Got a weird one - Helping a friend

[StevieC]

Got a weird one helping a friend tonight...

Both upstreams O2's switching normally and respond properly to rich and lean conditions at idle and pretty well match for both bank of cylinders and function normally above idle.
S/T is within 4% L/T is within 10%

Both downstream O2's stable above idle and match almost identically but one O2 post catalytic on one bank of cylinders goes extremely lean at idle after about a minute idling while the other sits at its normal stable level. While this is happening the up-stream for that same bank is cycling normally and matches more or less the other bank where it isn't going lean at idle.

The plugs on the vehicle all look like they have similar burn between cylinders and there is no misfire counter for any cylinder. The idle could be smoother so I think it might be operating a tad bit lean.

My thought is either a lying upstream on that bank of cylinders driving the mixture leaner than it should be at idle and thats why we can see it in the converter's readings, a fuel injector not putting out enough at idle driving the readings lower post catalytic, or there is an exhaust leak prior to the converter and it's drawing in some extra air which is why it doesn't show up unless it's at idle when the flow is lower. I don't believe this last one is the cause because the temperature in that catalytic is dropping at idle indicating a lack of fuel.

The reason it's a problem is the temperature will be correct on the catalytic converter on this affected bank above idle but at idle it starts to drop off as it begins to read lean.

I'd ask my dad but he's not in a great condition tonight... Thoughts?

2015 Jeep with the 3.6L Engine 68,000 miles (109,000km)

 

[Chris142]

Vacuum leak?

 

[StevieC]

Vaccum is stable at idle and the S/T & L/T looks good. I'm thinking either lying upstream or it's an exhaust leak pre-catalytic.

 

[Skippy722]

Could be a bad o2 sensor. The upstream one on my Caliber looked like it was reading not quite as lean as it should and not quite as rich as it should (0.3-0.7v it would bounce between) and not setting codes, but it was getting 20mpg. Although I had 90k on it, I threw a new NTK in since I had done the plugs and filter the year prior, and just did an oil change already. Bam, 25-27mpg again. Smoothed out the idle as well.

 

[StevieC]

I wish the sensors where the same on both banks so I could move it around and see if the problem moves.

 

[Rand]

so what exactly is the problem again?

 

[OVERKILL]

So you've imparted both a lean and rich condition on the engine and both upstream O2's track with it properly? If so, then that should rule the upstream one out. So it would seem to point to a leak, though that would be unusual on a 2015 I would think

 

[StevieC]

Originally Posted by Rand
so what exactly is the problem again?

As the catalytic temperature begins to fall the ECM bumps up the idle in an attempt to keep it warm. (Winter mode). I can confirm my Caravan does the same thing and did so today because it was brutally cold out today. His has been doing it even before today and he has been trying to troubleshoot originally thinking it was the electronic throttle body which are common on these engines where idle issues exist.

Originally Posted by OVERKILL
So you've imparted both a lean and rich condition on the engine and both upstream O2's track with it properly? If so, then that should rule the upstream one out. So it would seem to point to a leak, though that would be unusual on a 2015 I would think

Above idle it all looks good. The problem is the range of the upstream could be out. So it could be cycling but reading too lean in it's overall average and it's more apparent at idle. Or there is an exhaust leak pre-cat. I can't see any leaks or soot marks indicating a leak. Unfortunately it's too cold to do further testing for leaks.

I'm going to mess with it more tomorrow when we both get home.

 

[diyjake]

Is the check engine light on? What scanner are you using to get the readings?

 

[StevieC]

Originally Posted by diyjake
Is the check engine light on? What scanner are you using to get the readings?

No because it's still within the acceptable limits.

I'm using my Autel MD-805 but I have access to a Snap-On professional grade scan tool at my dad's 20 miles away.

 

[OVERKILL]

How are you making it rich and lean to test the O2's? Carb cleaner and a vacuum leak? Just curious.

 

[StevieC]

Originally Posted by OVERKILL
How are you making it rich and lean to test the O2's? Carb cleaner and a vacuum leak? Just curious.


Rich with propane and vacuum leak.

 

[OVERKILL]

Figured it would be something like that. If both track the same I'd have a hard time condemning one of them (the O2's). Is there any physical damage on the cat?

 

[StevieC]

Originally Posted by OVERKILL

Figured it would be something like that. If both track the same I'd have a hard time condemning one of them (the O2's). Is there any physical damage on the cat?


No damage to the cat and temperatures before / after match the other bank until it starts leaning it out then it starts to drop off. I'm thinking the upstream on the affected bank is lying and it just seems to have an issue at idle not when it has lots of exhaust gas rushing by it at other times and as a result the ECM is leaning out that bank based on the false reading it's getting from that upstream.

Looks like 02 sensor is the same for front and back however the cord length is different so I might try disconnecting it and see what kind of numbers I get from the catalytic running on the ECM's best guess. If the numbers are stable in the cat then it should prove the upstream 02 is the problem I would think versus extra air making it into the exhaust or a lack of fuel on that bank at idle.

 

[OVERKILL]

And you can't switch them bank-to-bank?

 

[StevieC]

Originally Posted by OVERKILL
And you can't switch them bank-to-bank?


I can move the one to the other bank because the extra wire is not an issue but the other side it will be too short it looks like. Thought of that. Always like swaptronics to see if the problem moves confirming the bad component where possible.

 

[OVERKILL]

Yup, exactly. If the problem follows the O2 then you know it's the problem.

 

[StevieC]

It's one of those, "now I have to figure it out because it will bug me" problems.

I just wish the weather was warmer to be messing with it outside. Although a warm engine over top of you makes it a tad easier, just not for the hands when you are trying to disconnect something.

 

[OVERKILL]

For sure, I feel your pain!

 

[StevieC]

Turned out to be the upstream oxygen sensor on that bank. We moved it to the other bank and the problem moved.

Funny enough after we moved the sensors around the affected bank was showing a code for Secendary Circuit - Insufficient Ionization on one of the cylinders so I guess it was running the mixture just off enough to hide this problem with the coil pack. Luckily it's on the easier bank of cylinders to get at.

Anyway, he's going to order an O2 sensor and coil over for the affected cylinder, we put it back the way it was in the meantime and so far no CEL's it's just dropping the Cat. temperature upon idle is all.