Backfiring under engine braking

[landy_v8]

HI guys,

Went for the first long run in my landrover last night and I noticed that when using engine braking it started to pop and backfire.

I am guessing this is either fueling or ignition related.

Any thoughts / suggestions?

Cheers

Jon

[Coops]

mine does this,
it's where extra fuel is being dragged in and its being ignited,
if it's not doing it when you accelerate or cruising i would not be too bothered about it,

Is your exhaust system leak free???
If it leaks this will cause the popping, as it will draw in cold air which helps ignite the fuel,

i'm sure i will be corrected if im wrong.

[ramon alban]

I'm taking a guess here - is this an Efi car? If so - two possibilities.

The system will have some sort of vacuum operated overrun fuel valve that cuts off the fuel supply when inlet manifold vacuum is high - as in rapid engine deceleration - preventing excess fuel from passing thro to the exhaust system unburnt, where it meets air and ignites randomly in the exhaust manifold and pipes.

Alternatively there is an overrun air valve which allows more air into the plenum under those high vacuum conditions to correct an otherwise over-rich mixture caused by residual fuel, that has already condensed on the inside of the plenum, vaporising in the increased vacuum, passing thro into the exhaust sytem unburnt, where it subsequently meets more air and self ignites.

Hence the random popping and banging.

If I guessed right, your overrun system is not working correctly.

In addition the kid had already pointed out that exhaust system leaks will have a similar effect as described because pure fresh air will mix with any unburnt fuel to cause the same symptom.

[landy_v8]

Its on twin SU's

Thanks anyway. I am going to have a look at the exhaust this weekend.

Jon

[JP.]

i'm sure i will be corrected if im wrong.

I'm shure I won't correct you cause I think you'r right.

Mine does the same and I must say I like it, especialy from the higher revs down

Even our V-Twins does it and in the dark sometimes with blue flames spitting out of the tails.

[Coops]

i'm sure i will be corrected if im wrong.

I'm shure I won't correct you cause I think you'r right.

Mine does the same and I must say I like it, especialy from the higher revs down

Even our V-Twins does it and in the dark sometimes with blue flames spitting out of the tails.

i like it on mine too the odd popping and banging out the tail pipes, does give the public a scare though

[jrv8]

Hi,
Is your Landrover engine fitted with the pulsair system ?

This system can cause severe popping in the exhaust if the mixture is too rich.
Jim

[Lewis]

My 3.5 flapper does this too - single sports system with one box, I think (haven't looked under the car much since I've got it ).

Unburnt fuel being burnt in the exhaust when you slam the throttles shut

[landy_v8]

HI guys,

Is your Landrover engine fitted with the pulsair system ?

No, its a bog standard 3.5.

My main concern is that as I will be using this offroad alot and engine braking is pretty important when crawling down a very steep slope, I dont want to end up loosing power with a big bang and effectivley free wheeling.

Jon

[ChrisJC]

It's a characteristic of SU's - they run lean on overrun. Remember the farting noise old morris minors make when changing gear?

There was a small valve in the butterfly on some variants to prevent very high manifold vacuum, but most people soldered them shut as they tended to give idle problems.

Chris.

[JC.]

Poppet valve.

[landybob_v8]

ive had that on every landy v8 ive owned or driven

same here i like it every gearchange the back would flash up

doesnt do it so much now mines an auto now