Odd oil pressure drop...

I drive a 67 396/325 Impala sports coupe,that i brought from the USA to France ,in 1990.
135000 miles on the odometer.

The car has not been driven,for ten years,so i decided to give it some fresh stuff before going back to the road.
New timing chain and gears,and a pair of head gaskets,among other treats.

After initial start up everything seemed to be ok.

It ran fine,temp and oil pressure ok. (i have added some extra instruments)

I let it warm up and iddle ,drove aroud the block a few times: no issues .


Then ,after start up,i noticed a drop on the oil pressure meter,and the oil pressure idiot light went on.

I killed the engine,and checked the engine bay.

No leak whatsoever,the oil level was at the low mark,as i was planning to change the oil after a short time.


I replaced the oil filter: same thing.
Oil pressure builds up on starting ,then falls down slowly to almost none...
I put a gage with an oil feed tube ,a new filter again ,and saw that pressure built up as the filter was filled then slowly falled down to zero.

This not temperature related,as the engine was cold.

I took the oil sump out,and checked the pump : it seems ok to me.

What could cause this condition?

Thanks for reading so far. Any suggestion welcome.
 
pressure is a measure of resistance to flow volume,
if the pressure drops is indicating EITHER the pump flow volume has been reduced ,
or the level of oil flow volume resistance DECREASED ,example a oil gallery plug got loose and fell out.
Id change the oil filter and oil, and pour in 5- quarts of fresh oil and see if that makes a difference as a first test,
if the pressure is near zero you should confirm that with a second gauge and if its not a defective gauge,
I know you don,t want too think about this, but I may be time to pull and inspect and rebuild that engine,
you will have extensive damage result if you run an engine with near zero oil pressure,
thus its time to find the cause before you run the engine to the point,
it starts throwing broken parts through the block or heads,

its sounds like a big job but having done that over a hundred times on bbc engine ,
its not that difficult, and the early 1967 396 is one of the easier engines too work on.
in my opinion , and experience and from lots of G.M. engine, race testing.
theres no need for oil pressure to exceed about 65 psi,
it takes power to spin the oil pump against that extra resistance, it induces extra wear on the distributor and cam gears,
and it does nothing to reduce bearing wear or increase cooling on the bearing surfaces, he should open some bearing clearances marginally
(maybe an extra half thousandth on the mains) to increase oil flow volume reaching the main bearings, and use a lower resistance oil pump bye-pass spring.
extra oil flow volume cooling the bearings and valve train will do more for durability than oil pressure exceeding 65-70 psi
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swaping in a new oil pump, would certainly not hurt and as a test its not that hard or expensive,
you should pull the engine apart and locate and correct the failed bearing or oil passage plug ETC.
personally with 140K or so miles Id take the time to install new cam,
crank and rod bearings and give the bore walls a good inspection,
if they look ok, a mild hone and new rings after cleaning the pistons and if it was my engine a new cam and lifters
, any flat tappet hydraulic cam approaching 140K miles is suspected of wear issues
. Id check the valve guides and seals and get a cylinder head rebuild while I was at it.
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assuming the proper bearing clearances and proper oil pump, was selected,ans oil pump pick-up installation were done,
the two most common reasons I see for low oil pressure are related to either the oil pump pick-up mounted too close to the oil pan floor
(3/8' clearance is about ideal) or trash/debris getting into the oil pressure bye-pass valve spring area


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when you inspect the bye-pass piston and spring or replace the oil pumps bye-pass spring, in the oil pump you must be certain it smoothly slides through the internal passage , if it binds it will cause issues with erratic oil pressure
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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
keep in mind that as oil temps increase the oil viscosity tends to decrease, thus cold oil, at lets say 70F might cause the oil pressure gauge to read 50 psi at idle but the pressure reading slowly goes down to 25 psi once the oils reached lets say 210F, this is normal and expected

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REMEMBER the oil pump bye-pass is designed to limit oil pressure reaching the oil passages in the block,to usually 65psi-75psi max, by opening and re-routing oil from the pressure side of the oil pump to the suction side of the gears one its forced open, this is not the same and has nothing to do with the oil filters oil bye-pass valve,
yes the BYE-PASS IN THE OIL FILTER DOES normally open on cold mornings, or under rapid changes in engine rpms, especially before the heat from the engine lowers the oils effective viscosity, Im fairly sure you have heard or remember hearing advise not to go doing stupid stuff like burn outs in your drive way until the engines reached and maintains operational temps
(usually oil and coolant has reached 190F-215F) well one big reason is that thicker oil viscosity when oils cooler ,takes a good deal more pressure to force thru the filter medium, once the oils fairly hot it flows better, and requires less pressure as it offers less resistance to flow thru the engines clearances and thru the filter, that in turn means that hot oil with its thinner effective viscosity tends to be far less likely to force open the filter bye-pass circuit and that means oil bye-passing the filter medium is far less likely to drag metallic trash held in suspension with it to reach the bearings to cause more rapid wear.the oil filter bye-pass is designed to bye-pass oil to the engine rather than forcing it thru the oil filter any time the pressure on the inlet side of the oil filter is higher than about 10 psi over oil pressure passing thru the oil filter, this in theory prevents the semi clogged oil filter from starving the bearing of oil.but it also means that during rapid rpm changes or before the oils up to operational temps some oil is routed around and past the filter, under the concept that dirty pressurized oil is far superior to no oil reaching the bearings.
MORE USEFUL INFO
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BE AWARE NOT ALL BBC OIL PUMPS FIT ALL BBC ENGINES, the LATER GEN V, and GEN VI HAVE A DIFFERENT PUMP due to main cap clearance issues (YEAH IM AWARE they can be retro fitted)
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spending the time too read through the links and sub-links will prevent a lot of wasted money and time and mistakes
http://garage.grumpysperformance.com/index.php?threads/another-496bbc.5123/

http://garage.grumpysperformance.co...alves-and-polishing-combustion-chambers.2630/

http://garage.grumpysperformance.co...tion-of-crank-durring-short-blk-assembly.852/

http://garage.grumpysperformance.com/index.php?threads/installing-an-oil-pump-pick-up-tube.1800/

http://garage.grumpysperformance.com/index.php?threads/basic-info-on-your-v8-lube-system.52/

http://garage.grumpysperformance.co...ing-parts-and-a-logical-plan.7722/#post-72126

http://garage.grumpysperformance.com/index.php?threads/precision-measuring-tools.1390/#post-68850

http://garage.grumpysperformance.com/index.php?threads/big-block-chevy-info.710/

http://garage.grumpysperformance.co...e-springs-and-setting-up-the-valve-train.181/

http://garage.grumpysperformance.co...g-and-installing-connecting-rods-pistons.247/


http://garage.grumpysperformance.co...402-bbc-with-a-4-454-crank-or-even-4-25.2165/

http://www.superchevy.com/how-to/148-0309-396-ci-chevrolet-chevelle-engine-rebuild/


http://garage.grumpysperformance.co...drops-at-15psi-on-fresh-327.16005/#post-96527

http://garage.grumpysperformance.com/index.php?threads/block-prep.125/

https://www.summitracing.com/search...-5l-396/engine-family/chevy-big-block-mark-iv

https://www.themotorbookstore.com/how-to-rebuild-big-block-chevy-engines.html

http://garage.grumpysperformance.com/index.php?threads/bits-of-396-bbc-build-info.3968/
 
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its always a good idea to verify machine work, theres always some machine shops that either do low quality or shoddy work, and you can,t expect an engine to operate correctly if the clearances or consistency of the bearing and lubrication surfaces are not correctly set, you would not bet the first or last guy to be told a crank was cut to a size that was not the true size or sold the wrong bearings for an application, you should always verify machine work, with both a precision measuring tool and plasti-gauge
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MEASURE CAREFULLY
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http://garage.grumpysperformance.co...l-pressure-switch-on-fuel-pump-circuit.16013/


http://garage.grumpysperformance.com/index.php?threads/can-i-get-it-polished.9214/

http://garage.grumpysperformance.co...guess-on-clearances-and-journal-surface.9955/

http://garage.grumpysperformance.com/index.php?threads/bearing-clearances.2726/

http://garage.grumpysperformance.co...tion-of-crank-durring-short-blk-assembly.852/

http://garage.grumpysperformance.com/index.php?threads/rotating-assembly-bearings.9527/
 
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Thank you very much for these explanations.

I already proceeded the way you suggest for the first steps: oil filter change ,oil filling to the high mark,use of another oil gage(mechanical),with the same results.
The pressure rises to 60psi,then falls down enough to turn the idiot light on...

I will take the mill out and search for the things you mentioned.
(I had the heads rebuilt with hardened seats,and new guides ,a few years ago.)

(By any chance,could the bypass valve on the oil filter housing cause this symptom?)
 
thats a very common question but the answers no!
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ALL THAT OIL FILTER BYE-PASS VALVE DOES IS ROUTE OIL FLOW PAST THE OIL FILTER
IF IT BECOMES SO CLOGGED WITH TRASH THAT THERES
A 10 PSI DIFFERENCE IN THE RESISTANCE TO OIL FLOW THROUGH THE FILTER
VS AROUND IT INTO THE BLOCKS OIL PASSAGES, oil enters the area over the oil filter in the block and is forced into the outer holes in the oil filter perimeter down through the case and filter element and up through the central hollow screw retention stud into the blocks oil passages, if the resistance too flow is too great the oil filter bye-pass valve routes oil around the filter directly from oil pump to the blocks oil passages.
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MORE USEFUL INFO
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yes the oil flows around the mounting stud,from oil pump to main cap to reach the engine oil passages, thru the oil filter
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failure to use the correct oil pump,mounting stud, bolt or nut or carefully check clearances when mounting an oil pump can cause problems
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ONE RATHER COMMON MISTAKE IS USING THE WRONG OIL PUMP STUD OR BOLT TO MOUNT THE OIL PUMP AS IF EITHER EXTENDS THRU THE REAR MAIN CAP IT CAN AND WILL BIND ON THE BEARING AND LOCK OR RESTRICT, SMOOTH ROTATION
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http://garage.grumpysperformance.co...-friction-and-pumping-losses.8966/#post-31978
obviously the first two steps in diagnosing any oil pressure problem you suspect is to check your dipstick for the proper oil level in the engine and then test the pressure with an independent test gauge to verify your cars gauge is correct, and you do have a problem , not a defective sensor or gauge
the oil pressure sensor location, is next to the distributor base,on most sbc engines on the rear of the block, but if your trying to track down an oil pressure issue it could be either a bad sensor, bad connection on the sensor or gauge, or a defective gauge, causing the high pressure reading OR it may be a defective oil pump, or bearing clearances or an oil passage plug, or the wrong oil viscosity, or a busted oil pump pick-up or incorrect installation of the oil pump or pick-up,defective oil filter or several other potential sources if its a low pressure issue as previously mentioned.

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broken , or improperly installed oil pump pick-ups can cause several major oil pressure issues
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It sounds like the 396 BBC Engine Main Bearings are Severly worn out.
Running oil clearances at .004.-.005".
Too much.
.002-.0025 desired.
Ditto same on the Rod bearings new.
 
2 other things to check:
the oil pump pickup might not be set to the proper clearance off the bottom of the pan.
the oil is not returning to the sump fast enough because the returns are somehow blocked (sludge, old silicone).
 
yes Ive seen several cases where guys failed to install the oil pump pick-up at the proper minimum 3/8"-to-1/2" off the oil pan floor clearance,
the result is the pump is starved for oil intake flow.
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almost every mechanics tool box needs a few basic measuring tools and supplies
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https://www.amazon.com/Claytoon-Set...d=1466872286&sr=8-17&keywords=plastilina+clay


http://www.utrechtart.com/Plastalin...currency=USD&gclid=CN3G75zOw80CFQgaaQodKbgFjA

http://garage.grumpysperformance.co...m-sure-your-convinced-its-the-oil-pump.11085/

http://garage.grumpysperformance.co...m-oil-pump-installed-now-no-oil-pressure.525/

http://garage.grumpysperformance.com/index.php?threads/bbc-oil-pump-in-a-sbc.2598/

ok, basics, the oil enters the lifter from block oil passages under pump pressure while the lifters on the cams base circle,
with near zero valve spring pressure on the lifter seat,
the hydraulic oil pressure lifts the push rod seat taking up the valve train clearance.
as the cam rotates the lifter moving up off the cams base circle causes the lifters push rod seat to momentarily move down compressing the trapped oil volume , as the push rod seat is compressed,down into the lifter body,
forcing oil trapped in the lifter up through the push rod,, the trapped , and now further pressurized oil is there for two different requirements,
first it lubes and cools the valve train, as it exits the rocker/push rods and flows over the rocker and valve springs,and second it quiets the valve train by removing the clearance, with a cushioned floating push rod seat,
that clearance is required for the valve train to function.
but the clearance results in a ticking sound like solid lifters produce,
adding a hydraulic supported push rod seat quiets the valve train,
and without the hydraulically supported push rod seat,the valve train requires periodic clearance adjustments ,
the hydraulic supported push rod seat is fully compressed, by the cam lobe forcing the lifter up against the push rod and the valve springs resistance,
by the time the cam lobe reaches peak lift ,
but at lower rpms all the oil is forced up and out of the lifter well before the lifter is near full lift on the cam lobe,
now it is possible that at extremely high rpms the lifter bleed down rate does not have sufficient time to vent all the trapped oil,
this is commonly thought to result in valve float,
but thats not likely, the inertial loads on the lifter and valve train not being completely controlled by the valve spring resistance,
and ability to seat the valve firmly and keep it seated as the lift returns to the cam lobe base circle,
and the lifter loosing firm contact with the cam lobe and or bouncing,
as it re-seats is far more commonly the fault.


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most of the GIVE or clearance or movement in a hydraulic lifter push rod seat is the result of pressurized oil flowing into and lifting the push rod seat, without the pressurized oil entering the lifter body,from the sides of the lifter body from the oil passages in the block, manually forcing the push rod seat down into the lifter body is basically a one shot deal that generally only going to compress in a minimal .060 -.08 once until oil pressure forces the seat back up.
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http://garage.grumpysperformance.com/index.php?threads/correctly-adjusting-valves.196/#post-90731
 
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I took the cover off on the oil pump pick up screen.
There was a lot of silicone sealant,and sludge and metalic debris...
Looking like copper...
 
excess silicone trash clogging the oil pump pick-up and internal oil passages in the block,
from guys putting too much or improperly located silicone sealant on gaskets,
is a common source of problems when the excess crap gets into the oil passages

http://garage.grumpysperformance.co...oil-passages-and-improved-oil-flow-mods.3834/

http://garage.grumpysperformance.com/index.php?threads/metallic-debris-in-filter.12364/

http://garage.grumpysperformance.co...k-after-a-cam-lobe-rod-or-bearings-fail.2919/

http://garage.grumpysperformance.co...oil-passages-and-improved-oil-flow-mods.3834/
 
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excess silicone trash clogging the oil pump pick-up and internal oil passages in the block,
from guys putting too much or improperly located silicone sealant on gaskets,
is a common source of problems when the excess crap gets into the oil passages

http://garage.grumpysperformance.co...oil-passages-and-improved-oil-flow-mods.3834/

http://garage.grumpysperformance.com/index.php?threads/metallic-debris-in-filter.12364/

http://garage.grumpysperformance.co...k-after-a-cam-lobe-rod-or-bearings-fail.2919/

http://garage.grumpysperformance.co...oil-passages-and-improved-oil-flow-mods.3834/
Thank you.
Great stuff you gave me links to.
 
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any progress?
I had hoped you would take a bunch of clear pictures of the engine , dis-assembly, cleaning, inspection, and rebuild
 
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I am new here. I just built my first big block. I have a 396, bored .040 over, balanced, dome pistons and a lot of good stuff in it.
Doing some reading I have seen it go back and forth about using a high volume oil pump with a stock pan.
My pan is a little over stock, 5 quarts, is that enough. I also have a windage tray installed. Thanks!


having a larger capacity oil pan is always an advantage ,
but maintaining control of the oil flow and its rapid return,
to the sump and oil pump inlet,
is vastly more important, than capacity,
theres no real advantage in use of a high volume oil pump in a nearly stock engine,
If you post the info on your car make/model,
I may know where you can find a baffled high capacity oil pan at a reasonable cost.

youll find a great deal of related info
reading links and sub-links on this web site


http://garage.grumpysperformance.com/index.php?threads/oil-system-mods-that-help.2187/

http://garage.grumpysperformance.com/index.php?threads/bearings-and-oil-flow.150/

http://garage.grumpysperformance.co...ilter-you-sellect-does-make-a-differance.117/

http://garage.grumpysperformance.co...l-pumps-pressure-bye-pass-circuit-works.3536/

http://garage.grumpysperformance.co...g-up-oil-feed-holes-in-bearings-shells.10750/

http://garage.grumpysperformance.com/index.php?threads/oil-accumulator.1280/

http://garage.grumpysperformance.com/index.php?threads/basic-info-on-your-v8-lube-system.52/

http://garage.grumpysperformance.com/index.php?threads/whats-a-windage-tray-do.64/
 
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