Inconsistently Lean 5.0?

[Arissett]

So now I’ve tracked the source of the vacuum leak so I thought. I have AC now and I’m running a lot smoother but now I’m lean on bank 2 still but only when the ac is off. So when you the AC is off my oxygen sensors are reading fine but I’m lean on bank 2. When my AC is on my oxygen sensors go insane and I’m no longer lean on bank 2. Here’s the Screenshots. What is this? 1999 5.0L V8 Explorer.

 

[C420sailor]

To me, that’s still a vac leak.

At idle with all the accessories off, you’re gonna pull the most vacuum. IAC is largely closed, throttle plate is closed. Turn on the AC, which is a half decent load, and the IAC has to open in order to maintain idle. Your vacuum is going to be lower.

With everything closed down and vacuum high, that small leak is going to really leak, and it will introduce a greater proportion of the air into the engine—and it’s unmetered (MAF never sees it). With the IAC open, and less vacuum, that leak may not leak as much, and it’s going to be a much smaller proportion of total air (as the IAC pulls in metered air that the MAF sees). Hence the smaller STFT with the AC on.

I could be very wrong, this is just what makes sense to me.

Makes me wonder if you’ve got a leaky intake manifold gasket on bank 2

 

[Arissett]

To me, that’s still a vac leak.

At idle with all the accessories off, you’re gonna pull the most vacuum. IAC is largely closed, throttle plate is closed. Turn on the AC, which is a half decent load, and the IAC has to open in order to maintain idle. Your vacuum is going to be lower.

With everything closed down and vacuum high, that small leak is going to really leak, and it will introduce a greater proportion of the air into the engine—and it’s unmetered (MAF never sees it). With the IAC open, and less vacuum, that leak may not leak as much, and it’s going to be a much smaller proportion of total air (as the IAC pulls in metered air that the MAF sees). Hence the smaller STFT with the AC on.

I could be very wrong, this is just what makes sense to me.

Makes me wonder if you’ve got a leaky intake manifold gasket on bank 2

How could I test for it? It’s a fairly cramped area.

 

[410Fortune]

smoke test is the best and only real good way to pinpoint a small vac leak especially on a 5.0

The egr valve has been known to do this

Intake plenum leaks on 5.0 on one bank not really super common.
If the intake plenum gasket is leaking you will typically see soot/buildup of dirt around the base where the air is getting past