Your main crank bearings and crank cap bearings are probablly worn well above spec which means that the oil flow is going throught them to quick and your not getting resistance to build pressure which means you need a motor rebuild
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Dear Bill, You will need to find a mechanic who can remove the oil pressure sending unit from the engine block and insert a "real" oil pressure guage to tell if you really have a problem or not. Lots of factory dashboard guages are not very accurate over time. If you really do have that low of oil pressure, you need to address that quickly.
It depends on your mileage. It's normal for the pressure to read high on start up and then drop some but your numbers are a little low on start up, usually on the high side of 40-45.You could check your pressure with another guage and see if it reads any different. If it does then you should change yours. But if your mileage is running on the high side of 200,000 then it's probably wear and tear. I would be inclined to try a heavier oil, 15-40 or 20-50. This should bring the pressure up and help with lubrication. You could also add something like Lucas to your oil to help with lubrication and pressure. It's also possible the oil pump is wearing out but unless the mileage is below 150,000 it likely won't do much good and a new pump won't like the heavier oil.
If you are unsure of the mileage you could do a compression check. If it's 100 or under the engine is worn.There is still life in it as long as the numbers are close together but it is time to start thinking about the next step.
I would stop driving it right away and test the oil pressure with a gauge. If at an idle when its cool you don't hit 30-40 psi....change the oil pump. The knocking is probably your main bearings. While your installing a new oil pump, put new mains in.
I have a 99 blazer and ever since I have owned it, it reads lower in pressure when idling, but will go up too 40 while driving. This is normal for the vehicle. If the pressure gets real low, and stays low, it means its time for an oil change.
As long as the pressure goes up when you give it gas, it is fine.
Lifers will rattle if oil pressure is low. Have you screwed in an oil pressure guage to check. Should read about 20 psi at idle and 40-60psi above idle.
Causes are nemerous from low oil level to worn out engine. You dont mention mileage. Normally very good engines.
No problem with the oil pressure, the norm is between 40 to 60. Anything lower then 40 you have low oil pressure and anything above 60 then you have high oil pressure which is caused by a blockage in the system.
What is common is the oil pressure gage fluctuating for the first 200 miles if the oil pressure sending unit is getting worn. Good maintenance will keep the engine clean and running good but the oil pressure sending unit is inexpensive and easy to replace. Next time you do an oil change you can have them change the sending unit.
Good luck and hope this helps. You can pick up the sending unit at Auto Zone and most auto parts store and will have the step by step to replacing it. If you do attempt to replace it. make sure you DO NOT USE any Teflon tape or silicon on the threads, they are tapered and need a good ground to the engine for proper readings.
If this is a high mileage vehicle, say 180k or higher, treatments are not going to help. As mileage increases, the tolerances and clearances in the mating parts increases. This will lower oil pressure. I have a 90 Safari with 192K. Oil pressure was identicle to yours. I put new rod and main bearings in it and now it never goes below 40 psi, even when hot and idling. I tried additives and all I got was more trouble, It started smoking and using a qt. of oil every 300 MILES. I put valve stems seals in and it helped alittle but not near enough. It is now parked and driven only when have to. This is a result of the previous owner of my vehicle not changing oil at regular intervals and overall poor upkeep of it.