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Runaway Elantra – how to lose your brakes.

OK, first you will not hear any BS here that the car drove away itself, BUT I gather some evidence that might explain how some people claimed their cars would not slow down.


I encourage you to do same experiment, but with huge caution. I am not responsible if you cause an accident. Think twice, and then think again, and again. I repeated this test a few times under different conditions with same result. So either my car is messed up or… it is really a case here.
I got an explanation why that happened, but this is at the end.

It will work if you have manual transmission or the sportmatic (whatever it is called – the manual shift in A/T). Now, accelerate to say… 50 KM/H (30 MPH) put 3rd gear and floor it. Now, keeping the gas in WOT (wide open throttle) push the brakes (little quick push) with the other foot (yes, two feet…) while keeping the gas to the metal. So, push the brakes a few times. After the 3rd time you will notice the brake pedal is hard. It still has brakes, BUT servo (brake booster) is gone. Do not panic, it only takes one movement to regain full braking power – release the gas.

How I learnt it? Well, I was changing thermostat and to make sure the engine will get hot quicker than at idle I put it in 3rd gear and going about 40 MPH started pushing the brake (going uphill) to warm up the engine as fast as I could. It was about 65C, but I wanted to reach 85C quickly. So after the 3rd push under WOT I realized the brakes are not same. They were getting rock hard. Sure, stepping on it with all force I had in my left foot did slow down even in WOT in 3rd, but not as efficiently.

Explanation – servo (brake booster) needs vacuum. Under WOT you do not have vacuum as the pressure is same before and after the throttle plate. Hence, nothing is being pumped out of the intake manifold. This could explain why, if the gas was stuck in the floor, the brakes failed. It did not have to be heat related, but it was enough to push the brakes three times to pump them out and then… panic attack.


I tested it at different engine speeds (1500, 2500, 3500 RPM) and in all cases brakes were pumped out. Also, at 1500 RPM, in 5th gear at speeds around 35 MPH with throttle about HALF way down – I could duplicate it. I did not test much more as I do not have many empty streets here and highways are busy, but I am afraid it might be a problem not only under WOT.
 

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I encourage you to do same experiment, but with huge caution. I am not responsible if you cause an accident. Think twice, and then think again, and again.
Interesting BUT!

I'll take your word for it and suggest that anyone reading this do the same.

Next time I'm out on the Bonneville Salt Flats I will give it a try.
 

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I'm not going to try that one but your logic sound correct. When a car has sudden acceleration the natural reaction is to push on the brakes. Do that a couple of times and you get panic mode when you loose vacuum pressure. I think I'll just turn the key to Acc.
 

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Why does this bring back that scary scenario in the 50's? My dad driving a 1950 Tudor Ford Deluxe with a flathead 6 cylinder up a mountain, raining like crazy, engine pulling the hill for all it's worth while the lack of vacuum kept those old vacuum-operated windshield wipers hardly moving. Couldn't see a dang thing. Going downhill was no problem.

Thank heavens that technology and engineering discovered a way to design and install wiper motors that ran on sparktricity! ;) Kids today missed a treat. That was scarier than any roller coaster ride at Cedar Point!
 

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Back when I was in high school my dad and I were in his Saab 9000 in the rain about five minutes from the school and the subject of vacuum operated wipers came up. Oddly enough at the next light we got next to a 50s Chevy and when the light turned green, he started and the wipers came to an instant halt mid-sweep.
 

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Discussion Starter · #7 ·
I'm not going to try that one but your logic sound correct. When a car has sudden acceleration the natural reaction is to push on the brakes. Do that a couple of times and you get panic mode when you loose vacuum pressure. I think I'll just turn the key to Acc.
Sure, I do not want make anyone to try it, although it is some funny/weird experience to realise brakes will not work... but I wanted to bring it up as a some kind of tutorial how and why under WOT one might not have brakes.

And in case of runaway car (going at max speed on highway) it is in WOT, what might explain the brakes issue.


On the other hand I was wondering if anyone could duplicate it...
 

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This doesn't really seem like an issue... my Camaro does this when I heel toe the car at autocross.

No vacuum, no break assist.
 

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My infamous 1980 Pontiac had the same quirk. Get the car up to 60 mph, step on the brakes, and then watch in horror as the pedal drops to the floor and you leave a 600-foot trail of brake fluid behind you.

Okay, maybe not the exact same quirk :)
 

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My infamous 1980 Pontiac had the same quirk. Get the car up to 60 mph, step on the brakes, and then watch in horror as the pedal drops to the floor and you leave a 600-foot trail of brake fluid behind you.

Okay, maybe not the exact same quirk :)
Thank you, Don! I needed this in the worst way, this morning. I'll be laughing about this for the entire week. "Humorous Post of the Week Award, goes to: Don67!!!" :trophy::clap:
 

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I thought you'd like that one Rich :)

There's nothing like bringing up the old days to put modern car problems in perspective...
 
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