Peugeot 306 1.4 1998 engine 'popping'

Our Peugeot 306 1.4 1998 is noticably 'popping' when sitting in neutral
at 2.5k rpm (most noticable then, but certainly present at lower rate at
2k and possibly lower revs). Car has done about 101,000 miles.

Engine runs otherwise fine, always starts first time and pulls away fine
- plenty of power etc.

When the car is at 2.5k rpm the popping is very quiet (from inside the
car) - not loud - but clearly there in the background. Noticable enough
for our 2 year old to say "pop pop"!! If I had to describe the sound
and frequency at these revs, it's actually not dissimilar to the
sound/frequency a bag of microwave popcorn popping! Certainly a 3+ pops
a second.

Oil, oil filter, air filter, fuel filter and coolant (including
flushing) were changed last month. Spark plugs were changed just over a
year ago and have a new set ready to go in when I get the chance. We do
around 10k a year. Most journeys are short to/from shops + work, but
generally at least once a week do journeys 20+ miles. Maybe every month
or so do 100+ mile journeys.

There has been a slight amount of mayo type gunk in the coolant -
although I understand that lots of stop/start journeys can make this
worse. Small amount of oil leaking from the front of the engine at the
head gasket level (not rocker cover) - but don't really have to top up
the oil much, if at all really (maybe every few months - less than 1/2 a
litre each time - min to max is 0.5l I think, and probably half that is
lost/replaced every few months). Coolant temp sits around 90 degrees
(as has always been the case).

I wouldn't be surprised if the head gasket needed doing at some point
(been leaking slighly like this probably most of the time we've had it -
had the car since Oct 2001).

I don't have a compression tester (yet) - and when I tried changing the
plugs before, I had difficulty in accessing the 4th - I think I didn't
remove enough of the gubbins sitting on top of the engine - so I got a
garage to change it just before an MOT. Once I've changed the plugs
myself (and confirmed I can get access to all 4), I might invest in a
compression tester and see what it might be.

Any suggestions as to what the cause of this popping might be - and is
it serious (eg. for the cat etc)?

Thanks

David

 

On an older car I'd say it was fuel burning in the manifold due to a crack
in it, but you shouldn't be getting unburnt fuel down the exhaust in an EFi
car. Possibly crap plugs or leads, but if you have platinum plugs the
interval will be 60k or so...

 

No leads on the car - coils mount directly onto the plugs it seems -
certainly nothing like my old C reg Polo etc which had flexible leads to
the distributor. And yes, it's electronic fuel injection.

D

 

Bollocks. You'll never get a 100% clean combustion 100% of the time, no
matter what fuel system is used. I've frequently heard of cars with cracks
in the manifold, or leaks in the downpipe, back firing. They aren't so
prevalent now with modern day exhaust systems, catalysts and exhaust baffles
to dampen the sound, but it still happens.

 

I never said it would be 100%, but you shouldn't be getting big quantities
of it. The air flow sensor should be able to make a good enough guess and
the lamdba will make 99% sure. If they didn't, catalysts would be dying
pretty quickly.

 

Might just be overfueling 'cos the ECU doesn't expect you to be sitting
at 2.5k whilst not going anywhere.

John

 

I think the popping is also there when driving under load, but obviously
less noticable due to road noise etc - easier to recreate when stationary.

D

 

There doesn't need to be. There are certain parts of the fuel map that will
always be out due to manufacturer tolerances. 2.5k while under 0 load is
probably one of them. So even if the lambda did detect overfueling, the
unburnt fuel will already have been out the ports, down the manifold and
sitting ready to be introduced to a little oxygen.

And they do. Things like faulty air flow meters, intake charge temp sensors
and shagged lambda sensors all attribute to less than perfect running. You'd
be surprised as to just how imperfect EFi can be, if you ever have a car
you're about to bin...try drilling a small hole in the downpipe and go for a
run Over-run is the best time to listen out.

 

If there wasn't some unburnt fuel then you wouldn't need a cat.

 

Actually not.
One of the reasons for the cat is to remove unburnt fuel.
The other is to remove CO/NOX.
Annoyingly, it needs unburned fuel to remove these, which can be a
problem for lean-burn.

 

I suppose if we're being pedantic, you only need CO to remove the NoX, but
even CO can go pop.