1999 Dodge Truck

Engine Performance problem
1999 Dodge Truck V8 Four Wheel Drive Automatic 118000 miles

Ok I have a 99 dodge ram 1500 4wd. My problem is As I am driving it my oil pressure drops to zero. If I tap the gas it comes back up to normal. Then drops off again. When I saw this I drove it about 6 miles home and haven't drove it since. Is this the oil pump or sending unit. Both are fairly cheap parts but where is the sending unit and do I have to lift the motor to get to the oil pump. Also the truck will not heat up the temp gauge doesnt even register and never blows out heat. Are these problems related? The engine sounds great no banging or tapping at all.
December 21, 2008.

Check the oil pressure with a mechanical gauge if normal replace the oil pressure switch/sensor

See below

Dec 22, 2008.