what cause one bank of cylinders to run lean
 

my workman has a V8 Tundra. He discovered it from a code reader. What can cause something like it? Weak fuel pump?

Bad injector would be my first thought. Or an injector wire failure. Next is an intake leak...

What is the exact code?

On a flat six 911 engine it can be cam timing. Not sure if this applies to a V8 Toyota.

Intake leak on the lean side can cause this on some engines (e.g. Intake manifold gasket leak)

Yes, listing the codes will help since the lean on one bank can be caused by one cylinder or all the cylinders on one bank. don't start throwing parts on the engine until there is more info. There must be a good forum somewhere that is similar to the Chevy HHR one I use that lists codes and what folks have done to fix the issues.

O2 sensor on that side isn't reading right?

Hummmmm, upstream left or right or downstream left or right O2 sensors? Since ********.com shows different part numbers for most of them, they might give different voltages. I would ask the mechanic to plug in his tester and see that each injector, coil pack (or what ever they have) and O2 sensor is reading. Do this while driving to get accurate real life readings, not idle. When I was putzing with a failing cat I plugged in my AutoEnginuity software and watched all those items, one injector was hanging open it seemed and caused the cat to overload from the rich exhaust.

If he is getting a P0420 code, good luck. There is a web site just for that. If you have access to a graphing OBDII reader that would help. You need to look at the pre-cat and post-cat O2 sensors and the fuel trim.

Dave

Leaning out. .....so mixture adaption on that bank is leaning out, most likely adding fuel but if a big vacuum leak it can't keep up and throw a code.
Look at the adaption numbers with the scanner, chances are that bank is adding 20%+.....if the adaption numbers are off you need to smoke test it to find the vacuum leak. .....

Blamo. We have a winner. However, I will also add this: It could also be that the threshold for triggering lean has been exceeded by use of oxygenated fuel. Cars have a pretty wide threshold for lean readings due to sensor wear, and on a V8 one side may run 1-4 points leaner than the other.

If one side was marginal (close to threshold) but still ok before oxygenated fuel, then using oxygenated will push it over the edge.

My BMW did this crap, and by putting a new MAF in it brought the correction readings down on both banks far enough - prior to the new MAF one side was borderline OK, the other was occasionally tripping the lean code- since the readings are never stable, they hover within a range. The new MAF brought the corrections way down so both sides were well within limits.

Look at the actual correction values on a scanner while running, if one side is just popping over the limit on occasion and both values are otherwise a few points within each other I'd start checking a sensor before tearing into the manifold.

rjp

Lots of valid ideas here...I'll add one more. A failed or failing cat can cause strange codes. I had failure of a cat on a Mercedes wagon that threw both rich and lean codes along with a misfire code on the left bank. I diagnosed it with an instant read laser thermometer. The rear cat on the left bank was cold due to the forward one being clogged. Replaced all the cats at once and cleared the codes and all has been great for the last 100k miles.