Will a manual vehicle stall at low RPM without being on the accelerator?

Question

Say you're in third gear going in a straight line with no need to stop. You let off the accelerator and downshift to second gear then coast with no pedals in use. Will the car reach a certain RPM and then stop decreasing, making the car crawl forward at a slow but steady pace? Will the car keep decreasing in RPM until the engine doesn't have enough torque to continue running, resulting in a stall? Does it depend on the vehicle in question?

Once you've started the car from a full stop, will the engine ever stall, assuming fuel isn't an issue and you don't touch the pedals?

I'm asking because my car will not stall in first, second or third gear on a flat surface. I'm just wondering if there's something wrong with my engine or transmission or otherwise. For clarification, I drive an Acura RSX Type S, so it has a pretty high strung engine which is where my concern comes in.

Answer 1

In my experience, most manual cars can move along in 1st gear at idle – on a flat(ish) surface. I've only tried this in the process of teaching people to drive a manual. My hunch is that in 2nd and almost certainly in 3rd or higher you'd stall most engines.

But the question is "does the engine have enough torque (and fuel) at idle to move the vehicle?" If it does, then it will keep going in whatever gear that condition is true. With a tractor (either variety) I'd bet that they can cruise along at idle in the higher gears as well, since they are designed to pull large loads.

If you add a hill, a car will almost certainly stall as it tries to climb it. Or not…

Your question inspired some experimentation, I had to go out for a drive this evening and tried driving in gear at idle. It worked, all the way up through 5th gear. Even going up a pretty steep hill in 1st. I think what is happening is that the ECU adds fuel to maintain idle RPM and so prevents the engine from stalling as long as the load is reasonable. I could cruise at about 25 MPH (40 KPH) in fifth on relatively level terrain, but the engine struggled to climb any kind of a hill in 5th.

Answer 2

Obviously in a manual car with the clutch engaged there is a direct connection between the wheels and the engine. Also while switched on most engines have an idle setting designed to keep them running under no external load.

Whether a specific engine will drag itself forward on idle will depend on its torque characteristics and the engine mapping, in some cases the ECU may actively try to prevent stalling and all else being equal a large capacity engine is going to be more inclined to do this than a smaller one .

While the car is moving under it's own momentum the engine can't really stall untill it comes to a virtual standstill as stalling is, by definition, the case where the engine no longer provides enough torque to overcome its own internal friction and compress the cylinders (although compression is a fairly substantial braking force itself).

Answer 3

I had never drive an automatic car, all my cars have being manual: depending on shifted gear, road angle and load, they keep moving without accelerating the engine. In fact, if I want to drive really slow on a flat road, I would shift 1st or 2nd and let it run by itself. If the engine has a carburetor, sometimes their adjustment makes this easier, if they are adjusted with a bit gas/air mixture. A trick for this is to gear on, but slowly release the clutch so the engine won't get scared :)

Answer 4

Each gear has a minimum speed that it can cruise at without any throttle input. So unless you have a lot of load on the engine, e.g. climbing hills, your car shouldn't stall if you don't give it any throttle, and it'll just cruise at that minimal speed.