My 1969 gs 350 has low pressure at idle after its warmed up, cold start has around 80 psi but drops to almost 0 psi at idle when warm. Where to start?
Any help will be greatly appreciated.

 

First some info please!
Mileage :
Approx part of the country you have vehicle (temp range):
What oil are you presently using (10W30) for example :
Recent work performed on engine, if any :

 

75,000 actual, Indiana, 10w30 no major work done!

 

75,000 is not a lot of miles for bearings to be worn out.
Does the engine begin to make unusual noises after warm up?
If not, Either invest in a decent oil pressure gauge and verify that those are the actual oil pressures or have someone check the oil pressure with another gauge.
If the oil has been in it for a long time it could have become diluted with gasoline that could be caused by a defective diaphram in the fuel pump.
Let me know what you find.

 

The motor sounds wonderful no weird noises at all, I put all new gauges in it still shows low pressure when warm. It also looks like it has a new oil pump but that could be defective as well but still with the high pressure at cold start wouldn't make sense if that were the case!

 

Do any of the oil pressure gauges contain a direct line to the oil galley and not an electrical sending unit? Worn oil pumps can and do behave like this when worn because the oil is thicker when cold. Sometimes a pressure regulating valve can get stuck in the open position and give the results you are seeing. That's why I always do an oil pressure check with a known good gauge connected directly to the oil galley. If you suspect the valve is stuck open , sometimes a good tap in the right place will allow it to move out of the stuck open position.

 

I replaced the sending unit with a direct fit analog gauge. Was hoping that the sending unit was bad, no such luck!

 

What is puzzling here , if there is a really low oil pressure the valves will get noisy when the engine gets warm. That does not seem to be happening , yet the extremely low reading should produce this. I would for no other reason than to be sure, get another gauge and recheck that reading right from the oil galley.

 

have you considered an engine flush? Sometimes a pint of kerosene can clean some trash buildup at the galley the gauge reads from. If that sounds too risky, maybe switch to Rotella 15w40 for a few thousand miles. The diesel oils contain a detergent that most "gas" oils cannot courtesy of the EPA. I've seen this work in L6 engines; may work in a v8 or v6.

 

The Rotella 15W40 certainly won't do any harm , it's one of the best oils on the market.
I can't help feeling you have an oil pump problem like worn internal gears or excessive gaps internally. Unfortunately the only way I know of is to remove it and inspect it.