Popping on deceleration
Discussion
simonharrod911 said:
I just spent a grand so this DOES happen!
I've previously spent thousands, trying to make this happen..
Popping on overun is caused by over fueling.. so that when you come of throttle, excess fuel is then burnt in the exhaust, causing the pops and bangs..
Modern management systems on most cars are so efficient, that this does not normally occur.. the exceptions being cars like TVR.!
I used to think it was cool on my Caterham on twin 40 DCOE Webers. Then an engine builder pointed out that it's actually not a good thing... I'm with UV on this one.
As for getting it fixed, well, it's going to be a mixture problem, but that could be caused by many things. Fuel injected cars with full mapping are pretty easy to diagnose though. Might be a sensor out.
As for getting it fixed, well, it's going to be a mixture problem, but that could be caused by many things. Fuel injected cars with full mapping are pretty easy to diagnose though. Might be a sensor out.
It's difficult to say for certain.
If it's a turbo, this is likely to happen, especially if you go from hard on the throttle into idle throttle and engine breaking quite quickly.
If the turbo is spun up to full tilt, and demand disappears then the fuel air mixture has to go somewhere. Often this is up the hot exhaust.
If it's just under heavy engine breaking, I reckon it'll be due to incorrect mixture.
The throttle plate is closed so little air enters the engine, but the suction from the engine is severe. If the engine is fuel injected, you'd think that the ECU would regulate the fuel appropriately. Obviously if it's carburetted, it's difficult to control. But either way, any fuel drawn through the engine collects in the exhaust.
I'd guess that under engine breaking the mixture is too far wrong to combust in the engine it's self, but the fuel can still go bang in the hot exhaust.
With little air movement through the engine, the mixture can potentially improve, relatively slowly, in the hot exhaust where it detonates just slightly lean, without the need for a spark.
There could be something wrong, maybe a sensor has packed up, maybe the ECU map has been fiddled. It could be a dribbly injector.
Whatever, the more severe the transition from throttle open to engine breaking, the more likely it is to occur. Also if you're going down a long hill under engine breaking, there's a longer period for the engine to achieve the right mixture in the exhaust.
Edited to add;
I'm sure there's plenty wrong with what I've said. Please feel free to correct/elaborate!
Further edited to add;
Of course it could also be clapped exhaust valves!
>> Edited by dilbert on Saturday 25th June 21:00
If it's a turbo, this is likely to happen, especially if you go from hard on the throttle into idle throttle and engine breaking quite quickly.
If the turbo is spun up to full tilt, and demand disappears then the fuel air mixture has to go somewhere. Often this is up the hot exhaust.
If it's just under heavy engine breaking, I reckon it'll be due to incorrect mixture.
The throttle plate is closed so little air enters the engine, but the suction from the engine is severe. If the engine is fuel injected, you'd think that the ECU would regulate the fuel appropriately. Obviously if it's carburetted, it's difficult to control. But either way, any fuel drawn through the engine collects in the exhaust.
I'd guess that under engine breaking the mixture is too far wrong to combust in the engine it's self, but the fuel can still go bang in the hot exhaust.
With little air movement through the engine, the mixture can potentially improve, relatively slowly, in the hot exhaust where it detonates just slightly lean, without the need for a spark.
There could be something wrong, maybe a sensor has packed up, maybe the ECU map has been fiddled. It could be a dribbly injector.
Whatever, the more severe the transition from throttle open to engine breaking, the more likely it is to occur. Also if you're going down a long hill under engine breaking, there's a longer period for the engine to achieve the right mixture in the exhaust.
Edited to add;
I'm sure there's plenty wrong with what I've said. Please feel free to correct/elaborate!
Further edited to add;
Of course it could also be clapped exhaust valves!
>> Edited by dilbert on Saturday 25th June 21:00
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