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What kind of engine demage can starting the car with engine runing cause?

I have a 2006 Matrix which runs so quitely that I inadvertently turned the key to start the car again while the engine was already runing. What kind of damage can it cause? I would assume that there are a lot of people who may have done the same thing.

Thank you so much.

It’s not an uncommon occurrence. It happens. The most damage will typically be to the starter and specifically the gear at the end of the starter that engages the engine when you turn the key. If I remember right, most manufacturers use a softer metal on the starter gear so it will be destroyed first since the starter is typically much, much easier and cheaper to replace than the corresponding gear on the engine crankshaft.

In essence, you probably shortened the life of your starter. How much? I don’t know. We could run some math, but most likely you won’t notice for years unless you make this a habit. If the Matrix has a tachometer, maybe make it a habit to check it before you turn the key. That way you know it is not running.

The worst that could happen is knocking a tooth off the flywheel in which case depending on the car the engine or tranny or both could have to be pulled to replace the fly wheel. More common is stripping the starter’s bendix gear. You’d be out a starter or have to have the starter rebuilt (much cheaper than new, and a better option since parts stores typically sell rebuilt starters instead of new anyway.)


Additionally, the starter drive gear (aka “bendix”), is mounted on a one-way clutch. That means if you engage the starter while the engine is running, you won’t have a big flywheel spinning a small starter at speeds that would rip the starter windings wide open. That ‘one-way clutch’ does allow for some amount of starting of an already running engine - for all definitions of “some”.

Occasionally ok…repeat offender daughter on older Mazda PU managed to damage the flywheel enough so that it required a little roll once in a while when the starter lined up with the missing teeth. Good to hear that teeth in starter are “softer”. Why don’t all cars have cut offs to prevent the starter from even engaging ? Doesn’t seem like a big deal add on and could save lots of aggravation.

My wifes 96 Accord and every car we’ve owned since if you turned the key when it was already running…NOTHING. The circuit was smart enough to know that the car was running and actually disable this operation from happening.

Thanks so much for describing the relationship between the starter and the flywheel. I will write down the date of the incident and keep an eye on my starter a few years down the road.

Thanks for sharing your experience, and it’s good to know nothing has happend to your car.

But Mike, there are those of us who prefer the simplest and therefore least likely to fail and easiest to repair systems. My truck will start without stepping on the clutch, my Jeep and Blazer will shift out of Park without stepping on the brake and start in gear. Simpler is often better for so many reasons. And I don’t mind using the key to lock and unlock the doors.