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sudden oil pressure drop

G

gruss

New Member
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Location
washington
these are the conditions i encountered. oil pressure about 40 psi going down the road, no problems. next day, same route, pressure suddenly drops to near 0, fluxuates from 0 to 20, 10, 0, and so forth. noticeable power loss probably due to the extra friction. pull the bottom end apart, crank has been turned under, looks in great shape. replace main and rod brgs with correct oversize. new oil pump. reassemble, start. oil pressure still as above. pull the oil pump drive, drive pump with cordless drill @ 2000 rpm w/ engine off. oil pressure same as above. oil level full, no leaks. any ideas why the very low pressure? stumped here. i see the oil flows from the pump to the filter to cooler then back to lube engine. with new correctly sized brgs why will it not hold pressure? the new pump is also hi volume too. anyone have a clue? im sure this isnt the 1st time this has happened with so many of these engines out there.
 
When using drill to run oil pump are you just spinning shaft or do you have an oil Pump drive tool that seals the top section?

Any signs of oil leak anywhere?

Cam bearings are most common interior failure.

Spin on a new oil filter& retest

Then make a small bypass hose to bypass the oil cooler hoses next to the filter. Then retest.

Possibly Pull valve covers and see if oil is coming out rocker arms.
 
Will L. said:
When using drill to run oil pump are you just spinning shaft or do you have an oil Pump drive tool that seals the top section?

Any signs of oil leak anywhere?

Cam bearings are most common interior failure.

Spin on a new oil filter& retest

Then make a small bypass hose to bypass the oil cooler hoses next to the filter. Then retest.

Possibly Pull valve covers and see if oil is coming out rocker arms.
Click to expand...
no oil leaks present on exterior.
oil filter is new.
so loop the inlet outlet of the cooler? we blew thru the lines with compressed air, they were clear.
when the engine was 1st started, it was a bit noisy but quieted down as oil started to circulate. when we pulled that plug to install the mech gage we verified it was an oil passage by ticking the engine over and oil gushed out.
 
If you cut a vac pump down for a pump driver , make SURE you plug the little tiny hole or you will take a bath .
 
Cut the old oil filter open and see what's in it. Inspect it first in case it came apart and plugged galleries and you have a warranty claim that cutting the filter voids. What did the old oil look like? Old bearing condition?

Any visible cracks in the main webs?

Note with an engine failure you can not flush the engine oil cooler - replace it. (You can try, but, after the second rebuild from debris in it you will replace it.)
 
I feel bad not being able to have a $20 fix for gruss the new guy. Like the news caster that on day 1 has to announce a natural disaster.
 
The end of the oil circuit is opposite diagonal where you put mechanical gauge, right front corner of block. Maybe move gauge or add another one to there and compare.

Having installed new lower bearings I am assuming a couple things that are better asked.
The bearings are select fit- as in can all be different sizes- did you check each location? GM said no turning this crank because of needed coating and since this one was done maybe machinist sized everything the same, but always worth checking.
The main webs are known to crack and could be the loss as mentioned by WarWagon, did you know this and inspect for it? Factory Harmonic balancer has 100,000 mile life, and when done causes the failure. Never buy the cheap ones btw, 30,000 mile failures.

I suspect an upper end failure like a spinning cam bearing.

Is this a turbo engine? What year, what block, etc ?
Any pics while it was apart? We might spot something
 
we replaced brgs with same size stamped. crank smooth, looked like it had just been installed. we didnt gage each as no noticeable wear. non turbo. didnt remove crank, wasnt aware of web issue but didnt see any cracks while in there but may be in there again. odometer shows 109000. we will move mech gage to the front, see what it shows. maybe blow out oil passages. also looking down the oil pump drive hole while driving with the cordless drill noticed a pin protruding about 1/4 in from the block spewing oil until the area flooded w/ oil.
this drop in pressure occurred suddenly from 1 day to the next, not gradually over thousands of miles.
 
On the back of the block near the heads, above bell housing area, there is a number on each side of the engine. These are the julian date code and block cast number. What are those numbers?
 
I think you are to the point of inspecting cam bearings.

If you rent/buy/make a primer tool so that the oil can go through the top end, you could remove valvecovers and see about oil coming out the rocker arms. Maybe with more of the engine disassembled you could spot something else somewhere.

Im just not sure what you could find that would account for an instant change that isn’t constant besides a spinning bearing or cracked main web.

If it were part of the oil filter bypass circuit maybe?
It’s worth ripping apart, but take pics each step so we could examine it with you.

How many miles did YOU put on the engine since the rebuild- NOT what previous owner told you.
 
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