Cold Engine (morning) Runs Rough Only on Reverse Gear!

[MDXwithSPEED]

Hello Everyone!

I have a friend with a 2012 Lexus RX350, 96,000 miles, (2GR-FE)

Last week she had stated to me that only in the morning when she starts-up the vehicle and put's the truck in 'reverse,' she feels the vehicle run rough and like it wants to shut-off, but when she puts the gear back on drive truck feels normal. The day she brought the truck over by my garage I hook up my scan tool and check all the vehicle's module's for codes... no codes are present! I looked at all the parameters, fuel trims, O2sensor's, MAF and everything looked normal. She told me that she wants to give the truck a tune-up... I said sure! 

Over the week I replaced the spark plugs, air filter, pcv valve and a oil change. I also cleaned the throttle body and it was not very dirty. The old spark plugs looked normal, replaced with OEM Denson same plugs that where removed. One's completed the repair, I test drove the vehicle for about 20 minutes looking for throttle body codes since I cleaned out that part and many times a code will set. I made sure that no codes where present and the truck drove well!

Today I followed up with her and asked her if she was still having the same problem? Yes told me yes! The truck, only in the morning when the engine is 'cold' and only when she puts the vehicle in 'reverse.' She still had the same rough running fell, but when she moves back to drive gear the truck is okay...

I am looking for some help here! Does anyone know what steps I should do next or has had the same issue with this model Lexus?

 

[Lexus Owners Club]

Just out of curiosity, would it be worthwhile parking the vehicle into the driveway/garage in reverse so that on cold start up the car can be shifted into Drive and see if the fault replicates?

[RX400h]

Did you check for a vacuum leak? By the way, I believe our Lexus required a sparkplug change at 120,000 miles so it doesn't surprise me that your friend's plugs looked fine.

I agree with Trevor that duplicating the issue is a great first step. That way you can poke around the engine bay while the rough idling is active.