odd 4age oil pressure issues. up and down in pressure.

[wiso]

I am not going to argue with anyone about what I have posted. This is what happened to my engine. If you don't believe me you are more than welcome to try it your self. The problem didn't start to show until the engine was running for about 20-30 mins so make sure you run it for that long. Even the engine builder was gob smacked when I showed him the issue and the simple fix.

Anyone is welcome to come and try on my motor and buy me a new one when they blow mine up. But every single other suggestion had not worked in my case.

I will no longer be posting in this thread. As my solution was found

[jondee86]

... massive massive air pressure is buidling up (it might have been vaccum I can't remember now) inside the oil system.

This it might have been vaccum is the interesting part. Basic physics will tell you that in a closed system (such as the engines lubrication system) raising the crankcase pressure by any amount will raise the gauge pressure by the same amount. BUT, lowering the crankcase pressure will result in a lowering of the gauge pressure.

Now, some people think that the cam cover breather pipe on a 4AGE empties directly into the intake manifold... not so. It connects with a channel cast into the side of the manifold and finds its way to an exit point in front of the throttle butterfly. In front of the butterfly the pressure is essentially atmospheric.

If, by some mistake, the breather was connected directly into the intake manifold, then the manifold vacuum (less than atmospheric) would be applied to the crankcase. Manifold pressure being at its lowest value at idle, and the volume of blowby gases being low, you could expect the crankcase pressure to be as much as 10 psi lower than atmospheric. So theoretically, you could expect to see the gauge reading rise by 10 psi when you took the oil filler cap off (or opened the breather to atmosphere).

Not saying this is what happened, but it is the only explanation I can think of that fits the observed result.

Cheers... jondee86

[trdee]

derp, i made a doodoo

[jondee86]

Ahhh.... I think you missed the part where I said "closed system". In this case the closed system is everything that is within the outer skin of the engine and touched by oil... sump, oil galleries, oil pump, cam covers. All of these share the same environment and are connected in such a way that the pressure before the oil enters the pump, and after it exits all restrictions, is the same. There is of course a pressure differential across the pump, and this differential will stay the same if the engine speed stays the same, or if the pressure within the closed system changes.

The one thing that is not within the closed system is the pressure sensor. It uses atmospheric as its reference, and measures the pressure within the closed system. In this case, it measures oil pressure on the discharge side of the pump. And as outlined above, if the vapour pressure within the closed system changes, the pressure in every part of the system changes by the same amount. The gauge reads the pressure relative to atmospheric, and will reflect the change in pressure within the closed system. That is, it will show the pressure generated by the pump plus or minus any superimposed system pressure.

In this case, I am suggesting that if someone had connected the cam cover breather to (say) a brake booster tapping by mistake, the closed system could have been pulled down to lower than atmospheric pressure. Then, when the breather tube was unplugged, and the system allowed to return to atmospheric, this was reported as an increase in pressure on the gauge.

The pump is just along for the ride

Cheers... jondee86

[knightrous]

So, in short it could be a pressure differential problem?
Like when you run a boosted motor and your fuel pressure regulator is references to atmosphere instead of manifold pressure, then the fuel injectors flow less fuel and your motor goes boomski.

[trdee]

Ahhh.... I think you missed the part where I said "closed system". In this case the closed system is everything that is within the outer skin of the engine and touched by oil... sump, oil galleries, oil pump, cam covers. All of these share the same environment and are connected in such a way that the pressure before the oil enters the pump, and after it exits all restrictions, is the same. There is of course a pressure differential across the pump, and this differential will stay the same if the engine speed stays the same, or if the pressure within the closed system changes.

The one thing that is not within the closed system is the pressure sensor. It uses atmospheric as its reference, and measures the pressure within the closed system. In this case, it measures oil pressure on the discharge side of the pump. And as outlined above, if the vapour pressure within the closed system changes, the pressure in every part of the system changes by the same amount. The gauge reads the pressure relative to atmospheric, and will reflect the change in pressure within the closed system. That is, it will show the pressure generated by the pump plus or minus any superimposed system pressure.

In this case, I am suggesting that if someone had connected the cam cover breather to (say) a brake booster tapping by mistake, the closed system could have been pulled down to lower than atmospheric pressure. Then, when the breather tube was unplugged, and the system allowed to return to atmospheric, this was reported as an increase in pressure on the gauge.

The pump is just along for the ride

Cheers... jondee86

DERP! Of course! Egg on my face lol. The pressure increase when the breather is blocked comes from blowby gases building up in the crankcase, not the oil pump (this is what causes dipsticks to pop out and main seals to blow their load etc). As you say, the pump is running in a closed system though, so it gives no fucks for this effect.

[jondee86]

@knightrous
Not really the same thing. You are talking about two separate systems (fuel and air) with a signal from one being used to control the other. But you are right in as much as the feedback signal from the manifold is used to regulate the FPR to maintain a constant pressure differential across the injectors.

A better example of what I was talking about above would be this. Build a small oil circulating system comprising an open top tank, an electric pump drawing oil from the tank, a discharge pipe with a gauge and a needle valve, and a return back into the tank. The gauge is a simple diaphragm type with line pressure on one side and atmospheric on the other.

Set it running on the bench and adjust the needle valve until the gauge is showing 15 psi. Take the whole thing into a pressure chamber and leave the door open... gauge still reads 15 psi. Close the door and pump the chamber up to 15 psi... gauge still reads 15 psi.... WTF ???

Now open the door and using a length of tube that passes through the wall of the chamber, mount the gauge outside the chamber. Set the pump running and the gauge reads 15 psi. Close the door and pump the chamber up to 15 psi, and what does the gauge read ? Yes, 30 psi, the 15 psi generated by the pump plus the 15 psi chamber pressure.

Cheers... jondee86

[trdee]

My boost/vac gauge generally shows -19"HG at idle. That is -9.3PSI

[wiso]

I pulled my throttle body off and now know exactly what happened here.

As stated in the original post the car ran fine for about 400km, after that the oil pressure started dropping. BUT I had the on an old engine running for a few thousand km's as I never separated the manifold and throttle when I rebuilt the engine. so really it did a fair amount.

Turns out my throttle body gasket did not have a hole in it for the breather lines to go from the manifold back into the throttle body. so technically it was blocked but the gasket had blown out there and was vacumn leaking into the breather line. So as Jondee said the issue was vac being applied to the breather line. Hence why is suddenly started happening.



It ran so long with a blocked breather but then suddenly got vacumn applied then all went pear shaped

[dnegative]

Not having a go, but this is why you buy genuine gaskets