Backfire

[Dave Meredith]

Just done a search on this without finding much so here goes:
I've got a backfire on my 4 litre Chim. Must be some unburnt
fuel getting into the exhaust. It's occurs mainly on start up but
once the engine has warmed up it isn't quite so bad. It will
drive along quite happily without backfiring but if you put the engine
under load it will backfire on demand. When I say 'under load' I mean
accelerating from low revs in a high gear or simply flooring the loud pedal.

I drove out of a car park recently and as I pulled away the car
back fired and triggered the alarm on a nearby car, so it is becoming
a bit anti-social!

I've done a check with ECU-mate and everything seems normal with no error
codes being reported.

I did wonder whether there might be a head gasket fault which might allow
fuel to cross from the inlet port to the outlet port. The strange thing is
that sometimes the car behaves perfectly. I drove 70 miles to Newbury and
had to be careful with the throttle to make sure the car behaved itself. On the
way back however, I couldn't get it to backfire no matter how hard I tried. In
fact it drove like a dream!

I replaced the plugs recently and ran the engine in the dark to see if there
was any arcing anywhere but there wasn't.

I would appreciate some feedback before I give up and put the car into a garage

Thanks all.

[shake n bake]

Do you have a standard exhaust or a sleeved type and have you changed/checked anything under the bonnet recently?
Is it a loud bang or just a pop and crackle type noise?

[Dave Meredith]

It's a loud bang and it's not been sleeved
All I've done recently is replace the water pump and timing belt tensioner

[zacherynuk]

I think a weak battery could cause acceleration misfires...

Try a full trickle charge and see how she goes.

[Pupp]

When changing the water pump, did you disturb the distributor or any of the vac gubbins attached to it at all? Plug leads pointing at the right cylinders?

[Dave Meredith]

No I didn't disturb anything
Sometimes the car will behave perfectly for 50 miles
so could it be a poor wiring connection somewhere?

[Pupp]

Certainly sounds ignition related...

[Trevor450]

Check the lambda probes. They don't always throw a code. The AFM can also cause an issue and not necessarily throw a code.

[anonymous-user]

I think you should keep it, for comedy value.

My dad used to have a Daimler Dingo armoured scout car which used to pretty much always backfire about 30 seconds after you started it, the neighbours loved it.

[Dave Meredith]

I think I see the comedy aspect of this but when I start it from cold in the garage
the backfire is so loud my dentures nearly fall out. Not a very relaxing experience to be honest!

I've had the plugs out. The plugs all look like this.
No oil or petrol or loose deposits. Just slightly dark.
Looks like it might be running slightly rich

I've read somewhere that if I remove the connections to the Llambda sensors the ECU
uses a default map. If the backfire then disappears it might indicate a fault in this area.
Is there a easy way of checking the air flow meter?

[blitzracing]

You may have a air leak in the manifold or front of the exhaust. This allows enough extra air in to allow any un burnt mixture to ignite. It could also be ignition related like cross firing between the HT leads (are they spaced apart), or a knackered dizzy cap or rotor. ECU diagnostics wont really pick up misfires.

[Malcster]

Timing?