: Unwanted engine/transmission braking in 2008 STS N*



STS Streak!
08-28-11, 10:29 AM
When I'm in manual mode, first gear, and going downhill, there is automatic braking that limits downhill speed to about 15 mph. Is there a way to disable this feature?

ddgm
08-28-11, 11:03 AM
Shift to a higher gear! How fast do you think it should go in first gear? On or off the throttle? Are you saying that the car applies the brakes? Or does it just top out in that gear.

Doug

STS Streak!
08-28-11, 12:21 PM
Doug,

I'm going down a fairly steep grade and if there was no "automatically applied" braking by one of the car's algorithms, I believe the car would proceed downhill between 20 and 25mph. As it is, once I toggle to first gear at the start of the incline, there is noticeable braking within a few seconds and this braking is applied down to 15mph, then it feels as though the braking is reduced, the car increases speed to about 18 - 20mph and then is automatically slowed back to 15mph. This process repeats itself over and over during the downhill journey. The downhill grade is constant over this stretch of braking. It is not steep enough to top out first gear. I would simply prefer to coast down in first without THE CAR deciding I was going to do that at 15mph.

Thank you for discussing this.

Bill

ddgm
08-28-11, 12:57 PM
I'm going to find a steep hill and try this, probably not today. It sounds like a rev limiter is taking effect.

Doug

STS Streak!
08-28-11, 03:39 PM
Doug,

Thank you. If you don't observe the same phenomenon, I've got some software on the loose somewhere and will need to know what to do! By the way, I've tried this with and without traction control engaged. Made no difference.

Bill

EChas3
08-28-11, 08:09 PM
I don't think there's an easy way to disable the rev limiter, if that's what is coming into play. I've always found 2nd gear low enough going down a hill. (What would you rather wear out; your brakes or transmission?).

If you press and hold the Traction Control console button for 5 seconds, both TC and Stability Control are disabled. (AWD's will still retain a low level of TC to prevent damage.)

STS Streak!
08-28-11, 08:56 PM
Thank you for the tip about holding down the traction control button for 5 seconds. I will try it.

Totally agree regarding brakes vs. transmission. The hill is too steep for second gear with the STS. Our Toyota Highlander takes it in stride coasting in first gear but the ratios aren't the same as the STS. I don't think there would be a problem coasting downhill in first in the STS if the automatic braking was not occurring. Before it kicks in, I am only at about 3,300 rpm. As mentioned earlier, after it kicks in, I'm destined for about 15mph and well below 3,000 rpm.

Will see if Doug finds a hill and is able to repeat in his car what I've described earlier.

I appreciate your thoughts.

Bill

Midnight
08-30-11, 04:34 PM
Could it be deceleration fuel cut off?

STS Streak!
09-11-11, 10:08 AM
Good morning and thank you for the reply. There is no indication of fuel shut off - none of the idiot lights illuminate and there is no need to restart the engine once I'm at the bottom of the hill. Although I don't know how I could determine it, there appears to be no indication that the car is somehow limiting the amount of fuel injected into the engine.

To review, here's what I've observed, consistently, as I coast down a hill in first gear: After downshifting (toggle) down to first gear when going about 20mph, the car automatically slows itself to approximately 15mph. At that speed, I feel the same sensation as if I let off on the brakes although I am not touching the brakes. The car begins to slowly accelerate to approximately 20mph. At that speed, I feel a sensation as if I put my foot on the brake pedal although I never touch the brake pedal. The car slows to 15mph, the let-off-the-brakes sensation occurs again and the car begins to accelerate to approximately 20mph. The process repeats itself several times until I am at the bottom of the downhill grade and there is not enough grade to re-accelerate the car. When at 20mph, the tach is at about 2400rpm; when at 15mph, the tach is at about 2000rpm.

Most of the people reading this may not have a local hill steep enough to recreate the scenario I have described. We live on a hill in Southern California. The hill is about 1,000 feet higher than the main road we use to go to work, shopping, etc. Between our home's location and that main road, the neighborhood road reaches as much as an 18% grade. It is on this steep part of the road that I've observed my STS "brake and un-brake itself" while in first gear. The hill is too steep to safely coast down it in the STS' second gear.

All that said, I started this thread to ask if anyone knew of a way to disable the braking logic since I would much prefer to coast down the hill, in first gear, at a speed above 15mph. Given the responses to date, my sense is that what I observed and described above, regarding engine/transmission braking control logic, is new to this forum's readership.

Thank you.

Bill

ffrcobra
09-11-11, 10:27 PM
I'm a long time lurker but as the previous poster mentioned this sounds like deceleration fuel cut off also known as DEFCO. Basically, this is a configurable parameter in modern engines to save fuel. If the engine RPM is above a set value and the throttle is closed, the PCM will shut off fuel to the injectors until the RPM decays below a set threshold. This is useful, for example when exiting on a freeway ramp. You take your foot off the gas and burn no fuel while you slow down. The side benefit is that you also get some engine braking. Once the RPM is below 1500 or so, the PCM exits DEFCO mode and all is well.

Now going down a step hill in a low gear causes the engine to speed up until DEFCO is invoked and all fuel is shut off. When the RPM decays to a point, the engine starts getting fuel again and the speed starts to build up. Then the cycle repeats itself. If the instantaneous MPG display goes through the roof during the phase when RPM is decaying, then you know that DEFCO is being invoked.

Bruce

dkozloski
09-12-11, 03:54 PM
Nearly all the stuff that's going on with the car during coastdown is mandated by EPA/CAFE and it's very unlikely that the car systems will allow you to change any of it without getting a CEL.

mckellyb
09-12-11, 06:15 PM
I'm a long time lurker but as the previous poster mentioned this sounds like deceleration fuel cut off also known as DEFCO. Basically, this is a configurable parameter in modern engines to save fuel. If the engine RPM is above a set value and the throttle is closed, the PCM will shut off fuel to the injectors until the RPM decays below a set threshold. This is useful, for example when exiting on a freeway ramp. You take your foot off the gas and burn no fuel while you slow down. The side benefit is that you also get some engine braking. Once the RPM is below 1500 or so, the PCM exits DEFCO mode and all is well.

Now going down a step hill in a low gear causes the engine to speed up until DEFCO is invoked and all fuel is shut off. When the RPM decays to a point, the engine starts getting fuel again and the speed starts to build up. Then the cycle repeats itself. If the instantaneous MPG display goes through the roof during the phase when RPM is decaying, then you know that DEFCO is being invoked.

Bruce

This ^ is the answer. Not a thing you can do about it, unless you want to hack the car's engine computer code.

caddyfat2
09-12-11, 08:20 PM
Is that what they call grade logic ?

dkozloski
09-13-11, 04:20 AM
2nd grade logic.

STS Streak!
09-17-11, 08:15 AM
Bruce,

Thank you for introducing me to DEFCO logic. I repeated the downhill excursion and monitored fuel consumption. You were spot on. At about 2000 rpm, while decelerating, the mpg went from 70 into the teens. The car began to accelerate until the tach read about 2500 rpm and the process repeated itself. It was the added engine braking because of DEFCO logic that slowed the car. I will live with this because I certainly don't plan to hack into the computer and manipulate any software. Case closed for me.

Bill