Oil pressure guage drops out

[tonymasone]

When driving my 89BII I notice that if I really get into it (5k+ RPM) the oil pressure gauge starts to radically drop down to low(no) pressure and when I let off it immediately comes back up to a normal level. My BII has 100k miles on it, and I thought I might be getting blow by, but I've driven this thing only about 50 miles a week for over a year on one oil change, and when I check the oil I haven't used any at all. There's no sign of white smoke in the exhaust, and the engine runs perfectly. Starts immediately every morning etc. I'm just wondering if it's a faulty reading on the gauge or if I should be concerned.

[tonymasone]

argh... bump

[drofman]

Check it with a physical gauge while driving. that will tell you. on the 5.0s i've seen cam bearings do this but not on 2.9s. did i scare you?

[tonymasone]

haha, well it is what it is I guess, in the meantime I'll just have to make sure I take it easier.

[tekatlarge]

Hey Tony I missed this post somehow.

My best guess is that you got some serious sludge and gunk partially restricting the oil pump screen.
I got some pictures around here somewhere from a motor with only 80 K on it and it was so gunked up it is a wonder it got any oil at all.

So here is the logical scenario of what is happening inside your engine. As RPMs rise the oil pump starts pushing more oil to the top end. The oil is slow returning to the sump due to sludged return holes.
With less oil returning you now get a "Low oil condition".

Because all of the crap and gunge from over the years settles in the bottom of the pan, it makes for the "Perfect storm" to partially clog the oil pump pickup screen. No you get turning high RPMs for even a fairly short time the pump is running full on at high RPM. The suction and demand of the pump exceeds the amount of oil that can flow because of a partially clogged screen. What happens when the pump sucks hard it tends to cavitate and you get extreme turbulence at the oil pickup. This effectively starves the oil pump due to aeration of the oil at the pump inlet. (Look ma no oil pressure)

This principle can best be illustrated using a milkshake and a straw. When you get close to the bottom of the cup if you suck on the straw harder you will suck air and get less and make strange gurgling sounds. If you suck on the straw with less force you will not suck air and you can get a lot more milkshake than sucking hard. When you slow down the process returns to normal and oil pressure is restored.!!

Class dismissed...

I can't make iot any clearer but I can say a motor with that many miles is just being beaten to death running it at high RPM.

[tonymasone]

Thank you for your explanation, this makes a lot of sense, I really just try not to abuse it much but I was hoping it wasn't a sign of bad rings or something like that.

On the same note, my 00' BMW M Roadster (I think most bmw's now have the same design, and more recently the newer corvette's adopted it) Has a vertically mounted oil filter/pump, and the intention of the design was to provide sort of a buffer for the oil as it travels into the engine, so during hard cornering or high rpms you don't get a lack of oil flow (At least this is what I read).

thanks again!

[tekatlarge]

You are correct about the pump mount methods. If I were building a road racer or real serious rock crawler I would go with a dry sump and oil tank. You could run upside down and not lose oil pressure.

[drofman]

I would still check it with a gauge, it may be something as easy as a sender

[drofman]

No offense, tek I think you are a very smart fellow

[tekatlarge]

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