No Oil Pressure

Tiny
AMARANTH
  • MEMBER
  • 1999 CHEVROLET ASTRO
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 128,985 MILES
My 99 Astro van (V-6) has been making this awful noise from the engine like all the lifters are losse or something. As I drive the oil pressure goes way down to nothing after I have been dring for a short time. And when I come to a full stop the check gages light comes on and the van starts to loose power like its about to die. I thought it was the oil pump so I replaced it and the van is still doing it and today when I came to a full stop it completely died on me. I don't have the expirence to correctly diagnose the problem.

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Tuesday, April 5th, 2011 AT 6:56 PM

8 Replies

Tiny
CARADIODOC
  • EXPERT
Suspect loss of oil pressure from worn engine bearings. A worn oil pump could cause low pressure but the bearings are more common. Since there has already been a loss of power, there's a good chance the crank shaft has been damaged, however, only an inspection will tell for sure. The power loss could also just be due to the collapsed lifters and the valves not opening fully to let fuel and air in.

Plasti-Gauge can be used to measure the bearing clearance for the crank shaft and connecting rod bearings. If there is excessive clearance AND there is no apparent damage to the journals, new bearings can be installed. They are available in.001" and.002" undersize which will take up a little extra clearance if desired. Check the back side of the old bearings to see if they are undersize already or if they are marked "STD" for standard size bearings.
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Tuesday, April 5th, 2011 AT 7:10 PM
Tiny
AMARANTH
  • MEMBER
Would that account for the decreased oil pressure after it has been driven for a short period?
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Tuesday, April 5th, 2011 AT 7:22 PM
Tiny
CARADIODOC
  • EXPERT
Yup. The pressurized oil dribbles out from the bearings and back into the oil pan. The tight crank shaft bearing clearances makes it hard for the oil to get out and drip back down, so most of it goes up through drilled passages to the connecting rod bearings, camshaft bearings, and the oil passages that the lifters sit in. Even though the oil is oozing out, as long as it is under pressure it is isolating moving parts from each other. When there is excessive clearance, the oil can run out too easily so while there won't be much pressure, there will still be a lot of oil flow. That can save the crank shaft bearings but it will greatly reduce the amount of oil that goes to the other parts of the engine.

The first casualty is usually one of the connecting rod bearings. To get pressurized oil it depends on the crank shaft bearings to not let the oil leak out easily. The bearing clearance is typically around.003" -.005", about the thickness of two sheets of paper. It doesn't take much wear to make it easy for the oil to run out when there is nothing to hold the pressure in. At higher engine speeds the oil pump moves a larger volume of oil which overcomes that excessive leakage enough to turn off the warning light. Most sending units turn the oil light on at around 6 - 10 psi which is dangerously low already. Somewhere around 35 - 45 psi is typical.

The pressure will drop more when the engine is warmed up because the oil thins out and can run out of the bearings easier.
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Tuesday, April 5th, 2011 AT 7:44 PM
Tiny
DAVE'S BIG BACKYARD!
  • EXPERT
If you also replaced the oil pickup tube when pump was replaced and it knocks when warm then yes the engine is done.
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Tuesday, April 5th, 2011 AT 11:02 PM
Tiny
AMARANTH
  • MEMBER
Is that the tube that comes out of the pump and into the pan itself?
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Tuesday, April 5th, 2011 AT 11:08 PM
Tiny
DAVE'S BIG BACKYARD!
  • EXPERT
Yep they somtimes get pluged.
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Thursday, April 7th, 2011 AT 9:29 PM
Tiny
AMARANTH
  • MEMBER
I'm thinking my engine might be shot. I forgot to mention that when I was changing my oil pump I noticed tiny metal shavings in the oil pan.
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Thursday, April 7th, 2011 AT 10:08 PM
Tiny
DAVE'S BIG BACKYARD!
  • EXPERT
Well if the motor knocked loud then its done.
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Tuesday, April 12th, 2011 AT 11:43 PM

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