Hey guys, not sure if anyone has come across a similar issue or not. I couldn't find any definite fixes for this with the few threads on this type of RPM drop in a search. This is not the RPM fluctuation that is normal upon downshifting.

2013 G37 7AT with 97,000 miles on it.
Mods: z1 intakes, Z1 HFC, Fast Intentions catback.

When I come to a complete stop in gear regardless of if the car is cold or warmed up, the RPMs dip really low and the car stumbles like it wants to die and then catches itself. It hasn't stalled all the way, but it acts like it wants to. The car isn't throwing any current or pending codes and drives perfectly when under throttle. It also idles fine after it catches from the stumble.

I've changed the spark plugs, checked all coils with no signs of arcing, checked for vacuum leaks with a smoke machine, cleaned MAFs, cleaned throttle bodies, performed idle air relearn, pedal position relearn, and throttle closed position relearn with no luck. Oil pressure is 24psi at idle, 62psi at 2,200rpm, so the gallery gaskets are still fine. I haven't checked fuel pressure, but given that I'm not tuned yet my fuel trims aren't super off. Bank 1 LTFT is 4% and STFT is -2%, bank 2 LTFT is 9% and STFT is -3%, so bank 2 is a tad lean, but I'm assuming that's due to not having a tune yet. This just started recently, car has been fine for about 7k miles after installing the above mods. Plugs all still look fine with no oil on any of the threads. I'm really hoping this isn't the start of the torque converter failing...

Any thoughts on what else to check?

 

Did some data logging this morning based off of a recommendation from some fellow 370/G37 owners.

The TPS's both show about 1.18% when the RPM drop occurs and they stay at this same value when the car recovers, so I don't think this is a throttle body or TPS issue.

The MAF's both drop to about 1.94 G/S when the drop occurs, and go to 2.2G/S when the car recovers, so the MAFs are showing some type of reaction to the issue.

The Battery voltage also drops when this occurs. While driving, battery voltage is showing about 12.6v, and occasionally will go to 13v, but it tends to live in the 12.4-12.6v range. When the drop occurs, I see battery voltage drop to 11.7v and then when the car recovers, it's at about 12.1v to 12.2v. So I'm not sure if this is a voltage related issue or something else going on.


In other notes, the car has started flaring really bad with the 3-4 shift when warm, which it hasn't really ever done before the last 1,000 miles or so, so this leads me to believe this could also be Transmission/torque converter related. No trans codes stored either.

 

Idle air control valve?

 

Do we have idle air control valves? I thought VVEL controlled the idle on these motors, but I could be wrong. I don't show any parts coming up for an idle control valve.

 

Was the idle relearn procedure done correctly. would hurt to try it again. Make sure your intake clamps aren't loose or maybe ruptured a coupling behind the maf sensors

 

Yeah, I did the relearn using the NDS III app and redid it just for safe measure. I'll check over all the intake clamps, and I'm also going to check for exhaust leaks at the cat to header flange to rule that out as well since Bank 2 is running a tad leaner than bank 1. Also going to try and pull the MAFs again and clean them really well just to make sure one more time.

 

Starting to wonder if this is voltage related. Voltage consistently dips below 12v every time it does this. Wondering if maybe the voltage regulator in the alternator is starting to go.

 

There is few forums on this topic and no one can figure it out. My car was doing it at times during the summer but ever since it got a bit cooler in Texas the RPM doesn’t drop. You can also find really old threads where people were having this issue when the car was basically brand new.

 

I'm experiencing the exact same problem as you on my q50s 3.7. Even the 3to4 shift flare on moderate acceleration but never in manual mode. I just got a new alternator recently but that didn't fix it. And I've tired almost everything you have as well and still no solution.

Have you fixed the problem with your car?

 

Try the same however throwing the car in neutral instead. It's quite possible that your torque converter could be staying stuck causing it to act like an engine issue.