1999 GMC Sierra 4+4 stuck in low gear and drive gear in park

SOURCE: low oil when engine warms up gmc sierra 1995

30-40 psi of oil pressure is fine for many vehicles including your GMC. I am assuming that the truck has more than 100,000 miles on it and still has the original engine in it. By rule of thumb you only need 10 psi per 1000 rpms in a typical street driven engine to give adequate oil pressure to keep the bearings from oil starvation. The oil pressure is typically higher when initial start up occurs because it is more viscous due to the temperature of the oil as the engines oil warms up the visconsity thins out and the oil flows more smoothly. The drop in oil pressure is a normal occurance. Above 45 is more than required for a daily driven street engine. I believe that the specs on that engine only list 30- 45 psi.

SOURCE: No power at fuel pump.

have you checked the fuses??

SOURCE: 2003 gmc sierra duramax 2500

Problem with Diesel Fuel System. Gmc 2002 duramax 2500 HD 4x4 will start 

SOURCE: 2004 gmc 1500 sierra heater control stuck on

Defrost is the default position. Is the control a autyomatic temperture control unit? ( you set the temp and select automatic) if it is the system is completely electronic. You push the buttons and the control still says defrost or can you select something else and the control shows it but it just keeps coming out the defrost? If the control changes and the air comes out the defrost anyway and the cheack engine light isn't on, then the problem can be blown fuses or an air door actoater not working. Does the dash emit a clicking sound? if it does with the covers off the bottom of the dash reach around touching the different pieces till you touch the idem that is clicking. this is an acutor motor, or blend door motor. they have gears in them that wear out and break the motors aren't repairable and you just replace the ofending motor. the door that is moves can be stuck. and may have to be freed up with blaster, WD40, or other rust buster till it moves easy. maybe after getting the door to move smoothly you can get the old motor to move the door wiithout having to install a new one.

SOURCE: LOW OIL PRESSURE ON 2007 GMC SIERRA 1500. CHANGED

have it tested using a manual gage if still low then main baring ore most likely bad that would mean a over hall if is really low and bearings bad should be making rattle noise when started up first few seconds

Testimonial: "thanks alot.truck is not making a rattling noise.we did check it with a manual gauge.it read about 10 at idol. press. gets better as you incr speed"