Hello all,

Ever since I bought my 08 G37s 6MT about two years ago I’ve had an issue with the idle. I’ve cleaned the throttle bodies and replaced them twice now and taken the car to the dealership. I’ve done the idle relearn, and ecu reset from the dealer. I’ve changed all my O2 and A/F sensors and I have no check engine lights. I’ve also done two vacuum leak tests.

The car start and warm up and will idle normally. As soon as I start to drive there will be really bad rev hang between shifts. When the car is in neutral the RPMs will raise to 3-4k and hold (at a stop). It does this more frequently when it is cold outside (sub 60 degrees). If I continue to drive the car for 15+ miles then eventually at some point the car will start to drive normally or if I pull over and do the idle relearn then it will drive normally. Usually the next morning the problem will present itself again after it sits for a while.

Does anyone have any idea what is going on here???

Thanks!

 

When you say you changed your sensors, did you also try to clean or replace your MAF sensors? If they are dirtied or damaged that could be the culprit regarding a high idle. Excuse me if you already checked that.

It could also be a bad coolant temp sensor, but I don’t have enough experience personally with that failure in our cars to confidently say that’s what it could be- but it’s a possibility.

Couple questions that can help us all figure it out-
+Is your car tuned, or was it previously tuned?
+Any intake or exhaust modifications? (If you’re running the OEM intakes, check for cracks in the plastic post-MAF tubes; they are known to crack)
+How many miles are on your car?
+Last spark plug & coil pack change? (if applicable)
+Have you checked for any exhaust leaks?

 

I actually haven’t cleaned/replaced the MAF sensors. They haven’t given any CEL so I figured they were fine. As for coolant temp sensor, I’m not sure either. I just recently changed the thermostat if that has anything to do with it.

To my knowledge the car has never been tuned, definitely not while I’ve had it.

The car has K&N air filters. It has high flow cats from Kinetix, and a Magnaflow axel back exhaust with the mufflers removed. The cats have never given a check engine since putting them on.

The car is high mileage at roughly 174k miles.

I changed the spark plugs less that 10k miles ago. I kept the same coil packs. The car doesn’t have any misfires and drives completely fine when it is idling properly.

I haven’t checked for exhaust leaks but I don’t hear one from the car. I’ve been told from an Infiniti shop forman that any leaks or vacuum issues before the A/F, O2, and MAF sensors would trigger a CEL due to the sensitivity of the car, and so I took him at his word.

I hope these answers help. Thanks!

 

I would side with Goldbug in checking / cleaning the MAF sensors (if they are dirty, they might not cause a code / CEL) and checking around the corrugated portion of the intake tubes for cracks.

As for mods, if the prior owner installed a ported upper plenum, it can cause idle issues like you mentioned, especially if it wasn’t tuned.

 

I will double check the MAF sensors and clean them just to be safe. There were no cracks the last time I checked. I did a smoke test and there were no leaks. The plenum looks stock to me.