Popping when engine braking

[quee2646]

Hello all,

The other day, I was in third gear with no throttle going down a steep hill when I heard a popping sound - like someone flicking a sheet of paper. As a novice, could someone explain whether this is 'backfiring'? Is this okay, or does this indicate a problem?

Thanks

[mogbob]

"Coasting " downhill in gear , with no throttle.. ...depending on the steepness of the gradient...means that the sheer weight of the car is turning the rear wheels faster than the engine speed. In gear , the rear wheels turn the
half shafts , which through the differential , turns the propshaft , through the clutch the fly wheel on the back of the engine is rotated. The crankshaft , attached to the crankshaft is turned which pushed the pistons up and down.
In your scenario the pistons are being "forced up and down mechanically " rather than by the normal method of an exploding mixture of petrol and air ( being ignited by a spark from the spark plug ) pushing the piston down on the power stroke of the 4 stroke cycle. The outcome is everything is out of sequence, there will be unburnt fuel
in the engine which will ignite at incorrect times ( for a normal sequence ). No wonder your engine is making funny noises.... it's a slight back fire and protesting ! Either brake and reduce you road speed or accelerate slightly so that the engine is propelling the car ( you don't want the weight of the car forcing the engine to rotate unnaturally without any throttle being applied.
It is certainly NOT good practice and if you continue it will possibly damage your engine. I have assumed that the engine is fine when driven normally with no back firing.

Bob

[oliver90owner]

Certainly not ‘back-firing’ - the engine is still rotating in the normal direction.

This is termed ‘over-run, not coasting (that is allowing the car to roll without connection between engine and transmission.

It wiill likely be unburned fuel/air mix, from the combustion chamber, being burned in the exhaust - because the fuel to air ratio is not conducive to flame propgation within the combustion chamber allowing other, not fully combusted gasess, to burn in the exhaust system because the gases are still burning after the exhaust valve opens on one or more cylinders. Likely the carburettor is not adjusted optimally.

Modern fuel injection engines avoid this by fully turning off the injection system when slowing down.

[beero]

I thought that this was what Minors were famous for!

[liammonty]

I'm afraid I disagree with Mogbob on this one - I have never heard of a case where using engine braking caused damage to a well-maintained engine, though the reasons he gives for the 'popping' I do agree with. It's worth checking you don't have an exhaust leak anywhere, as this can exacerbate backfiring on the overrun (as per Oliver's comment - not coasting!).

Excessive use of engine braking in a modern car is bad practice as it's unnecessary with today's decent disc brakes, which are very resistant to fade and overheating. It is sometimes necessary to utilise engine braking in a Minor, for example as you were while descending a steep gradient, if you don't want to reach the bottom only to find you have no brakes due to overheating. I'd caution against relying solely on the brakes whilst going down steep hills in a Minor, even with well-adjusted and serviced brakes - they work well, but have their limits!

[Myrtles Man]

Hence the warning signs on some very steep hills: 'Engage low gear now'.

[palacebear]

I stand to be corrected here, but I seem to remember being advised many years ago that exhaust popping (not backfiring) on over-run was likely caused by excessively lean mixture... or am I way off-track? I had a 1300 Marina (HS4 carb) which suffered the symptom but seldom returned less than 40mpg.

[Myrtles Man]

Sounds reasonable but I wonder if it's more likely to be a result of the vagaries of ignition timing/valve timing interaction, as the rather spectacular tendency for some racing motorcycles and cars to spit flames out of the exhaust on the overrun also seems to suggest over-fuelling as a likely contributor.

[oliver90owner]

When the carburettor butterfly is totally closed, theoretically there would be no air flow through it and so no fuel transferred to the engine. Practically, of course, there is always some air flow, and the mixture should be very weak - but it must supply sufficient for tick over. At overrun the vacuum in the inlet manifold will be at maximum, so air may, or will be drawn in from wherever it may leak - the throttle plate is one particular point in any carburettor and especially if there is any wear. So the fuel/air mixture will be weakened. At the same time the inlet will be drawing in air fom the engine, down the valve guides.

As far as compression is concerned, the engine will draw exhaust air from the exhaust on valve overlap, cylinder compression will be relatively poor if there were very little gases to be induced (that vacuum in the inlet side would ‘compress’ to zero volume, in theory.

That all means there is far less gases entering the system commensurate with engine speed, than normal (throttle plate open for normal drive). Any flame front, if at all, in any cylinder would be reduced by all these factors. This will result in very little exhaust flow but with some fuel invariably passing. Any opportunity for a flame path to a reasonable amount of fuelled gas will result in a minor combustion in the exhaust side at times.

Hope that sort of explains it. Certainly not comprehensive, but do forget any change in timing of valves, pistons or sparks - apart from the dizzy which will probably have full vacuum adjustment under those conditions. The engine will be driven at the same speed as it would if driving the rear wheels. - unless your clutch is slipping (very doubtful!!).

Only engines with no throttle plate will create any useful compression on overrun. Some truck engines use that as a means of extra braking effort - look up Jake brake for info.

RAB

[quee2646]

I wasn't expecting my post to draw quite this range of debate! Although I'm still slightly confused...

As liammonty and Myrtles Man mentioned, I only rarely use engine braking (in the Minor) to save the brakes on long/steep hills. Is this occasional practice okay? Is the 'popping' to be expected? or is it a sign of some fault?

Thanks for your patience.

[myoldjalopy]

Problem is, we can't hear the noise! However, it is unlikely to be backfiring as that would be very loud! Have you altered anything recently? You suggest it is a recent issue that wasn't occuring before. As mentioned above, a weak mixture will cause popping or spitting on the overrun, as would water in the fuel, although that is unlikely. Ignition faults like dirty or wrongly gapped plugs or points can cause popping noises. Maybe you have a slight leak at the manifold?
But there is nothing wrong in running a well-maintained and properly adjusted car downhill on the overrun - I do it frequently as there are loads of hills where I live and have no noises other than the classic Morris Minor trumpeting!

[oliver90owner]

Is this occasional practice okay

Whilst a lot of gear trains are designed to ‘drive’ from (normal) input to output, the safety issue is important and should never be ignored! Going down hill with reduced braking can be a disaster in the making!

How many of us simply ‘slack off’ the accelerator and slowly decelerate under mild engine braking when approaching a driving obstacle? It is, among other things, called economical driving.

I drive an automatic and still prefer to change down through the gears as I approach stationary traffic or a corner. One should always be in the right gear for the speed and conditions. I hate auto boxes changing gear at inopportune moments. I remember the firstbtime I ‘kicked down’ in a 3 litre Capri, to overtake. Definitely best to be driving in a straight line whilst doing it!

The minor gearboxes are not now considered high tech, so I, for one, would not be worrying about changing down a gear while slowing, rather than dissipating all the energy as heat through the brakes.

RAB