2018 Dodge Grand Caravan 3.6L - LTFT

Hello everyone:

I have a 2018 Dodge Grand Caravan with the PentaStar V6 3.6L. It has about 5,000 miles on it and it has been flawless until just recently.

I seem to be having a slight shaking from the engine at idle that is more pronounced when it's in gear. It's not enough to set a misfire counter but I have noticed that bank 2 LTFT at idle is about -8% - 10% whereas bank 1 LTFT is at 0.

When above 1500rpm the LTFT's match and when under load. (Example going up a grade at constant RPM on the highway) then as I approach level ground the LTFT will begin further leaning out bank 2 than what bank 1 is reading.

There is no stores codes, no pending codes and the misfire counters remain at 0.

When the vehicle is open loop the slight shakiness is not there or it can't be felt if it is.

I was thinking maybe a lazy 02 sensor on bank 2 with it being so new. The vehicle has excellent power and will get up and go without hesitation and it's returning 24.2mpg with mostly highway driving and limited city.

The vehicle is under warranty but I'm afraid that the dealer will not do anything because the MIL isn't on and there is no stored / pending codes and the LTFT is less than 25% so I'm trying to troubleshoot this myself.

Thanks in advance for your help!

Your motor isn't broke in yet at 5k over the next 20k your motor will get higher compression and more efficient burning.

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I appreciate the reply.

The LTFT matched for both banks when it was new when it didn't have the slight shake to it. I would love to hook up a scope to the ignition coils but it's almost impossible on this engine without taking the plenum off.

This is a really great post! Your open loop observation is a key piece of evidence. How is it during cold open loop vs hot engine restart open loop (which may only be a very short time)

Thanks for getting back to me.

Cold Open Loop it runs pretty well and there is no slight shaking to the engine.
Hot Open Loop (very short indeed) - no slight shaking to the engine.
Hot Closed Loop when coolant is reaching 218F just before fans kick in, when it's in gear is the worst for the slight shake that I feel.

Graphing Bank 1 and Bank 2 02 sensors shows normal waves and pretty much overlap each other. Bank 2 where I have the LTFT showing it's running too rich with a negative fuel trim seems to have the odd hick-up in this rich-lean cycling back/forth and the volts although close between the upstream 02 sensors don't quite match.

I was concerned I might have a coil pack or spark plug breaking down but it's impossible to scope them because of the Plenum setup.

The injector modulation is just as about identical as you can get among them all.

Thanks again for your reply. Really appreciate what you do and I'm a big fan of your YouTube channel along with SMA.

Update:

So I was watching the Bank 1 and Bank 2 upstream 02 sensors on the highway and when I let off the gas completely and the ECU shuts down the injectors I noticed that on Bank 1 sensor that it pulls down to 0V and stays there where as Bank 2 gets close to 0 but there are minor waves still at maybe 0.02 volts sort of thing.

I'm thinking that this 02 sensor is the culprit so I'm going to disconnect and drive it that way and see if it shows complete 0V or biased voltage to determine if it's the 02 or the wiring from the 02 to the ECM.

Solved the problem...

It was the upstream oxygen sensor on bank 2 (front). Replaced with another OE NTK sensor and the fuel trims immediately fell back into line and were almost identical to the other bank of cylinders. The idle smoothed out dramatically.

The way I verified the sensor was the problem was I disconnected it from the ECU and put it on a volt meter and I could see it was switching rich/lean but the range was incorrect when I created a lean and rich condition manually and it would also sometimes generate negative voltage on the signal wire instead of pulling down to 0 only. Never saw that before in any of the 02 sensors I have replaced.

I also tested the wiring to the ECM by grounding the signal pin and it bottomed out at 0 and stayed there without going negative.

So to anyone looking at just the scan tool it would appear the sensor is operating properly because it had good waves from rich/lean but they were in fact incorrect and the sensor was lying. When the voltages would go negative on the sensor the scan tool would just report 0 volts but the multi-meter on the signal wire showed it in fact going to negative voltage.

So what does this all mean? It was actually running that bank of cylinders much leaner than it should have been because the ECU was getting a false report from the 02 sensor telling it that it was running richer than it was. Which is why the LTFT for this bank had a value of -10% or so and the other bank operating properly had a LTFT of 0%

After the sensor was replaced both banks are now reading -1% and -1% (Running on ethanol free fuel)

Just wanted to update this thread for anyone else with similar problem and because this is good learning with regards to never trusting your inputs until you have verified they are accurate.

Very nice fix.

Do you recall what the downstream sensors were reading during the fuel trim imbalance? Wondering if there was a discrepancy between the upstream and downstream on the same bank that would have suggested a lying sensor.

Both downstream were reading about the same although bank 2 would drop a tad bit leaner ever so slightly than bank 1. Keep in mind that my problem with the upstream was only running a little bit lean and not all the time and the CAT's are new so it most likely would absorb any minor changes in upstream exhaust gases without changing the readings in the CAT where the downstream sensors are, all that much.

Cheers,

Steve

Very interesting discussion - I need to pay more attention to readings like this as I slowly learn what they mean.

Sorry for the late reply. I have been super busy at work and some other stuff going on in family life.

This is a great resource as is the forums at: www.bobistheoilguy.com/forums/ubbthreads.php