rough idle, bad mpg, replaced a lot still nothing

[xKnownShadowX]

Car:
89 240sx M/T
Engine:
KA24E no emissions

Mods:
3inch test pipe
3in exhuast
4-2-1 header
injen intake

Replaced:
o2 sensor
spark plugs
cap
rotor
wires

I replaced all that and my car still seems to be either running too lean or rich.Pulled the new plugs out after one drive and they were coated white on the tip.Not half of it like it should be the hole pin thingy.My gas mileage sucks completely and my car backfires a lot.Timing is done and set to correct position too.

My idle sucks to I tried adjusting the TPS but if I move it the car rises to 2000RPM and doesn't go down until I got back to where it was.The idle screw on the TB doesn't do anything, neither does the two screws on the IAC.MAF seems to be working fine but I still need to adjust TPS I suppose.I looked up how to do it but there are two plus the one with 3 pins and the one with 2 pins.What one do I adjust back probe?I tried them all and none read .45v or did I look up wrong voltage?Is it suppose to be done in mili volts or volts?

I read up somewhere that the intake was giving the OP's car too much air but I dunno if that's possible.Any ideas?

I have no leaks that I know of since I removed all vacuum lines.The only vacuum ones I got is the Brake booster, from intake to FPS to SCV.Thanks I'll update later or tomorrow just beginning to get frustrated.

[jeu510]

do a cylinder leak down test.....sounds like a bad head gasket burning coolant ....

[s13loverdrift1]

Did you check if one or a couple of your injectors are leaking? That could be it just pull the rail add pressure and see if they leak I think there is a write up somewhere. The backfire, MPG, idle, and if it is running rich (you should smell gas) could be explained by a fuel injector leak do you have a CEL?

[xKnownShadowX]

Okay sorry for late update.After putting new o2 sensor I been watching my MPG and it got better within 2days or so.IDK how that worked but it did.The backfire issue I was told could be from exhaust leak which I got 2 since I didn't put gaskets on due to being in a hurry.

I got no CEL light on but the IAC still won't adjust anything.

The headgasket is new but maybe it did go out at some part perhaps?

I also noticed my valve cover gasket leaking so I gotta replace that since it makes it hard to see where the leak is from after it drips down the block.Thanks will update once I seal up my exhaust system.