise identical conditions.

i've done a somewhat "extreme" version of this before: using a PF52 size filter in place of a PF47 size and adding a quart more than what the full line on the dipstick indicates. i never ran into any issues that i could detect(no air in the oil, no excess leakage, etc), but i can't think of many benefits of it either. mostly for track(or "track") cars that deal with a lot of time spent in situations that might expose the opening in the pickup tube to air.
 
Actually, adding too much oil (amount of "too much" varies by engine) causes foaming from being beaten by the counterweights and bearing caps. Foam is like pumping mayonnaise. It does not want to flow, especially through the filter, so the pump creates pressure instead of flow.

Adding more than the fill line is bad.
 
Originally Posted By: widman
Actually, adding too much oil (amount of "too much" varies by engine) causes foaming from being beaten by the counterweights and bearing caps. Foam is like pumping mayonnaise. It does not want to flow, especially through the filter, so the pump creates pressure instead of flow.

Adding more than the fill line is bad.


You'd probably have to overfill the crankcase on a 5 qt sump by more than 0.5 qts to cause any foaming issues.

As far as oil pressure, the gauges on these trucks are not idiot gauges. They may be inaccurate, but you should be able to compare between oils as they are relatively consistent in operation.
 
Originally Posted By: widman
Actually, adding too much oil (amount of "too much" varies by engine) causes foaming from being beaten by the counterweights and bearing caps. Foam is like pumping mayonnaise. It does not want to flow, especially through the filter, so the pump creates pressure instead of flow.

Adding more than the fill line is bad.

Happened to me 20 years ago. Coincidentally a C1500 w/a GM 4.3L Vortec V6. Overfilled by a little over 1qt because I checked on a hill like a dope. Driving along after topping off I heard sloshing, shortly after the engine began to lose power, then it stalled. Rolled to the side of the road and started it up. Oil light, stalled again. Checked the oil, it was a milk shake. Had it towed home. Got lucky it didn't seize. But it had a tick after that that never went away.

Rather be down 1qt than over 1qt, lesson learned.
 
You have an interesting theory on why oil pressure is now higher.
I suppose it's possible.
Keep the thing half a quart overfilled and see if the higher indicated oil pressure persists.
I doubt that the gauge or sender just coincidentally began reading differently when you overfilled by half a quart.
 
Originally Posted By: widman
Actually, adding too much oil (amount of "too much" varies by engine) causes foaming from being beaten by the counterweights and bearing caps. Foam is like pumping mayonnaise. It does not want to flow, especially through the filter, so the pump creates pressure instead of flow.

Adding more than the fill line is bad.

I got to call ya on that one, oil will aerate and become full of bubbles, loosing pressure but it will recover when the motion is stopped... If it turns into the kind of mayonnaise I'm imagining, there's moisture involved...
 
More then likely you're seeing placebo effects.

It read higher, by error, and now you think its higher, so much so that you're overly paying attention to the gauge and either noticing its higher at times or fooling yourself subconsciously into believing it is higher.

Even quality aftermarket gauges aren't perfect, oil pressure isn't really an 'exact science' to say the least. As long as its not way high or way low, you're doing great. I doubt the extra oil is actually increasing pressure any.
 
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In my 40+ years of operating trucks I have found any factory gauge to be suspect for accuracy, but generally accurate if operated in tandem with a real mechanical gauge for a day or two to develop a "feel" for it's inconsistencies.

They tend to slowly develop faults over time due to the sending unit. When you replace that you must then use the mechanical gauge again to determine the new sender's accuracy.

To the OP, I think the oil temp theory is the most logical, but there's too much conjecture involved to give a concrete answer as so many here do so easily.
 
I would drain out some oil and use it to fill to the proper level. Then I would check the OP again and see if it is your old normal. Then let us know what you found out
 
I'm not draining any oil out of my truck. It burns a quart of oil in 600-1000 miles anyways.

I doubt the oil temp had anything to do with it. It's been about 4-8 degrees Fahrenheit the last 10 days in the morning. The oil pressure has always been 30psi on a cold start and 27 while warmed up at idle. Always. Never changed.

And now with the same weather, same coldness outside, with a 1/2 quart over the fill mark on the dipstick, the oil pressure is reading 35-38 at start up and 32 at idle.

When I burn this half quart off, I'll see where the oil pressure goes.

I do not think it's temperature related. I think it's mechanically related....perhaps.
 
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