I had a valve job done on my 3.0L V6 4Runner (167,000 miles) because the engine was running rough and getting lousy gas mileage (13mpg) the #3cylinder had low compression (50ish)(all others were 155 to 165) and when I took the head off the head gasket was fine but the exhaust valve was burnt. Someone suggested replacing all the fuel injectors (not because of any testing and they are not cheap) so instead I moved the #3 injector to the #2 position and went on my way.
30,000 miles later. Engine running lousy again. #2 and #4 cylinders low compression (50 and 70) all other cylinders good. Adjacent cylinders, low compression. Head gasket right? Nope Head gasket was perfect, but both the #2 and #4 exhaust valves were burned and all of the other valves were tighter than specs (less than.006 thats the smallest feeler guage I had) Toyota specs range from.007-.013. Everyone explained this problem occuring in such a short time as due to inaccurate machine shop work.
Had the valves, guides, etc replaced and valves adjusted by a highly recommended machine shop. They adjusted the valves in the shop to specs.
I replaced the piston rings and bearings. Slapped the thing together and am now getting a code 52 (knock sensor) which Toyota says is common when you disconnect the sensor to do the engine work. I also got codes 25 (lean condition)and 26 (rich condition)while driving around to break the engine in. I cleared the codes and drove 50 more miles and the 52 code remains but no 25 26 code. The truck seems to miss at around 2500 rpm in all gears but most notably 2nd and 5th and is getting 13 mpg again. Before all of this it got 18 mpg. I'm going back in to replace the knock sensor tomorrow but wanted to get some feedback from folks as to any other things I should be doing. Sorry this is so long but.