i have a problem with the oil pressure in my '96 a4 (low oil pressure when engine gets hot), and it seems a tough one. i've posted it in the a4 forum, and the only advice i got was to post the problem here, since i might find some guys with more experience around here.

so if you think you can help me, please read my post on the a4 forum (link below)

thanks a lot, Stil<ul><li><a href="https://forums.audiworld.com/a4/msgs/1636212.phtml">https://forums.audiworld.com/a4/msgs/1636212.phtml</a></li></ul>

 

I'm thinking you may have some sort of bypassing that is leaking the high pressure side back to the low pressure side or maybe a restrictor somewhere in the line that came loose or leaking. Two things that I can think of that can bleed the oil back into the pan are the pressure relief valve and the oil filter - I hope you replaced that one too as I didn't see you mention it
The pressure relief valve is on the cover of the oil pump (inside the oil pan) On my 12V, it is part of the cover that houses the oil screen - oh btw, make sure that the screen is clean.

When you replaced the fuel pump, I am assuming it is the pump body itself only - which has those 2 spider gears, and maybe not the lower cover which has the pressure relief valve? But if you replaced all that, we need to look elsewhere. Speaking of which, that lower pump cover needs to be sealed well. Any leak in that gasket means the oil flow back into the pan.
And as far as the oil pumps go, if the spider gears are tight and they turn, they will generate flow and they are just positive displacement pumps. Low pressure just means they face no restriction.

What is the pressure when it is cold? It should be close to what you see when it is hot, but not by a large margin. Typically cars should see from 20-40 psi at idle (about 1.3 to 2.8 bar). On my 12V, I saw as high as 90-100 psi at mid-high RPM (~6.9 bar). If it's a mechanical restriction problem (or lack thereoff), at cold you should see slightly higher pressure than what you said (0.7) and goes down with temperature as the oil gets warmer is is becoming more fluid (less viscous).

If it's a big jump, then there is something opening electrically, or something that leaks pressure when it expands due to heat - like a gasket, seal or valve.

What oil are you using? what is the temperature in your area?

At cold and idle, where is the oil level on the dipstick?

At hot when pressure is low, where is it?

I doubt you have a bad engine. Just something inside it that's faulty.

However, it may be getting worn fast when the oil pressure drops below critical levels.

 

thanks for your reply

about the oil pump - i did replace it as a whole, including the pressure relief valve, so the problem must be somewhere else. (and, as a parenthesis, there's no gasket between the pump covers - it's just aluminium-on-aluminium sealing)

now, the oil pressure when cold goes as high as 5 or 6 bar, when the rpms go up. at idle, is about 2.5, if i remember correctly. i had it checked at the local dealer too, and they said it's ok (they had the service manual at hand, so i assume they knew what they were saying...).

the oil level - at idle - is at the upper mark on the dipstick. when the oil problem occurs, the oil level is below the lower mark - but that's only if i take a look right after switching the engine off. if i leave it for a few minutes (as recommended) it goes back up. on the other hand, if i leave it for a few minutes, the warning goes away, and (for a while) the engine seems to work ok.

now, about the oil. i've always bought oil according to the manual, so it shouldn't be an issue... the problem first occurred during the winter, when the temperatures are in the minus 15 (at night) to plus 5C (about 5-40F). i don't recall exactly the oil i had then, but it must have been something like a 5W40. now, the temps are in the 15-30C range (60-90F), and i have a 20W50 mineral oil.

you said "i doubt you have a bad engine". what would that mean? worn out? or with a faulty part that's beyond repairing?

thanks again, Stil

 

i forgot to answer the question you asked in the subject

yes, he meassured the oil pressure by replacing the sensor with a manometer. as the dealer guys do it the same way, i assume it's the right way to do it.

 

replacement when maybe there is just a little thing broken, loose or out of alignment or spec in it.

I still don't know how the pressure can drop from 2.5 down to 0.7 bar from cold to hot. Is there anyway you can borrow the gauge and keep it hooked up in there to observe how the pressure behaves when the engine is getting warmer?

If it slowly drops with temperature, definetely something is expanding with heat causing the drop on pressure. If it changes all of a sudden, hmmm, I can't of why it would do that.

I'm thinking of where your oil pressure goes as yours doesn't have a turbo right? So, it should just flow into the block, then into the crank shaft bearing caps then out to the crank bearings via small holes. If you had a turbo, it would flow out to tiny piston squirters via a bolt on the crank bearing cap. Each bolt has a pressure relief valve on there.

Then there should be another port going up to the head for the CAM bearings. I can't remember where the oil flows to fill the lifters, I think on the side of their walls. Other than than, it should just flow back into the pan.

I'm curious if you are seeing oil starvation when the oil pressure is low. Adding some oil when you see the oil below the dipstick level (or just jacking up the rear to put all the oil at the oil pump) should be a quick check to see if the pressure goes back up. If it's oil starvation, then we'll have to figure out why it doesn't flow back into the pan as freely. There could be a clog in the system as opposed to an unrestricted path. Weird problem. Hope you get it fixed without too much money and time

 

i don't know the engine inside-out, i only have an idea of it's schematics. so i'm wondering what are the parts that can be worn out and ruin the oil pressure. here's what i can think of: oil pump, crankshaft bearings, camshaft bearings, the camshaft chain tensioner (since it's hydraulic, just like the valve lifters) and the lifters (would they compromise the pressure?). anything else? i've had all these checked and i replaced the ones seeming defective (oil pump, tensioner, crankshaft bearings).

now, the only other hypothesis i have involves a crack somewhere in the block or in the head. however, since i don't know the oil circuit, i don't know where to look for such a thing. do you have any idea where such a crack could develop, and if it's possible to look for it?

to me, this second hypothesis makes more sense. that's because the engine failed all of a sudden, it didn't die out gradually. also, the whole issue started last winter, during a period when temperatures were at about -20C at night. of course i had anti-freezing liquid in it, but who knows?

now, about your idea with the oil being unable to flow back in the pan. i thought about this too (especially because the engine was quite dirty when we opened it), but because the problem only occurs when the engine is hot, i thought this wasn't the case. i mean, i can drive for miles and miles (such as up to 100km or even more) if i keep the engine cool (slow on the gas, on not very hot days). now, if some oil channels were closed, shouldn't this happen all the time? and i can't think of any situation when the oil pipes would become clogged only when the engine warms up.

as i've said, event if i'm right (but i might be wrong) and there's a crack or some loose bearings somewhere, i still don't know where to look for. so, if you have any idea: where could a crack develop that would short-circuit the oil system? or what bearings can become loose? or what mechanism (such as the oil pump pressure relieve valve) can fail and ruin the oil pressure?

and, btw, thanks a lot. and i really mean it, it's a very nice feeling somebody is actually trying to help you as opposed to the guys at the dealer shop - their reaction was simple ("change the engine") without even trying to think what could be wrong... now, i'm an engineer myself (even if my area is computers), and i just can't resign that easily in front of a mechanical problem... if i change the engine without even knowing what's wrong, well, i'm sure i won't sleep many nights...

thanks again, Stil

 

I haven't mentioned a head gasket because it doesn't seem to be blown from how you describe the symptoms.

Here's the thing that's weird. Even if one of the cam bearings are loose for example, that gap it created will not even be big enough for the pressure to drop drastically. The oil pump flows a large volume - It's meant to feed a turbo also, which yours doesn't even have, so there is extra capacity left for it. It probably doesn't have oil squirters so again there is extra capacity for it.

On my 12V, I tapped next to the oil pressure switch to feed a turbo, and the overall pressure didn't even budge.

So, with that, I rule out small cracks on the block or head or even the gasket. They won't be large enough to ooze out the flow and drop the pressure drastically.

Here's another thing I was thinking. If for any reason there is enough pressure buiding at the high side of the pump - either due to a head gasket leak, maybe the sum of the parts leaves low pressure overall from the pump. I'm not sure.

Also, just in case, maybe open the oil cap when the oil light is on and see if it hisses pressure out. That could be an indication of excessive pressure build by either the gasket or excessive blowby - piston rings. Other than that, it's pretty weird. Hope something comes up soon

 

So metal expands when it gets hot, and contracts when it gets cold. The only thing I can see happening is that some port/line is still clogged/congested to the point that when the motor heats up that line/port expands enough to fully close it off. I know you said you had the entire motor cleaned out, but maybe something was missed.

Now the oil pump is an electric pump correct? If so, try hooking up a voltmeter to it and run the same test as you did with the pressure sensor. You may be losing electrical power to the pump when it heats up for some reason.

Don't know how much this will help, but its an idea