I just purchased a 2000 GMC Savana 3500 with the 5.7 litter engine with 92000 miles on it. The temp gauge shows that the engine is running between 100 - 160 degrees, seems too cold. The oil pressure gauge is showing that at idle is around 15 psi, when driving is at 30 - 40 psi. However, after a long road trip and sitting at idle for about 20 minutes, the temp went up to about 190 degrees. I drove it down the road and as soon as I came to a stop light, the oil pressure dropped to 0 and the "check gauges" light came on. I pulled over and popped the hood. The engine seemed fine, no knocking and was idling smooth. I then turned the engine off, waited 10 minutes and check the oil. The oil level was good and it was clean. Need help!! Is this a thermostat issue, an oil sending unit issue, or is there a bigger problem? I am getting ready to install about $15k in equipment in this van tomorrow. What should I do!!??

SOURCE: low oil presure on gmc 5.3

Oh my!! I'm guessing the "oil pressure relief valve" is stuck open. Does the motor sound OK no knocking or ticking? The "oil pressure relief valve" is built into the oil pump.

SOURCE: low oil pressure 93 GMC P/U w/350 motor w/205,000 miles gauge

While engine is running, open the cover where you add oil to engine and see if you see a good flow of oil, if good flow, could be as easy as replacing oil sending sensor. got to realize that if poor maintenance history your engine may not be building enough pressure at iddle to move your hydraulic operated valves, only when you rev it up it will pump enough to run smooth. if running engine too long, you may cause more damage to other engine components such as crankshaft and cam shaft bearings. if you can spare a few bucks, have a good mechanic check your oil pressure with a mechanical gauge. another cheap option is to replace oil pump. last resort if you love your truck as much as we do,replace engine or rebuild it. At 205,000 this is a normal symptom, other than timing chain problems, most engines don't make it that far. hope this helps!