The gauge pegged to the right at first. I replaced the sending unit and now the gauge works. But when running it stays left reading no psi and the light is on. When reving the motor a little the needle bounces but still shows no psi.
What could it be and how should i go about fixing it?
Did you use a thread sealer on the threads when you replaced it? You shouldn't. It can mess up how the sending unit works in relation to the gauge. It should seal fine anyway without thread sealer.
Not quite true. All NPT threads need some kind of thread sealant to avoid leakage. The crest of the threads will not mate perfectly with the valleys, and the resulting gap can form a spiral leakage path.
The key is that you should not use thread sealing tape (e.g. Teflon tape) on threads that also form part of an electrical circuit; the tape may form an insulation barrier between the male and female threads. For these threads you need to use a liquid or paste thread sealant.
Is that all you did... replace the sender? Or is this result after some major engine work like a rebuild and/or oil pump replacement? If all you've done is replace the sender, then I'd suspect the sender or the wiring to be the cause. On the other hand if you're getting low oil pressure after other work it would help if you said so. For example the symptoms you're getting are also what happens when you forget to put in one of the little oil gallery plugs deep inside the engine during a rebuild.
Nothing was removed from the engine. I got it as a long block. Pulled the crank, rods, and pistons. Had it honed and washed. Put new rings and bearings in it along with an oil pump. The pump is working. If i use a high speed drill on the shaft it pumps oil. It seems to be getting oil through the motor but the gauge reads 0.
I don't mean to belabor the point, but your post is a little confusing when you say nothing was removed from the engine, yet the crank, rods, and pistons were pulled. I assume if you took out the pistons, you also had the heads removed since compressing the piston rings to re-install the pistons from the bottom isn't easy. Assuming that when you say you had it "washed", you mean it was professionally hot tanked and not just pressure washed, the machine shop would have removed any oil gallery plugs to help flush the internal oil passages. (I can't say for sure if there are any in the 3.4, but I would assume there are.) In case you're not familiar with what I'm talking about, here is a 2.8 showing where they would go... they're very easy to overlook when rebuilding. I overlooked one once... that's why I know what the symptoms are.