Modern cars equipped with "start" buttons...

"Pete C." wrote in news:4ac61051$0$25077$ snipped-for-privacy@unlimited.usenetmonster.com:

Point 1: In a panic situation, most non-car-people won't think of the engine over-revving. What I believe they also won't do is think of putting an automatic shifter in neutral. Automatics really only have two controls that are regularly used: Gas and brake. It's only natural that one of those two would be employed as the obvious solution to a runaway automatic-transmissioned car.

Point 2: Modern engines CANNOT over-rev themselves.

Automatics are an enabler for incompetent drivers who would never be able to get a manual transmission vehicle out of a parking space.

This is actually false. Diesel engines can only control engine speed by controlling fuel delivery and there are several possible failure modes in modern diesel engines that can supply uncontrolled fuel to the engine causing it to over-rev to destruction.

Most modern EFI engines including Diesels have a rev limiting feature such that it may go to the red zone but not high enough to blow or damage the engine if it is in good condition. If it is running away, you may not have enough vacuum to operate the brake booster. IIRC, your vehicle has a hydraulic assist booster. In any case, even a blown engine is preferable to a high speed crash in which the vehicle probably won't survive any better than you.

Lugnut

A slide plate to stop the air flow into the engine, whether gas or diesel, would quickly stop the engine.

An old ''Trick'' to dislodge dirt in a balky carburetor is to rev the engine and use the palm of your hand to stop the air flowing into the carburetor. cuhulin

The manual seems to say otherwise. Downshift is considered one of the ways to slow the vehicle. The difference in speed can make a big difference.

I was talking about the option of setting maximum speed below the top speed.

I think none of the models with the problem is diesel. Even over-revving is an issue. That might be the last thing we worry about in that situation.

You are misinterpreting what it's saying.

Downshifting a vehicle with it's engine at idle / closed throttle will provide additional braking action from the engine compression. This is not the case with a runaway engine at wide open throttle, and downshifting under those conditions will reduce the service brakes capacity to slow the vehicle by increasing the runaway engine's torque to the wheels.

That would be unsafe. There are valid reasons for needing to go over the speed limit at times and not having that ability can cause accidents and deaths.

Higher speed lower torque is better than lower speed higher torque? I don't know about that.

How so? If that's the case all cars not able to go faster than the speed limit should be made illegal on the highway.

I'm not aware of any commercially available passenger cars that aren't capable of going faster than the fastest freeway speed limit in the USA (75 MPH, IIRC)

If there *were* one, I wouldn't buy it. If a car is so weak that it struggles to reach 75 MPH, it's probably also dangerously slow to accelerate.

nate

Some electric or old cars are like that.

Not necessarily but that's besides the point. 75 MPH is much better than 120 when it comes to safety.

80 MPH here in TX.

Yep.

Alrighty, let's revisit physics here...

An engine with the throttle closed produces a braking force when it is overdriven by the vehicle's drivetrain. This braking force increases the more it is overdriven. Higher gearing like 1:1 will produce lower braking than low gearing like 3:1. This means that downshifting will provide more engine braking assist to the service brakes.

The situation is entirely reversed if the engine is stuck at wide open throttle and therefore producing excessive drive power, not compression braking. In this case the service brakes are trying to stop the engine in addition to the vehicle mass. A lower gear amplifies the torque from the engine making it harder for the service brakes to stop the engine and vehicle. A higher gear gives the engine the least torque amplification making the load on the service brakes less.

I remember when Texas and Montana didn't have a speed limit.Except in the cities and towns, of course. cuhulin

Don't know about that, but Texas does indeed have 80 MPH on the more remote highways. While the clueless city dwellers probably freak and think an 80 MPH speed is crazy, the roads in question are stereotypical desert highway, flat, straight, wide and with wide level dirt past the shoulders, and 80 MPH is very safe on such roads.

Yes, however there are still conditions where the PCM can loose all control of fuel and not be able to prevent over-revving to destruction. I've seen a writeup of one such condition which occurred due to failed oil seals on turbo bearings allowing diesel diluted engine oil to collect in the CAC until it started to get sucked into the engine, more being sucked in as RPM increased until failure.

Yes, hydroboost, though I expect you'd loose that at excessive RPM as well with the pump cavitating or failing.

Well, the point is that unless the vehicle in question is a Lamborghini or similar, it won't be accelerating so quickly that you can't apply the brakes and stall it.

I still remember when Illinois didn't have a speed limit. When you left the city limits the signs said "resume safe speed".

Montana was actually not that long ago. From 1995 through 1999 Montana reverted to "reasonable and prudent" before instituting a 75 MPH speed limit. So even I remember that :)

nate

It is kind of a moot point though....if the car is accelerating when you do not want it to and you get the car to slow down by trashing the engine, you still come out ahead(read: alive).

A modern -gasoline- engine really cant over-rev unless you get it up to some ridiculous speed in a high gear, and then downshift and let the clutch out to force it above the theoretical maximum RPM. With a slushbox I don't think you could over-rev it even if you tried, as the valve body/computer control would not allow the transmission to downshift in such a way as to over-speed the engine.

The two fuel-injected cars I have will ground all 4 injectors at

7300rpm, give or take a few hundred RPM. I'm sure just about every other fuel injected car out there today has a similar fuel-cut RPM programmed into the ECM.

Chris

The few diesels I've encountered had just such a lever under the hood.

Yeah, but doing that also floods the everlivinghell out of the engine. Sometimes you get lucky when you try this trick, but most of the time you end up with a flooded engine that still needs a carburetor overhaul....

Chris