Hi
Why is it that in new cars you're recommended not to perform push starts in case it damages the catalyst? I'd have thought it would only be the same as starting a car on the starter motor?
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Mike Farrow
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I thought it was something to do with overrich fuel or unburntfuel or.....something fuel.
It has something to do with petrol I'm sure!
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Adam
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Adski's correct.
www.cats-direct-shop.co.uk/technical-information.p...p
and
www.nationwide-catalytic-converters.co.uk/catalyti...m
Both offer the same advice.
4. Never attempt to bump or tow start your car. This causes unburned fuel to be injected into the cat, which makes the monolith overheat and melt.
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Adski's correct.<<
I know - I'm scared too.
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Adam
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I think they are being over-cautious. I would have thought any brief burst of excess petrol would be burnt off or vapourised and expelled down the exhaust pipe long before it could harm the cat. A prolonged spell of over-fuelling would be another matter.
Caravans don't push cars downhill if their brakes are properly adjusted. In fact the reverse happens. You start down a steep hill a bit apprehensive that the outfit is going to run away, but instead there is a reassuring restraint from the caravan or trailer as its over-run brakes kick in.
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Hi guys
I understand about the extra petrol injected, but why is this different to turning a car over on the starter motor? Does the ECU know when it's a starter turning over the engine and changes the fuel metering or something else I'm missing?
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Mike Farrow
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Well on my 1.3 Fiesta, as the oil light used to take a while to go out, I would floor the accelerator when starting in the morning. This meant it wouldn't start and spun the oil pump up a bit. It still passes the emissions etc so I guess the cat is ok.
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I suspect the real difference they are warning about is not that the ECU can detect what mechanism is turning the engine, but that given the circumstances in which a car usually needs a bump start, it is likely to be slow to fire and so will simply have churned over a lot before any combustion takes place.
A "good" starter will catch on a bump start as quickly as with the starter, but if the engine is in poor condition, the battery flat, electrics possibly damp, the car may have bumped halfway down the hill before it coughs into life, pumping through petrol all the time.
I still think it would have cleared though in the first 2 seconds of running.
There is actually a potential mechanism for the car "knowing" how it is being started, in the form of the 3rd terminal on the starter motor solenoid. That's the one that bypasses the ballast resistor in an ignition system with a ballasted coil. I don't know whether this is ever used though.
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Thanks Cliff, I suspect you're right.
I suspect then that if you're in a non-start situation due to low battery then a push-start would safely work as long as you're confident there's enough juice for a spark and you haven't just been flooding the engine by turning it over with a dying (but not dead) battery.
I only ask after my dads car wouldn't start because the starter motor was taking too much power from the dying battery and not enough was left to power the ignition system, and I wondered whether a push-start would have saved the day!
Thanks again.
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Mike Farrow
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Personally, I think it's been blown out of all proportion, the catalyst is nearly always cold when push-starting ocurrs, so any fuel will have long diffused into the gases before the catalyst gets up to the temperature where it oxidises.
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I agree that, in some instances, there is no difference between starting using your starter motor, or by bump starting. Like cliff hinted at before, the distance the car is pushed before it fires is the most important thing about bump starting a car with a 'cat.
This is why my Volkswagen handbook states (IIRC) that push starting my old Polo (which was fitted with a 'cat as standard) is an acceptable method, as long as its not pushed for longer than 50m.
Therefore i wouldn't hesitate in doing so, but only in an emergency. But I must admit, I had never thought about the possibility of the battery being so flat, the ignition system wouldn't work! But as soon as the engine speed was brought to about 500rpm, the alternator would probably produce just enough to allow the engine to start. In doing so, this would raise the engine speed to about 900rpm, and thus run as normal, so i suppose it would work either way.
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Literal answer - its too much of a stretch for them to reach the bumper?
Being serious, on the occasional cold damp morning that my Micra was reluctant to start (the well known overfuelling problem) it could take a few minutes of churning away in short bursts before it would get going.
This never seemed to do any damage to the cat as the emissions were basically the same at each MOT, and if anything actually improved slightly.
Of course, this was done with the engine and cat cold, so any excess petrol would have been blown out by the time it got up to temperature. If everything was hot and it was struggling to start then it would be a totally different story...
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Yep, I had to drive my Golf 20 miles or so with a failed coil pack and hence no spark on one cylinder. Far from ideal I know, but this didn't harm the cat on my car as the subsequent MOT proved.
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