CleviteKid
02-01-2004, 15:14
Dr. Lee:
I have a 1984 GMC 3/4 ton with a 6.2L engine that has 90,000 miles. The truck is in excellent condition and the engine starts/runs fine (no knocks) but the oil pressure has dropped to less than 10 PSI after reaching operating
temperatures. The mechanic tells me that the block is cracked and suggested a new engine.
Is there any other possibility?
Harold M.
* * * * * * * * * * * *
Hello Harold,
Yes, there are other possibilities. One is that the sender for the electrical oil pressure gauge is not working correctly. If you have access to some medium pressure hose (100 psi rating is fine) and a mechanical oil pressure gauge, you can unscrew the electrical sender (at the back of the engine, in the "V" between the cylinder heads - take the air cleaner off for a look-see) and screw in a 1/4" pipe thread adapter to your hose, and see what the mechanical gauge says.
Another: worn bearings. If the oil clearance is too large, there is not enough resistance to oil flow and the pressure drops as the oil heats up and thins out.
Yet another: debris blocking the pressure relief valve in the oil pump open. Since you have to drop the oil pan to inspect the bearings, it is easy (for someone who knows how) to pull the oil pump off, take it apart, clean it, inspect it, and reinstall or replace it. A worn pump will also have trouble developing pressure with hot oil. A look at the gears and the housing will tell if they are new and shiny or old, worn and scratched. And while the pan is off, you can do your own inspection for block cracks. Clean off the block webs adjacent to the main bearings - wipe them down with rags wet with mineral spirits until the cast iron looks very clean, then go have a beer or two and watch an hour of TV. When you come back, if there ARE cracks, you will see black lines on the metal where oil has seeped out of the cracks and left its trace on the clean iron. No black tracks, no block cracks.
For diagnostic purposes, if it is almost time to change oil, you can put TWO bottles (14 ounces each) of STP into the crankcase to see if that builds pressure. If the pressure comes up, probably bearings or bad oil pump. If no change, bad oil pressure sender (easy to fix) or a badly cracked block (replacement engine indicated).
Look for these clues, and let us know what you find.
Dr. Lee :cool:
I have a 1984 GMC 3/4 ton with a 6.2L engine that has 90,000 miles. The truck is in excellent condition and the engine starts/runs fine (no knocks) but the oil pressure has dropped to less than 10 PSI after reaching operating
temperatures. The mechanic tells me that the block is cracked and suggested a new engine.
Is there any other possibility?
Harold M.
* * * * * * * * * * * *
Hello Harold,
Yes, there are other possibilities. One is that the sender for the electrical oil pressure gauge is not working correctly. If you have access to some medium pressure hose (100 psi rating is fine) and a mechanical oil pressure gauge, you can unscrew the electrical sender (at the back of the engine, in the "V" between the cylinder heads - take the air cleaner off for a look-see) and screw in a 1/4" pipe thread adapter to your hose, and see what the mechanical gauge says.
Another: worn bearings. If the oil clearance is too large, there is not enough resistance to oil flow and the pressure drops as the oil heats up and thins out.
Yet another: debris blocking the pressure relief valve in the oil pump open. Since you have to drop the oil pan to inspect the bearings, it is easy (for someone who knows how) to pull the oil pump off, take it apart, clean it, inspect it, and reinstall or replace it. A worn pump will also have trouble developing pressure with hot oil. A look at the gears and the housing will tell if they are new and shiny or old, worn and scratched. And while the pan is off, you can do your own inspection for block cracks. Clean off the block webs adjacent to the main bearings - wipe them down with rags wet with mineral spirits until the cast iron looks very clean, then go have a beer or two and watch an hour of TV. When you come back, if there ARE cracks, you will see black lines on the metal where oil has seeped out of the cracks and left its trace on the clean iron. No black tracks, no block cracks.
For diagnostic purposes, if it is almost time to change oil, you can put TWO bottles (14 ounces each) of STP into the crankcase to see if that builds pressure. If the pressure comes up, probably bearings or bad oil pump. If no change, bad oil pressure sender (easy to fix) or a badly cracked block (replacement engine indicated).
Look for these clues, and let us know what you find.
Dr. Lee :cool: