NL2013

While cruising on the freeway with the cruise control set to 70 mph, the system will slow the car on slight downhill sections of the roadway.The tachometer will momentarily drop about 250 rpm for one or two times.Is this normal?Has anyone else experienced this with their Lexus?I’ve driven the same stretch of freeway with several other vehicles (Ford, Mazda, Infiniti, BMW) and the cruise control system is seamless….they better hold the set speed and no feeling of “the brakes being applied” when the engine is throttled back.
2013 Lexus GS350 F Sport

Mike728

How much speed do you lose? All modern cars I've driven let off the gas on a downhill stretch, but not enough to drop the speed. It usually drops slightly, initially, when going up hill and has to downshift to maintain.

er34

First of all, the 250rpm drop is probably the lockup clutch which optimizes fuel economy whenever you let off the gas, not just when cruise control is on.

Second, if you feel "the brakes are being applied", that's actually not the wheel brakes, but the car downshifting to engine brake the car from rolling too fast - though when this happens, you should see the tachometer go up instead of down. Not sure if it is the weight of the car, but I find the GS tends to gain speed on steeper declines much faster than my other cars, and it keeps rolling further without throttle as well.

MustangSal

If the speed is set to 70, than it's going to try and maintain 70 by manipulating performance to slow it down. If you want to coast, knock off the CC and drive it, or speed the car up.

If you're saying it is slowing the car down, on downhill grades...to what speed is it slowing you down to? Assuming that also means once the land flattens out, it is speeding the car back up to 70?

Pls clarify.

Handi

I have noticed this also and it is a little annoying but it does hold your speed.
From what I have seen it is as er34 describes, it down shifts the transmission so the motor slows your speed.
When you see the RPM's dropping the car has slowed and the transmission is up shifting so the car can continue on it's set speed.

MH6

My 2013 AWD Luxury does that exact same thing, however there is no downshifting, it is simply reducing engine RPM a few seconds at a time to maintain the set speed going down hill. The GS coast's extremely well, especially down hill.

I don't care for that about the GS, all my others Lexus over the years have not operated in that fashion. I had it checked and was the same on other GS as well.
Mine goes a step further.....when the speed is set, the speed its fine on flat areas but when entering a slight hill, speedo drops about 1.5 mph and maintains at that level and then regains slowly on hill and then once on flat areas it goes back to original speed. Had the dealer check and they did not see a problem (of course). I am going to take back in the next couple of weeks and look over this again.

Seppo

I noticed the cruise control's behavior on my drive home after getting my CPO 2014 GS350 AWD, especially the herky-jerky feeling on downhill sections of highway. Most of the time the speedometer remains within an acceptable tolerance, but on irregular occasions the speed varies by as much as 5 mph from the setting. I have had many years of cruise control on my own Volkswagen, Audi, and BMW cars, they responded with utter smoothness and precise speed control. A couple of rental or business vehicles, mostly older GM or Chrysler/Jeep models had abrupt responses trying to hold on to the setting speed, or which varied a lot from the setting.

I have concluded that refined and seamless cruise control is just not one of the GS strengths, and have adapted to it. Now it only bothers me when for whatever reason the car's speed varies more than five MPG from the setting, although that doesn't happen often, and there doesn't appear to be a distinct pattern in the circumstances that it occurs. Turning cruise control off and back on again seems to rectify the big variations in speed. But since I drive a lot in the mountains, downhill sections always have that slow down-stutter-speed up thing happening.

NL2013

Thanks to all for your replies! You've confirmed what I have suspected......the cruise control function on the GS350 is not as refined as some other cars. My warranty is about to expire and I wasn't sure if I should make a big deal out of it at the dealer.

To answer a few questions, the system does work ok other than it allows the speed to drift up and down by as much as 3-4 mph. When going down hill, it will pick up about 3 mph and then correct by dropping the engine RPM for about one second which slows the car by about 1 mph. This is noticeable to me....I feel it slowing the car and these are not big hills (there are no big hills in Fort Worth, TX). As I have stated, many other cars on the same stretch of freeway operate much better....they hold the speed by +/- 1 mph and you do not feel the system slowing the car. It is a bit annoying to me since I expected better functionality from a premium car. I guess all cars have their strengths and weaknesses.

MustangSal

"better"...according to you.