[ike84]

Blue -

Along with unmetered air and low fuel as causes for single bank lean conditions. You have to add misfires. They can cause a lean signal because the oxygen is not being used up in the combustion process. So the O2 sensor sees excess oxygen in the exhaust. which is in a misfire case, unburned fuel. But the O2 sensor only sees the oxygen. So the ECU sees a lean condition. The other thing is valve train issues. anything from valve timing to valves not sealing at the seat. Or weak springs, or bad lifters. On an engine with four cams like ours. I believe the cams have a few degrees of adjustability by loosening sprocket on the front of the cam turning the cam and retightening the hold down bolts.
So you could have just one cam on one bank out of time and cause a lean condition.
So four ways that I see a lean condition could happen on a single bank the M96.
For what ever reason ECU's don't always report both banks as having a lean condition even when there is a common vacuum leak.
Sometimes they report just one bank. like the P1128 and no P1130.
I have seen it numerous times on other makes.
Now why they report both banks one time and only a single bank at others. Is beyond me.

I agree the most common cause of those two codes is kinda split between MAF issues and common vacuum leaks. Then exhaust leaks.
Yep the fuel pumps are pretty robust and as long as good clean fuel is used and maintenance is done. There are usually no problems with restricted fuel filters. So fuel delivery issues are down the list when I am trying to diagnose a lean condition. That is true on just about any make of car for me.
Good point on the LTFT I think the limit is something around 20%-25%. All your points on the fuel system are spot on.

As far as diagnostics of lean codes:
First thing I like to do If possible is:
Cold start and just let it idle and warm up.
This way I can listen to it,observe RPM,and sounds weird but feel it.
Then a road test.
If it is a suspected vacuum leak then:
let the engine idle so it steady's out if it will.
Then slowly bring the RPM up to about 2500-3500.
Here is my reason for doing that:
If it is a common vacuum leak:
Lets say a 1/4" hole in the intake.
and at idle let say your closed throttle is equal to the 1/4'' vacuum leak.
you have a 50-50 or a 1 to 1 ratio of metered air to umetered air.
As you open the throttle and bring the RPM up that ratio changes.
If you have a 3" throttle body at WOT the ratio is now 12 to 1 in favor of metered air.
So when I bring the RPM up slowly to 2500-3500 the engine smooths out and runs better and better because the Total amount of air entering the intake is being metered more and more accurately as the throttle opens more and more.
If the engine will not smooth out. It is probably not a common vacuum leak.
Sometimes I do the above with a scanner that can show fuel trims.
With a common vacuum leak or a single bank vacuum leak fuel trims will be positive.
I run the RPM up to 3000RPM and hold it as steady as possible for 60-90 seconds.
If it is a vacuum leak fuel trims will start to come down to more normal readings. (All other things being in correct condition).
I hope you can make sense of the above.
Those two tests are very simple and together they are very effective in telling you if the issue is a vacuum leak.
If those two tests are done correctly and you don't get the described results. It's 99% not a common vacuum leak.
If I determine it is a common vacuum leak find it fix it.
If I still have a lean condition:
Then the Vacuum gauge test to see the overall condition of the engine. Valve train,rings, everything I can get from two or three tests with the vacuum gauge.
If I have not found proof of a lean condition at this point.
Then I will look at sensors to see if they are faulty.
Then the fuel system.


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