95 Monte Carlo Idle Fast and Have low oil pressure

Sounds like you need to replace thermostat assy and add new antifreeze and as far as your oil gauge, replace the "oil pressure sending unit". That should solve your problem

Oil level ok? When was the last time you changed the oil filter? Try changing the oil filter

It is hard to say without being able to check the vehicle. It is possible that gasket material, silicone, etc. ended up inside the engine duing the repairs and is now stuck in your oil pump screen. It is also possible that you really do not have low oil pressure and the gauge is "lying" to you.

The way to find out is to remove the oil pressure sending unit and install a manual gauge. Take an actual pressure reading to see if the car's gauge is accurate. If it is not accurate, you most likely need a new sending unit. If you find that the oil pressure is actually low, then it is time to pull the oil pan and look inside to see what kind of damage or debris is in the oil pan.

First step is to remove the oil pressure sending unit and replace it with an oil pressure gauge to make sure you don't have an engine oiling system problem. If you have good oil pressure you probably just have a bad sending unit.

If the oil pump was bad, the noise would get worse as the engine heats up. You probably have worn out lifters.

Could be a bad sending unit (sensor) casing the problem, or maybe something else:
You say that the "gauge is gong back and forth" when is this happening? At idle, then I would suspect the sensor. But other items might be the oil pump pressure regulator or a cracked pick-up tube, not likely, but possible.
Now if the gauge pressure is fluctuating while driving, there could be a host of problems such as: oil pump or main bearings wear or some internal leak.
The best test at this stage would be to connect a manual oil pressure gauge to the engine to see what exact oil pressure is reading.
Don't drive the car until you get this fixed. You could turn a small problem into a big one if the engine truly is not getting oil. A cheap repair could go to 3-4 grand if you fry the bearings in the engine.
Comment back with your mileage and further description of the problem if you want more help.
J (Working on Small Blocks since 1967)
(2.21.2011 00:42 PST)

Hi,
you do not state the figures for the low oil pressure. There are various reasons for low oil pressure in an engine.
One fact is correct, if you continue to use the vehicle with low oil pressure being developed, you will be looking at an engine replacement or full repair in the very near future.

Good luck.

yes it is. this is typical of most cars - the oil pump is gear driven, as your RPMs go up so does the oil pressure.

i have to be honest, it sounds like a cheap oil filter.

does the oil pressure gauge show low oil pressure?is it low oil pressure light? or low oil light? 2 sensors on this vehicle.make sure you get a chevrolet oil sensor,aftermarket sensor do not do well,make sure your using 5w30 or 10w30 wieght oil,if you are putting out over 20 lbs at idle in gear,i see no problem,should go up to 40 or better on accel,if it is low oil pressure and the gauge conferms,possible engine wear