I have a 1993 Grand Cherokee that give me this headache, hope someone can point me to the right direction. When you start the truck up the oil presure gauge goes up to about 40 then when you start driving it the pressure drops to zero and you start hearing this tapping noise on the motor. I have replace the computer on the motor and still it continues.
Thanks in advance for your help
Sounds like it is running out of oil at the oil pan pickup. The clicking
noise may be the lifters operating with low oil pressure. ( I'm sure that
you have checked the oil level). Sometimes if the engine is really sludged
up inside, the oil can be pumped up into the valve covers and can't drain
back to the pan to the oil pump pickup. The oil pump pickup can also be
really sludged up and not letting enough oil into the pump. During idle
less oil is being pumped so the pressure can stay up.
Remove the oil filler cap and stick your finger around the inside of the hole. If it has a large thick greasy build up, sludge may be the problem. Being what sounds like the lifters clattering when you drive would be an indication the low oil pressure is real. Driving the truck this way could do bad damage.