Dangerous malfunction with no computer code to clue in mechanic

On my daily commute in stop-and-go traffic, on a bridge, headed downhill, suddenly neither the accelerator nor the brake pedal would engage in my 2008 Elantra. Cars in front of me had sped up, then ground to a halt, and I was coasting downhill toward them, at a pace too fast for the handbrake to stop me. I couldn’t downshift either (the car has automatic transmission, but shifting to low lets the engine slow you down). With nothing else to try, I shut off the ignition and then turned it on and, amazingly, all was back to normal. The dealer didn’t find any trouble codes in the computer so they say the car must be OK to drive. But I don’t feel safe driving the car, or selling it to anyone else for that matter. Anyone got an idea of why this might have happened and what I can do to fix it?


“suddenly neither the accelerator nor the brake pedal would engage”

Please clarify

Are you saying that the brake pedal was “locked” and you couldn’t physically depress it?

Is it possible the engine turned off for some reason? That would explain the symptoms, the accelerator wouldn’t work of course, and the brakes, you probably have a brake booster, and so with the engine off, you would have to press considerably harder than normal to cause the car to stop.

Was the gas tank near empty? Is all of the routine maintenance suggested in the owner’s manual up to date? If not, maybe tow it to the and shop and start there. They may spot the problem in the process of getting all the maintenance up to date.

I also have to think the engine died on you while going down the hill. The warning lights in the dash should have been on if that is truly what happened. Did you notice if they were on before you restarted the engine?

It sounds like you’re saying the car simply stalled. If so, if this happens again, you still have brakes. You just need to push the pedal very much harder than normal.

There is nothing electronic that can cause your brakes to totally fail. You may have lost your vacuum assist, making the brakes harder to push, as others have said. All cars since power-assisted steering and brakes were developed have been required to include a purely mechanical linkage that will still work even with no power–it’s just harder to brake or steer with the engine not running. Most vacuum brake boosters also maintain enough vacuum ‘pressure’ for at least one partially-assisted stop, even with a dead motor, assuming they’re working properly.

It might be worth getting up to about 30-40 mph on a straight road with no traffic, putting your car in neutral and turning off the motor to experience non-assisted brakes and steering, to familiarize yourself with the extra effort required and how the car responds, in case this happens again.

It does sound to me like your engine stalled, like everyone else has pretty much suggested.

The OP can count this as one more vote for an engine stall being the only possible cause of this problem.

Because the brakes operate hydraulically, rather than electronically, they continue to function, regardless of any electronic problem. The only difference in brake operation would be as a result of the engine stalling, which would result in the loss of power assist for the brakes, and there is a major difference between “the brake would not engage”, and simply having to use more effort than normal on the brake pedal.

The bottom line is that, if the driver had simply stepped down much more forcefully on the brake pedal, the brakes would have worked. Once the owner/driver learns how to use the brakes when power assist is lost, the only problem will be finding out why the engine stalled.

…and a faulty ignition switch seems possible. If it opens the “run” circuit the engne will shut off just as if you urned it of. No code would be stored because the computer only senses that the engine has been shut off. Ignition switch problems are sometmes intermittent, thus hard to firmly diagnose.

Some makes/years of cars have a reputation for this and recalls have been issued. Some Hondas, for example.

Remember that anytime you turn off the engine with the key, the steering wheel will lock. So if you do an emergency off-on while underway, keep this in mind.

Thanks to everyone for your thinking on this still unresolved mystery. Actually, during the incident the accelerator and then the brake pedal depressed completely to the floor without resistance or effect! Shanonia, thanks for the tip about the ignition switch. I’ll ask the garage to check.

If the brake pedal went all the way to the floor with no effect and the brake fluid level is OK, probably the master cylinder is bad. But more likely you didn’t didn’t push hard enough, and it felt like it was all the way to the floor. See oblivion and VDCdriver (above.)

To echo shanonia’s response, I have to point out to the OP that, while a master cylinder problem is always a strong possibility when someone loses their braking ability, worn-out master cylinders do not “go back to normal” when you turn off the ignition and then restart the engine.

When there is a loss of power assist for the brakes, the pedal feels “hard” or unyielding. If somebody has never experienced this before it is very possible for them to believe that the pedal reached the end of its travel (i.e.–“went all the way to the floor”), when, in reality, it just needed to be pressed with much more force than normal.

When people are in panic mode, they frequently don’t accurately observe or assess what is happening, and that is natural because they are panicked. As a result, I believe that the OP is mistaken regarding the brake pedal “going to the floor”.

Obviously, pushing the gas pedal to the floor will have no effect on a stalled engine, and this reality, coupled with the supposed loss of braking ability leads me to–IMHO–the only possible conclusion, namely that the engine stalled.

Thanks, everyone. Another problem with the same vehicle, related or not to the incident discussed above…
With car fully stopped, my foot firmly on the brake pedal, gear in Park (automatic transmission), when I take my foot off the brake the car sometimes violently bucks forward once or even twice. It has even happened when the ignition is off. Variation: Car is fully stopped, I shift into Reverse to back up and the car bucks forward a couple of times (in one case, hitting a car parked in front of me). I’m precise while shifting and am careful to ensure the vehicle is fully stopped before changing gears to Reverse or Park. I’ve observed carefully and don’t believe this is the result of driver error. Any thoughts?

How the car can buck forward while the ignition (engine) is off is beyond me. From that and other things you say are happening it would make me park the car. It is too risky to drive around in my opinion. Some of the things you describe might be having trouble due to faulty grounding in the electrical system. As for the ignition being off and the car bucks I can only think the ignition power wasn’t getting switched off and the engine was really running at the time the incident occurred.

Everything that the OP has told us so far leads me to believe that the ignition switch is faulty.

I agree with you @VDCdriver. Most of the problems mentioned could be explained by a faulty ignition switch. Though the problem with the reverse shifting doesn’t seem to fit that explanation as far as I can see. Perhaps there is more than one cause for the problems on this car. I’m glad Soobs don’t have these kind of issues.

Thanks again to all who weighed in. Much appreciated.