Toyota Repair: 94 4runner code 25, valve shims, power steering pump

Question
i just replaced my valve cover gaskets and now i have a lean code. i have checked all the vacumm lines for leaks, i have replaced the intake hose that connects to the throttle body, because the old one was dry rotted and cracked.i replaced the pcv valve and the hose connecting it to the plenim because the hose was cracked. i also found a vacumm line from a vacumm sorce to a vacumm switch{i dont know if thats the proper term} on the back of the power steering pump was cracked. ireplaced all of those items reset the ecm by pulling the efi fuse for 30 seconds.then i found that when i go up ahill or put a load on the engine and then stop at light the check engine light comes back on. so i replaced the o2 sensor since there is 164000 miles on the vehicle. but the light came back. i have gone over all the vacumm lines and the intake entirly with brack clean and carb clean and have not found any vacumm leaks. iwas told to check valve clearence. could that be the problem? im stumped.

Answer
A lean code can be triggered by an engine misfire or a combustion leak that may be caused by a tight valve or a loss of compression, I would recommend doing a compression test on all the cylinders to see if any of them are low, a reading of 160-200 psi is normal, each cylinder should be within +/- 10%, any cylinder that is low may need to be checked for a tight valve, these engines were known to have some valve problems, if there is a tight valve it can be adjusted by installing different size valve shims but this may only be a temporary fix, the best way would be to remove the heads and do a complete valve job.