'92/'93 Elantra baffling problems

I have a '92 Elantra five speed with a '93's 1.8L engine ('92 had a 1.6L). Problems first appeared in winter, with the car running rough when it was cold. It has progressed into something that contiues to baffle mechanics.
Okay, here are the clues. It runs way too rich which fouls the plugs, but seems to miss less or even not miss at all when I'm accelerating, but I cannot accelerate even remotely fast because around 3800K the engine goes into spasms, bellows black smoke, and backfires. The idle oscillates occasionally, but I have checked the throttle position sensor and it is fine, and have even replaced it for good measure, with no result. It's not the air flow sensor (tried 3 good ones) or coolant temperature sensor (tried two good ones). I've bought new spark plugs, fuel filter, and air filter, and the resistance on the plug wires is normal. The car dies now every time I clutch it, and I am getting almost exactly HALF the gas milagel I was getting (175 miles to a tank vs. 350). The check engine light is not coming on, but the light works for sure (unplugging cirtain things proves this). The dealer looked at it and said the culprits were a faulty EGR valve, EGR thermal switch, and AIC motor, all of which I have replaced to no avail. Also, the O2 sensor does not seem to be working, but if it isn't working shouldn't the check engine light come on? Also, unplugging the air flow sensor lets me accellerate past 3800K without spasms, but I've check the harness (along with the harnesses to all the other stuff) and voltage and resistance reads normal, and replacing the sensor doesn't work.
Where should I look next? Vaccuum lines? ECU? Has this happened to anybody else? Is it because I put a California 1.8L in a federal '92 Elantra? I can't afford to fuel up every other day. Please help me!

I have a '92 Elantra five speed with a '93's 1.8L engine ('92 had a 1.6L). Problems first appeared in winter, with the car running rough when it was cold. It has progressed into something that contiues to baffle mechanics.
Okay, here are the clues. It runs way too rich which fouls the plugs, but seems to miss less or even not miss at all when I'm accelerating, but I cannot accelerate even remotely fast because around 3800K the engine goes into spasms, bellows black smoke, and backfires. The idle oscillates occasionally, but I have checked the throttle position sensor and it is fine, and have even replaced it for good measure, with no result. It's not the air flow sensor (tried 3 good ones) or coolant temperature sensor (tried two good ones). I've bought new spark plugs, fuel filter, and air filter, and the resistance on the plug wires is normal. The car dies now every time I clutch it, and I am getting almost exactly HALF the gas milagel I was getting (175 miles to a tank vs. 350). The check engine light is not coming on, but the light works for sure (unplugging cirtain things proves this). The dealer looked at it and said the culprits were a faulty EGR valve, EGR thermal switch, and AIC motor, all of which I have replaced to no avail. Also, the O2 sensor does not seem to be working, but if it isn't working shouldn't the check engine light come on? Also, unplugging the air flow sensor lets me accellerate past 3800K without spasms, but I've check the harness (along with the harnesses to all the other stuff) and voltage and resistance reads normal, and replacing the sensor doesn't work.
Where should I look next? Vaccuum lines? ECU? Has this happened to anybody else? Is it because I put a California 1.8L in a federal '92 Elantra? I can't afford to fuel up every other day. Please help me!

This is a very interesting situation...unplugging the MAF the ECU should fault out. You say the O2 sensors appear to not be working? Have you tried replacing them? I know there's more to this than I am focusing on right now but this almost sounds like your ECU's firmware is screwed royally. You talk about a 1.8? did you perform this transplant? Did the harness get swapped out with the motor and ECU...? Has the timing been checked? Has the timing belt been checked? a timing belt that is stretched out or has worn teeth could really mess up your motor.

Jason

i've had a similar problem in my 94 elantra 1.8L CA. And have not been able to figure out what is wrong with it. i've replaced the fuel filter (not sure if i put it in right, does the thicker end go on top or bottom?) plug wires are fine. my car doesnt backfire but is slow to shift (automatic) and revs up to 4000 rpm before it'll shift gears. If anyone out there has heard of this before, let me know. thanks for the help.

big bad joe

After you installed the mass air flow sensor did you discconnect the battery for 15 sec .

if not you may of fixed the problem and not reset the adaptive learn ,

as a car get older it learns to run with old sensors when the sensor finally goes bad and you change it the ecu tries to run the engine thinking the sensor is old .