Oil pressure plateaus at 40 PSI

DorianL

DorianL

solid fixture here in the forum
Staff member
Strange - when hot, my pressure climbs quickly 40 PSI (2000 ROM) and then it takes proportionally much more revving to get over that. At 3000 RPM it start paking over 40 PSI. Since it is a new build - I am a bit reluctant to go much over that.

Cold idle: 60+
Hot idle: 15+

10w40

Is this normal? (I used to run 20w50)
 
GM performance told me years ago that on BBC they looked for 10lb per thousand rpm. The 540's in my Donzi had 20 lbs at idle (950 rpm), and had roughly 70 lbs at 6500, at that speed it was hard to concentrate on exact numbers in 3' swells! :mrgreen:
 
pressure is a measure of the resistance to flow,both the oil viscosity and temperature effect that resistance to being pumped under pressure thru bearing clearances
viscosityvstemperature400.gif

clearanceflow.jpg

if your oil pressure peaks at less than 60 psi, the first thing Id suggest is checking with a separate oil pressure gauge and sensor because its hardly rare for those sensors, or gauges to be defective.
FLOW IS THE KEY HERE!
if your oil levels low that can also cause the symptoms, so verify oil level in the cars engine, before you go nuts. one simple test is simply to add an additional quart of oil and see if it makes a difference in the oil pressure or where in the rpm range a problem shows up.
now if the oil pump pick up is mounted too close to the oil pan floor it restricts oil flow rates entering the pump and as the rpms increase so does the flow up to the point the oil flow can,t enter the pick-up due to the restricted area between the oil pump pick up and the oil pan floor.
this will produce the exact symptom your describing, and can happen at 20psi, 30psi, 40psi,, depending on the pick-up to oil pan floor clearance or bye-pass spring which may be weak and opening early or clearance between the pick-up and oil pan floor,but its not the only possible cause., and its the source of the myth that high volume oil pumps pump the oil pan dry, because high volume oil pumps sit lower in the pan and if you don,t carefully check oil pump pick-up too oil pan floor clearance theres a good chance swapping to the longer pump restricted oil flow into the pump.
the next thing to do is swap to a thicker viscosity oil (save the old oiil) and a new name brand oil filter,like WIX or MOBILE 1 or PUREOLATOR because oil filters can be defective or get clogged, if the pressure increases, on both gauges to well over 40 psi, to 60 psi you know the gauge is reading the resistance and its not the oil pumps bye-pass because if thats defective, or the pumps intake pick-up clearance, is restricted, the bye-pass circuit will still open at 40 psi. and the pumps still restricted, if pressure goes up, with the thicker viscosity, and new oil filter theres a good chance its the bearing clearances, or the old oil filter was restricted, if you, now drain the oil and put the old oil back, but keep the new oil filter, and it still has the higher oil pressure it was obviously a restrictive filter. if it returns to the 40 max psi, theres a good chance its a restricted pump inlet or defective bye-pass circuit, or excessive clearances


heres a few related threads to read

viewtopic.php?f=54&t=3536

viewtopic.php?f=54&t=150

viewtopic.php?f=54&t=1800

viewtopic.php?f=54&t=3194&p=9385&hilit=scope+clearance#p9385


BTW

THIS TOOL MAKES CHECKING THE OIL PUMP TO PAN CLEARANCE EASIER, drain your oil , and you can use the flexible scope and look at the oil pump clearance to the oil pan floor, the oil pump pick-up and its brazed joint,the cam, pistons and bore walls etc, thru the oil pan drain hole, or pull a spark plug and inspect the valve or piston condition

scopeinternal.jpg


http://www.tooltopia.com/provision-pv618.aspx
 
Thanks, thanks...

So far:

idle - 15 psi
1000 - 20
1700 - 30
1788 - 35
2400 - 40
3700 - 45
4500 - 40
 
DorianL said:
Thanks, thanks...

So far:

idle - 15 psi
1000 - 20
1700 - 30
1788 - 35
2400 - 40
3700 - 45
4500 - 40
Click to expand...
I'm not the tech that Grumpy is, but in my experience that drop off at that rpm was most often related to restricted flow, especially if the symptoms were relatively similar warm or prolonged hot driving. In my boat I once had that problem in one engine due to one pick up tube being a few thousandths longer than it should have been. Oil pressure was similar in both engines until about 4500, then the one started to fall off and get erratic somewhat. I was afraid it was the rotating assy. But I found the oil in that motor was foaming at higher rpm operation when I pulled the stick to look at it. So I lifted that one out and pulled the pan, measured the pickup depth from the lower block rail and then measured the pan depth, I called the supplier to verify length, it wasn't a lot too close but enough. he sent me a new pick up assy and the problem was cured.
 
update: gauge seems inconsistent.

While driving, idle is sometimes 10 PSI sometimes 25.

I brought gradually the RPM up with the vehicle parked. Pressure rose till 45 PSI. As I very slowly brought the RPM higher (4000 - 5000) the needle seemed to dip here and there - like venting. It never went above 50. I am thinking I am hitting up against the the bypass.

But first - out goes that gauge!
 
IF IT TURNS OUT TO BE the bye-pass spring it will open if the springs weak, DON,T SHIM IT, get a NEW spring that fits your pump ,the large end faces out.
if it turns out to be the oil pump p ick -up being mounted to close to the oil pan floor naturally that will need to be changed.
btw what did you find , changed if anything,when you added an additional quart of oil?
if its a defective sensor or gauge thats obviously the easiest to cure!

http://www.summitracing.com/parts/SUM-122170/
122170_w.jpg


viewtopic.php?f=54&t=1800

pressureREG.JPG

you should NEVER shim an oil pumps pressure relief spring as it may prevent the piston it holds from moving down its bore far enough to allow it to open the bay-pass passage, that allows the pressure on the high pressure side of the oil pump from bleeding off back into the intakes side of the oil pump
oilspring.jpg

Many pressure relief springs have one end larger than the other end,the spring always mounts with the larger end facing away from the bypass piston, if installed reversed the piston can,t move far enough to completely open the bypass circuit passage and pressures skyrocket, under some conditions
 
Why 60 psi at cold idle? Doesn't that mean it's the clearances? It can't be the bypass. Could still be pick up.
 
if it goes to 60 psi cold its not the bye-pass if you increase the rpms with it cold and the pressure drops as rpms climb its almost certainly caused by a clogged pick up screen or a pick up screen thats come loose or fallen off or its mounted too close to the oil pan floor and its restricting oil flow because even if it was the bearing clearances in the main and rod bearings as the rpms increase the pressure might be lower than ideal but it would steadily increase with the rpm increase, so your getting closer to finding the cause
 
Hmmm. Will do the test and bring the rpm up when cold.

A buddy of mine might have one of those scopes.
 
Hmmmm. Seems like I need to drop the pan short of getting my hands om that scope which is unlikely.

I could try putting my old pump on there until I get the pick up of néw pump sorted out
 
while your checking put some modeling clay under both pick-ups and see the effective clearance difference then see if the old pump clears the problem, in an ideal world the pick-up to pan floor clearance is a minimum of 3/8" and 1/2" is ideal

TW-383-oil-pump-5.jpg


viewtopic.php?f=54&t=1800
 
Ummm, yes and no... Replaced the pump this weekend and it seems the problem is still there.

I am thinking it may be the pressure gauge. Grmbl. Feel like a rookie.

If it is not that... then I am at loss.
 
I assume you verified the pick-up to oil pan clearance? and changed to a new name brand oil filter?
the idea that you can 'pump the pan dry" with a high volume oil pump, in a properly set up chevy engine has been proven false dozens of times, thats a myth that myth been repeated for decades and the reason its gained its supposed verification as fact is simply that guys install a high voluum oil pump in the same engine that they just removed a stock oil pump from and find that they reve the engine and the oil pressure starts to go up then tops out or even drops off , the instant and wrong conclusion drawn is that the high voluum oil pump sucked the oil pan dry, but what really happened in almost every case was a bit simpler.
high volume oil pumps have a longer body that moves the oil pump pick-up closer to the oil pan floor and if the clearance is not checked and corrected to maintain the required 3/8"-1/2" minimum clearance it severely restricts the flow into the oil pump and pressure drops of as a result of the restriction on the intake side of the oil pump pick-up.
your typical small block oil pump has a 4 bolt cover, a 5/8" pick-up tube diameter and the pick-up presses into the cover , it has the smaller 7 tooth gears, and because the pick-up tube presses into the cover swapping from a standard to a high volume pump, with its longer impeller and hoseing casting, tends to move the pick-up about 1/4" closer to the oil pan floor
oilvbn1a.jpg

oilvbn2.jpg

ChevyOilingPluga.jpg



http://www.mellingselectperformance.com ... -Oil-Pumps
 
There was nothing to be seen in particular in the pan. The mesh screen was finer on the pump in there and the oil was a little silty - nothing dramatic. The pan to pickup clearance was fine.

Anyway, I decided to put the old pan and oil pump are back on there and... the symptoms are identical.

I guess that means it is either the gauge or the clearances are excessive. I cannot imagine it is the clearances cuz I checked them and the were where they should be if a little tight even. Unless those damn crank spacers?

Grmbl
 
Ummm, oil filter... on order. Everything takes a couple of days in Belgium... to a week!

And yes I carefully checked the pick up clearances.

Plus fresh oil.
 
DorianL said:
There was nothing to be seen in particular in the pan. The mesh screen was finer on the pump in there and the oil was a little silty - nothing dramatic. The pan to pickup clearance was fine.

Anyway, I decided to put the old pan and oil pump are back on there and... the symptoms are identical.

I guess that means it is either the gauge or the clearances are excessive. I cannot imagine it is the clearances cuz I checked them and the were where they should be if a little tight even. Unless those damn crank spacers?

Grmbl
Click to expand...

I hope the spacers had the correct oil feed holes aligned and were installed correctly Ive seen spacers where the bearing oil feed holes were restrictive , but I have not seen them spin or cause problems if the oil feed holes were ok
 
But why 60 psi cold? Hmmmm. I think at this point oil filter and a new gauge.
 
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