I'm hoping someone here can help me out. I have a 97 toyota avalon that I am trying to raise from the dead. I've done a bunch of work to the engine and now I am left with a p0300 code as well as p0301 and p0305. I have replaced all spark plugs and wires with new ngk plugs and wires and still the same codes pop up. This engine was junk when I got it because the previous owner never changed the oil regularly and when he did he used cheap oil. In any case, I had to replace the front cylinder head due to lack of oil eating up the camshaft. I've also changed the timing belt and water pump plus pullies. I have changed out the valve cover gaskets and spark plug seals as well as intake gaskets and plenum gasket. Yesterday, I noticed oil in the spark plug tubes for the front cylinders that the coil packs go in so I took the spark plug seals off the front valve cover and added some black RTV silicone which is now keeping the oil out and sealing like it should. The back cylinders do not have any oil in them at all. I figured after I got the oil out and cleaned the ignition coils the misfires would go away but they still remain. Any help appreciated!

 

Here is the list of possible problems.
Ignition system
Injector
Fuel line pressure
EGR
Compression pressure
Valve clearance not to specification
Valve timing
Mass air flow meter
Engine coolant temp. sensor
Open or short in engine wire
Connector connection
ECM

Do you have access to a scanner that can check the rear cylinder bank 02 sensor output for rich/lean mixture?