1990 yj 4.2 inline 6, carbureated. fouled a plug a few weeks ago, was running rough so I changed the plugs and wires, it was the 2nd plug, all black and nasty.. ran great after that., now...its not running rough like it was then, but it doesnt want to idel at lower rpm's. and occasionaly the idle will kicitself up now and then. I'm gonna check the plugs again, but i really dopn't think its the same problem. doesn't feel the same. any ideas? could it be a carb problem? one guy i talked to mentioned a throttle positioner?
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To find out if it is the coil pack, swap #3 pack with another cylinder and see if the fault changes. If not, you should check the compression on cylinder #3. If that is OK, I would suspect a bad injector.
First try shooting some starting fluid in intake manifold, and if fires up but still won't keep running, my guess next would be your looking and checking out your fuel pump. You can check that out my removing a line and see if fuel is pumping out of the out put of fuel pump. Because it it carburated its pretty easy to trouble shoot. It's going to be in most cases spark plugs , fuel pump.
Did you replace the plugs with the same plugs (Manufacturer) as the plugs you had in there. Some cars are very picky with plugs. ie: some Japanese cars will only run well with NGK Spark Plugs. Try to use only what Subaru recommends in your engine for the best performance.
P0300 is multiple misfires.
P0171 is lean burn on left bank 1 means drivers side
02 sensor is faulty. Your vehicle is over fuelling that is why your are getting P0300 code and the rotten egg smell. Good idea is to replace both 02 sensors.