"Only uphill? Perhaps the change in gravitational force direction is causing a shifting of something creating a restriction in fuel flow to the engine.

A few suggestions so you can provide a hopefully useful description of the symptom(s) so the dealer's service techs have some good, solid symptom information to work with in diagnosing the issue.

1. How step a hill slope is needed for the symptom(s) to occur? Does the hesitation/rough running vary (and how) with different hill slopes? Is speed a (frequency/severity) factor in the hesitation/rough running symptom.

2. Does the hesitation/rough running occur under cruise control, driver manual control, or both?

3. If you park (out of traffic) on a hesitation/rough running hill slope, does the idling engine have some sort of hesitation/rough running symptom? (If not, you might try to find the steepest slope in your area t see if something really extreme has an effect.)

4. Does the hesitation/rough running show up in the tachometer display? If so, take a short video (or more) to show the service department some concrete evidence of your complaint. (If you can hear a sound effect of the hesitation/rough running include that also.)

5. Look around your dealer's location of features with slopes similar (or greater) than the hill slopes that your experiencing the hesitation/rough running problem. (Hills, freeway on ramps, multilevel parking garage ramps, etc. - somewhere local to the dealer for the service techs to do slope hesitation/rough running diagnostic testing.) Depart early for your dealer service appointment, so you can verify a dealer local slope testing candidate does indeed cause the hesitation/rough running issue.

6. Have a fairly good idea of where and when you've bought gas for your ES. That's mostly so you've thought about it in case questioning about "bad gas" arises.

In brief, the reasoning behind the above suggestions is to develop a good description for the hesitation/rough running issue and the definite conditions under which the issue occurs."


Yesterday, prior to coming Lexus service department appointment, I had my independent mechanic replace my original engine air cleaner. Since doing so, the hesitation problem has not reoccured. I'm hoping that this may have been the cause. The old, original engine air cleaner was only moderately dirty and had only 12,000 miles on it (though 2.5 years of use).
I'll continue driving for a week or so before I pop the corks but this is encouraging and it may just have been one of those things, so to speak.
Thanks again for your help.

 

I've been changing my engine air filter every 25K miles and it's barely dirty even then.

 

Maybe you drive through a very dusty area, like a roadside construction crew. How was fuel economy before and after?

 

Under 20 mpg lately. Lots of of road work and dust around here this summer May have caused the problem. I dunno. I’ll track it now though, post new air filter and see if there’s any difference. Important thing is since new air filter, the symptoms seem to have gone for whatever reason!.