Paradoxial drop in oil pressure with throttle... ???

[ed]

As in title, but perhaps not quite so simple...

Engine is now 260km old. on second oil change still running in running a 20-50w mineral oil (nothing flash) bottom end clearances are about 3thou BE and mains, 18thou rod thrust.

initial oil pressure when fresh oil change was 25psi hot idle, then about 70psi at 6000rpm hot.

this oil change is now 260km old, and hot idle is presently 20-25psi, but now only 50psi at 6000rpm. i can only think the oil is starting to break down a little? theres no way a whole bunch of tolerance has opened up in the engine to cause such a noticeable drop? any thoughts? cheap shitty mineral oil? im changing to a 20-50 semisythetic HPR30 tomorrow night as the next stage of run in prior to full synthetic, so will be interesting to see what the oil pressure does then

more interestingly, and the point of this thread, imagine cruising at 4500rpm, minimal load, 3rd gear, oil pressure is a stable 45-50psi. now... mash the pedal into the floor WOT, long enough that the engine notices, but not long enough that the revs start to pick up or accelerate the car... WHY does my oil pressure drop??? it drops a good 10psi to 35odd psi momentarily as the engine goes into full load WOT condition, but there has been NO change in rpm etc??

why is it so?

i can understand if it were a drop in oil pressure as RPM increases on full noise as oil gets expelled out the journals with RPM and the pump struggles to keep up, but with NO rpm change? why would the oil pressure wander down?

cheers
ed

[oldcorollas]

inneresting..
when you mash pedal, all that will happen, regarding oil, is that the crank gets pushed down further...

the oil inlets to main bearings is also on top of shell , so... maybe it is the opening of the top clearance? seems rather odd tho. even with thatm it shouldn't be shedding so much out the side of the bearings that it actually causes oil pressure to drop.

if it was crank case pressure backing up, from blowby, it should increase oil pressure if anything.

do you have that exhaust sucker still on? you think it could provide 10psi suckage?
if it was crank case vacuumage, then it will suck oil out of everywhere, and increase shedding rate....

can you put a vac gauge temporarily on the crankcase or cam covers or something?

[rms]

Ed,
Electric oil pressure gauge ?
Voltage drop due to fuel pump/injector load increase ?

Have had my share of transducer problems due to voltage variation.

[c2888]

Would VVTi solenoids changing the cam phasing show any sort of drop like that? If you have it and are still running it of course.

[ed]

erol, yes, elec pressure gauge. noidea what voltage is doing though. pump is on ful time, only inj duty cycle would change. not sure if that ould be enough... manual guage correlate on the dyno would e a good cross check though, and easy to organise

stewy, exhust thingy is still running. no ideaif it could pull 10spi, i doubt it

my inital though as that it was a crankcase pressuefluctuation. but a weried one.

suppose i should do a whole bunch more double checking of the various system pressures....

[ed]

Would VVTi solenoids changing the cam phasing show any sort of drop like that? If you have it and are still running it of course.

vvti isnt functioning/activated yet.... will be as of sat PM tho!

and stewy, your sentiment re: downward thrust on the crank opening up the top of the mains journals is echoed elsewhere around the net. none better than this quote (yes i realise its the intahweb)

Hi.

If the pressure drops when the engine is under load, I would suspect excessive main bearing clearance. I always test for this by driving the vehicle at highway speed (just light throttle) just cruising. Then accelerate hard for a few seconds. For example, if the speed is about 55 at 2800 rpm with little load and the oil pressure is 40 lbs, hard load and the pressure drops significantly at the same relative rpm I would suspect main brg clearance being somewhat high.

My theory for this is that on most chevy engines the upper half of the main brg is groved, the lower half is not. When the load is applied it attemts to push the crankshaft away from the upper brg and creates a major oil pressure leak. When the load is removed the crank returns and the leak is reduced.

Just some thoughts
Jeff

i wonder if a combination of my (deliberately) larger bearing clearances, my somewhat lowish 'around the streets' rpm and shitting mineral run in oil are manifesting themselves here

[thechuckster]

you should be able to log voltage thru your ECU

[oldcorollas]

injectors should only really pull an average of 1 amp each.. so not much current.
MAX 10 amps at maxy duty cycle you would think..
compared to pump, fan, and whatever else pulls 10-15 amps..

onyl issue would be if the pressure gauge earth (block?) was getting interfered with by the injectors.. but they earth thru the ECU anyways...unlikely tho?

the shitty oil could be the problem.. lower shear or whatever you call it, even tho it has the same "trickle" thickness??

bearings? i hope not you have grooved suckers?

[allencr]

Gauge that needs a voltage stabilizer?
Flotsam & jetsam & shop swarf & smegma has gone through everything & chewed up the pump and/or the pressure relief valve bore and/or seat.
Pickup too close to the bottom?

[30psi 4agte]

My vote is gauge for the sudden drop.

As for the total drop of pressure at 6000 ........the outside temps have been pretty hot lately this would cause the oil to thin out further possibly causing a little less pressure vs when the days are cooler...... but 20psi???
What were the outside temps like when you first had the car running vs the current days we have been having ed?

[LeeRoy]

Get your hands on a mechanical oil pres gauge.

[jeffro ra28]

I doubt the pressure drop is due to the crank floating downwards. Bearings are made with a "pinch" at direction of load. In other words, the bearings are at their tightest dimension (usually where clearance is measured) when straight up and down and the dimension at its largest when approaching the bearing cap and block join/faces. I think this is what creates what engineers call "dynamic wedge".? Which should eliminate te crank floating downwards.

Besides, it can only move .0015" downwards before the crankshaft meets the bearing. So that leaves an approximate margin for the crank to float downwards of .001"which leaves .0005" for oil clearance.

Remember, forces are pulling crank upwards also, you have a cam timing belt yanking of the front of the crank, and the weight of the pistons and rods jerking the crank upwards on exhaust stroke. So it all sort of evens itself out..... sortoff

Any engine i had anything to do with that had .003" clearances had a big fuckoff 5 stage dry sump pump. I cant explain why its happening, but perhaps your oil pump is playing catchup permanantly?

Generally, increasing engine load will increase oil pressure, not decrease it.

Perhaps a long shot, but by mashing the pedal you are instantly reducing vacum within the inlet and chamber before the engine accelarates and catches up. Could this have a direct affect to bearing loads or crankcase pressure?

[ed]

well see how the oil change goes and also see what happens when i finallyget up to the 60w or 70w synthetics i plan to run

all else fails, a dry sump is a kinda expected endpoint, but not on this engine

[Draven]

randomly off-topic... has yavuz worked with your vvti at all yet? Mine needs a tweak in the near future,and I'm looking at you as my guinea pig

[auto351]

Have you checked the plug which connects to the oil pressure switch?
I go with an electrical problem cause by block movement