KA24E. White smoke, car idles like crap. HELP PLEASE

[steezy]

please someone help, i got a 90' 240sx w/ a SOHC ( ka24e ). I replaced my fuel filter and now the car runs like crap. i checked it, no leaks. I dont think the fuel filter has anything to do with it, it might have just been a coincidence.

What happens is the car starts up and then idles real low, almost dying. After about a minute or so, the rpms start to jump all the way to like 2k and then back then, from then on the car just jumps all the way to 2k+ and all the way down to almost dying.

i thought it was a injector leak, so i replaced the o-rings, still runs like crap. Also there is white smoke coming from the back of the motor near the firewall. I have no idea any more, if anyone has any idea what it might be, or any input please let me know asap. i need this car running.

- also the coolant level has gone down, but i dont know if it has anything to do with me cracking the overflow res. recently. ( dont know if that has anything to do at all, just putting that in there. )

[schema]

Sounds like a blown head gasket, coolant getting in to a cylinder, causing all kinds of rukus. White smoke from burning coolant (and can I take a guess and say the car is a little loud, perhaps from an exhaust leak, hence seeing from behind the firewall?)

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