2002 trailblazer oil pressure drops when stopped returns accelerating

It sounds like you have some internal engine wear. What causes the pressure to drop when you are at a stop is the engine RPM drops. When the RPM drops the oil pump does not turn as fast, when the oil pump doesn't turn as fast there is less oil volume, when there is wear in the bearings and even in the pump itself there is more volume of area that needs oil than there is available causing there to be pressure loss. The only fix for this is to replace the worn parts either by rebuilding your current engine or replacing it with a good salvage or new engine.

Sounds more like an engine idle issue than a oil pressure issue. If the engine idle drops too low, the oil pump cannot provide sufficient oil pressure. If there is no oil light when the engine is idling normally, then there is no problem there.

bad oil pump low oil make sure you have enough oil in your engine

It sounds like the oil pressure switch has failed. It is only an on/off switch that trips at about 15 - 17 PSI, the reading displayed on the dash is created by the computer based on the pressure switch being closed and the engine RPM.

First we would need to know when this occurs. Asuming the oil level is at the full mark!. If the pressure goes up during acceleration and down at idle when engine is fully warmed up, then this is normal. An engine requires 10 psi of oil pressure per 1,000 rpm ( ex. 20 psi at 2,000 rpm, 25 psi at 2,500 rpm, and so on ) . If the gauge is moving back and forth erratically at idle , i would say the gauge is bad. If it drops to 0 while accelerating I would say your oil pump is bad. If from time to time it seems to work and other times it seems not to work, I would say the oil pressure sending unit for the gauge is bad. If the pump failed you would start to here a Knocking noise from the engine in a short period of time. Use the above scenario's to narrow down what it may be.

Sounds like it could be low on oil PRESSURE. Or possibly bad bottom end (main bearings). A lot of times when the clearences are excessive in the main bearings, oil pressure will drop considerably once the oil is hot and has a lower viscosity. Sorry about the caps, just wanted to clarify the difference.

poor oil circulation may need either more oil or a new oil pump, any other symptoms?

It could just be the pressure sensor, as this is cheaper get it looked at first.

Hope this helps
Regards
Dan

Does the oil light come on or the knock occur during the engine warm up after a cold start?
There could be a couple of problems causing the issue. You could have an oil pressure pump or pump pressure regulator sticking. I would have a mechanical oil pressure gauge installed and verify oil pressure at idle and 2500 rpms to make sure oil pressure is in normal ranges.
You could also have excessive piston rod bearing, crankshaft bearing or cam shaft bearing wear that will cause oil pressure to drop too low to prevent the rod knock your hearing at 2500 rpm.
You said a knock and not a ticking noise,so I don't believe a lifter is the issue at 2500 rpm.
Start with the oil pressure check and go from there.
Let me know.
Regards,

How many miles on it? Could be the oil pump if the oil level is full.

When the gauge is at 0, is there any noise from the engine (tick, knock etc?) I would have someone put a mechanical gauge on there and read true pressure.Your driving pressure seems OK but if the low number is "real" something is wrong!