Nissan Pathfinder with misfire issues.

[HAROLD3533]

Good morning. Thank you all who may take the time to read this. I have a 1998 Nissan Pathfinder XE with 165K miles. It is my nephew's truck, he purchased it from a friend of mine several years ago. It had been maintained by my friend very well but since my nephew has owned it not as well.
He has had some issues with misfire problems and after the shop where he is located couldn't figure it out I brought the truck to my home to work on it. While I'm not a mechanic by trade I do alot with jeeps, hemi swaps, etc. But obviously this 3.3L is a completely different animal.
Brough the truck to my home. It had an intermittent misfire but ran well and was drivable. Did some work on it. Pulled the upper plenum and replaced the plugs, wires, cap, rotor, and installed new injectors. I also had to replace the EGR solenoid under the throttle body since one of the nipples for the vacuum lines broke during disassembly. Got it back together and now it has a steady misfire with white smoke. Had several codes at first including an EGR malfunction which turned out to be the solenoid I replaced wasn't plugged in completely....my fault. Now just has a P0300 code and white smoke. I did swap in a new distributor with no change and I'm leaning toward a faulty injector. They were Nissan OEM injectors,but got them online so I'm wondering if just bad luck...or possibly junk parts. I have not pulled any plugs to see if any are wet with fuel to narrow it down to which cylinder.
I did a radiator pressure test and the truck did fail it but I don't think it is a head gasket as the failure was very slow over several minutes with the cooling system leaking at the water pump. That being said...a timing belt and water is now added to the list to do.

Thanks, any suggestions would be great.

[VStar650CL]

If the smoke is constant then it could be an injector leaking or stuck open, but it could also just be a bad o-ring. Did you lube the o-rings when you installed the new injectors? If not, there's a good chance one or more of the rings is deformed and causing a constant drip.

[HAROLD3533]

Yes, lubed all the orings. As I seated them in the rail and replaced the cap, I rotated them slightly to make sure they seated well.

[HAROLD3533]

I have read people talking about seeing issues with coolant and oil levels and those being indicators of issues.
In this application when I looked at this truck before it was brought to my house the oil was low but it had massive valve cover leaks which I fixed once I had it. I told him to add oil before driving it to me and he literally, at the suggestion of his stepfather, dumped an entire gallon jug of oil in it. So when I got it, it was atleast 2 qts over. It has been since drained and refilled to proper level with a new filter.
The coolant was low because his expansion bottle was broken. Have since replaced that. Coolant was at correct level, no evidence of oil in it or bubbling seen when running. Gonna have to drain it to replace water pump and timing belt.