2015 V6 Auto doesn't want to accelerate past 45-ish MPH

[Kobra337]

**PROBLEM SOLVED IN REPLIES, THANK YOU**
Issue started today. Noticed I could turn on cruise-control, but could not set a speed. Then as i tried to accelerate up to the 55mph speed limit, the car refused to go any faster, even if i continued pushing the pedal to the floor. If I took my foot off the gas and tried accelerating again, I could go 5-or-so MPH faster, but the issue would just continue.

NO CEL, NO WRENCH ICON, NO warning lights whatsoever. The only indicator that would show up was the occasional "hill assist not available" thing.

There are no weird noises, no funny smells, no vibrations throughout the car. No RPM/engine stutters.

The odd bit is that if I stop the car and floor it from a complete stop, it will accelerate like normal until 45-ish MPH. No stuttering, RPMs just climb as normal.

68k miles, changed oil 2.5k miles ago. For the last good month and a half, I've only been using cruise control, no spirited driving except the usual "need to pass this slow car". I checked oil on side of the road, nothing looks out of the ordinary.

I don't think it's fuel related, as I've been on the same tank of gas for about 3 days going 40-50 miles each day with no issues. However, I'm not saying that it's not a possibility. I've seen some threads "similar" to this one say a MAF sensor could be loose or faulty, but I'd figure that would throw a code after 40 minutes of driving.

Car is bone stock, no mods, no altercations. 2015 V6 Automatic.

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[ugstang17]

Is the car shifting gears? Does the RPM climb while the speed stays about the same indicating trans slippage? Is the problem only hill related or all the time? Have you had the trans fluid checked? Trans fluid is hard to get to on the 6R80 not sure what trans in on the 6 banger but guessing it is no easier to access. So that needs to be checked. Trans codes may not be seen on the dash. Have a scan done on the car for codes.

Any chance you have accidentally reprogrammed the mykey function (or grabbed a spare key already programmed) which allows you to reset the key to limit speed for use by a teenager or valet. Check that as well.

 

[Kobra337]

Is the car shifting gears? Does the RPM climb while the speed stays about the same indicating trans slippage? Is the problem only hill related or all the time? Have you had the trans fluid checked? Trans fluid is hard to get to on the 6R80 not sure what trans in on the 6 banger but guessing it is no easier to access. So that needs to be checked. Trans codes may not be seen on the dash. Have a scan done on the car for codes.

Any chance you have accidentally reprogrammed the mykey function (or grabbed a spare key already programmed) which allows you to reset the key to limit speed for use by a teenager or valet. Check that as well.

Car shifts gears fine, I can even still use the tap-shifters. It indeed does have the 6R80. RPM does not climb when the issue presents itself; it just won't accelerate, almost like i'm not even pressing the gas pedal. I'm not sure why the "hill-assist" thing even popped up. I wasn't on a hill, I was stopped on flat ground when it decided to pop up.

The speed thing is very odd to me. I only have one key. From a stand still, if I try to accelerate, all is fine until that 45mph range. It just won't go anymore unless i take my foot off the gas and "pulse(?)" it. It's weird trying to explain. If I take my foot off the gas pedal, wait a second, and then press it again, it will accelerate a little more but I have to do that repeatedly to get to 60-65 mph.

It's baffling to wrap my head around. Doesn't want to go above 45mph, but if I floor it from a stand still I can get to that at full RPM? shit's whack.

 

[CTH621]

Check your brake lights if they are on even if you're outside the car and not stepping on the brakes. It could be your brake light switch. Blue small piece of sensor/switch right above the brake pedal. Because if the car detects you are on the brakes and on the gas at the same time, it puts the car in limp mode. i.e. not over 45mph.

 

[Kobra337]

Check your brake lights if they are on even if you're outside the car and not stepping on the brakes. It could be your brake light switch. Blue small piece of sensor/switch right above the brake pedal. Because if the car detects you are on the brakes and on the gas at the same time, it puts the car in limp mode. i.e. not over 45mph.

This is exactly what it was! Thank you. $14 part that I changed out in less than 5 minutes.

 

[Cobra Jet]

Check your brake lights if they are on even if you're outside the car and not stepping on the brakes. It could be your brake light switch. Blue small piece of sensor/switch right above the brake pedal. Because if the car detects you are on the brakes and on the gas at the same time, it puts the car in limp mode. i.e. not over 45mph.

Yep - and for future reference if anyone comes across this thread, here’s the link to the brake light switch thread:

https://www.mustang6g.com/forums/threads/brake-light-switch-near-pedal-failure.122100/

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