Can a Vacuum Leak EVER Cause a RICH Code?

Alright guys, I need your help to settle a fight I'm having with a guy online. We all know that a vacuum leak will cause a lean condition, and therefore cause a lean trouble code. But is there any time where a vacuum leak can cause a RICH code? I say no, the guy I'm arguing with says yes. His logic is that the computer, once it detects the lean condition and sets a lean code, will add more fuel and make it run rich (sort of true I guess, it will max out fuel trims to try and get the mixture right), but he says it will intentionally make it VERY rich so as not to overheat or damage the engine from running too lean. And because it's running so rich, it can throw a rich code. Even though the computer TOLD it to run rich in the first place, the computer will still be concerned about this rich condition and throw a rich code, or something like that.

So, what say you guys? I can admit when I'm wrong, but his arguments are totally nonsensical to me. He thinks I'm just being stubborn and refusing to admit I'm wrong and that it's "widely accepted knowledge" that vacuum leaks cause rich codes.

A failing evap purge solenoid - as a special case of vacuum leak - is able to cause both rich and lean codes: when stuck open after refueling massive fuel vapors will be sucked into the intake and can trigger a P0172/P0175. Once the canister gets depleted the condition inverts and the resulting lean mixture may trigger a P0171/P0174.


With regards to your example: If the pcm reacts to a preceived lean condition the goal of this reaction is to keep fuel control. If the pcm is able to maintain fuel control the engine would not run rich .....

With regards to your example: If the pcm reacts to a preceived lean condition the goal of this reaction is to keep fuel control. If the pcm is able to maintain fuel control the engine would not run rich .....

So you're saying that, if the pcm detects a lean condition, it will add fuel to compensate (positive fuel trims), and it will continue to add fuel until the lean condition is gone (assuming it doesn't max out fuel trims), and therefore will never run rich, or set a rich code, correct? Because that's exactly what I told him. And even if it did max out fuel trims, it did that because the engine was STILL running lean despite the maxed out fuel trims, so obviously it isn't running rich. He doesn't seem to understand fuel trims at all; I don't know if he's ever even heard of them. I sent him a link to a Scanner Danner video on fuel trims and he called it irrelevant and didn't want to watch it. He said that vacuum leaks are very unpredictable and could cause the engine to be VERY lean one moment, and not so lean the next, and the PCM wouldn't be able to compensate quickly enough, so to avoid damage by running too lean, it just runs extremely rich all the time while the lean condition is present (but if it's running too rich, it's NOT running lean, but the PCM knows it's still too lean, so it makes it too rich and...I dunno, try not to think about it too much).

He said if the fuel trims max out and it's still too lean, it goes into open loop and richens the mixture even MORE (if the PCM were capable of richening it even more, then the fuel trims wouldn't be maxed out, would they? Also, if it went into open loop, how would the PCM even know it's running rich to begin with?! Open loop, by definition, means the PCM isn't paying any attention to the O2 sensors. So even if the O2 sensors were indicating it's running rich, the PCM wouldn't pay any attention to it and therefore would NOT set a rich code! Also, if it was in open loop and continued to run stupid rich, how would the PCM ever know if/when the lean condition is fixed and it could go back to normal? It would stay in open loop forever until someone cleared the code and would be incapable of clearing on its own.)

By the way, thanks everyone for your suggestions so far. You guys mentioned some conditions I hadn't thought of.
Now, for context, this all started when someone was asking for help with his 2000 Ford Ranger 4.0 OHV V6, which was throwing a P0172 Rich Bank 1. He suspected a cracked intake or intake gasket leak, and I replied that that would cause a lean code, not a rich code. And then this other guy started arguing with me that an intake leak could in fact cause this code, and here I am.

That's a MAF car and will not run rich with a vacuum leak.
Looks like you found a better forum anyway.

Exranger06 wrote:

Alright guys, I need your help to settle a fight I'm having with a guy online.

This is where you went wrong. :silly:

His logic is that the computer, once it detects the lean condition and sets a lean code, will add more fuel and make it run rich (sort of true I guess, it will max out fuel trims to try and get the mixture right), but he says it will intentionally make it VERY rich so as not to overheat or damage the engine from running too lean.

I have seen a couple cases where an ECM/PCM reacted to an severe fuel control/misfiring/overheating problem by forcing Open Loop and richening the mixture above stoich. But that kind of strategy is rare in my experience, AND won't set a rich code as a result.

Now, for context, this all started when someone was asking for help with his 2000 Ford Ranger 4.0 OHV V6, which was throwing a P0172 Rich Bank 1. He suspected a cracked intake or intake gasket leak, and I replied that that would cause a lean code, not a rich code. And then this other guy started arguing with me that an intake leak could in fact cause this code, and here I am.

They're both gonna be disappointed when they don't find any vacuum leaks. :lol: Or they find some leaks, fix them, and still have a rich condition.

In that specific case, the only way I can think of that a vacuum leak may cause rich running is if the hose going to the fuel pressure regulator is unplugged/cracked/broken.