Toyota Repair: code 25 and intermittent check engine light, air fuel mixture, maf sensor

Question
QUESTION: Ted first of all thanks for giving this a look,and hopefully i can describe this problem the best i can.
I have a 94 v6 4x4 pickup w/ 190,000miles. initially went to the dealer and got the code 25 and 21 and couldn't get an inspection sticker until they played w/ it a little. new maf sensor 700$ couldn't do it, got used. Changed the plugs and put in new o2 sensor. code 21 gone but still have 25.
So the truck really wasn't running that bad and I had the sticker so I forgot about it and lived w/ the engine light for awhile until the light started going off. This usually happens when its very damp or raining. When its off the engine runs terribly up to 3000rpms when it kicks into high gear and has all the power it needs. this only happens under load and is worse on hills.
So I talked w/ a friend at the dealership and he suggests new cap,rotor,wires and plugs. Done and while in there i check the compression. All cylinders read 170 except the middle drivers side at 140, squeezed a little oil in and boom 170. I'm getting depressed, and I'm starting to forget what my Q is. Any advise on the code or the piston rings would be awesome  

thanks

Colin

ANSWER: The code 25 indicates a lean condition of the air/fuel mixture, since the #4 cylinder shows 30psi lower than the rest that's where the problem may very well be, the valve clearance on that cylinder should be checked first, this could be due to a tight valve.

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QUESTION: so what should i specifically ask the dealerships service guys?

Answer
Have the valve clearance checked on the left bank of the engine which includes the #4 cylinder, if there is a tight valve it can be adjusted using different size adjusting shims, if the valves are not tight then I would suspect that a valve is leaking or there is a compression loss due to piston ring wear, the fact that the compression came up after injecting oil makes this a possibility.