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Old 06-12-2008, 09:57 AM   #20 (permalink)
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Okay, so bump starting might not be that bad for the clutch, but what about the catalytic converter. The owner's manuals for both of my vehicles say that the cat might be damaged if I push start my vehicle. Is that the same as bump starting or are they talking about something different?
If the ignition is on and the engine starts right away that is fine. If some defect creates a condition where fuel is sprayed into the engine with no spark to ignite it, that fuel will ignite in the cat if the cat is hot. Do that long enough and the cat housing will melt.

I'm sure one of you engineer types could come up with the forces involved, 1st gear start vs low speed bump start in 5th. How much load the clutch carry for each.
I don't think the clutch is the issue. There is a thrust bearing, often at the center crankshaft journal. This bearing resists the push of the throwout bearing when the clutch pedal is pushed in. When the engine is stopped, there is no oil pressure (obviously). The thrust bearing is getting no lubrication. Every time the engine is restarted, that thrust bearing wears a little (is is just a plain bearing shell, no balls or rollers). This is also true on cars that make you press in the clutch to start them. But usually cars are not started and stopped that often over the life of the car. And you don't want to have to go in there to change that thrust bearing. That said, I do bump start by car sometimes just for the practice.

would I be better off to pop the car out of gear then just a quick clutch to put it back in or will holding the clutch in for the period of the glide be safe on the clutch/tranny?
If you keep your foot on the clutch pedal a lot you will wear out the clutch throwout bearing. If you are in neutral, (clutch pedal out,) a lot with the engine and transmission turning at different speeds, you will eventually wear out your pilot bearing, but that is not too common.

it seems revving the engine as you slow down would use more fuel even if you're not on the throttle.
No, because the throttle body or carb butterfly valve is shut. So the engine is not getting air, and not using extra fuel. (unless you are driving a car with a carb from the 1960s). This is a case where the engine speed is high only because the momentum of the car is spinning the engine. Not because the engine is using fuel. Another advantage of slowing the car with engine drag is it encourages smoother driving, which is less wear on all mechanical parts.
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