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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

Reply to
David Hearn
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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...

Reply to
Doki

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
Reply to
David Hearn

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.

Reply to
Sandy Nuts

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.

Reply to
Doki

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

John

Reply to
John Greystrong

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
Reply to
David Hearn

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.

Reply to
Sandy Nuts

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

Reply to
Duncanwood

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.

Reply to
Ian Stirling

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

Reply to
Duncanwood

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