Smoky exhaust

BX Tech talk
User avatar
1K Away
1K Away
Posts: 1383
Joined: Tue Jul 05, 2005 7:54 am
Location: Usually in the garage
x 12

Smoky exhaust

Post by Jaba » Mon Feb 05, 2007 10:17 pm

I was travelling through the Alps last week with a mate following me. When we got to our destination he told that my TD is emitting blue smoke - not all the time- but whenever going downhill on a long descent down a mountain pass. This would typically be in a low gear for 10 to 20 minutes.
It was bad enough to make him switch his deutschmobile's air intake to recirculate.

I have subsequently checked my mirrors and cannot see any smoke so it is not a huge cloud that is being chucked out. The car performs normally does 10 miles per litre and does not use much oil. If it was a GTi then I would confidently diagnose valve stem seals with a small possibility of piston rings too.

I am no diesel expert but I thought that diesels did not produce the vacuum to suck oil down the valve stems.
Anyone any helpful thoughts ???
The Joy of BX with just one Citroën to my name now. Will I sing Bye Bye to my GTI or will it be Till death us do part.

Over 2k
Posts: 2112
Joined: Fri Jul 01, 2005 2:58 am
Location: Hampshire UK

Post by jeremy » Mon Feb 05, 2007 11:47 pm

That's right and I think its right to say that all TD's (and maybe N/A as well) have bronze valve guides and no seals. Seals were introduced later, probably mid 90's. Search this board - and you'll see this isn't a problem.

When your engine is on the overrun there is no fuel being injected - so nothing to ignite any lubricating oil getting into the cylinders or the back of the valves. This happens on fuel injected petrols - but even carb ones don't burn very hot then which is why you get that characteristic cloud of blue smoke out of a petrol engine when you accelerate at the bottom of a hill.

Diesels tend to be bad liars - so if there was bore/ring wear I'd expect it to be reluctant to start and to fume excessively from the crankcase. By this I mean that there would be oil all over the engine as it blew out of joints you didn't know it even had - and if you removed the filler cap with the engine running you would have to stand back in order to breath.

I'd have thought that climbing the Alps would have burnt out any muck collected in the exhaust but I suppose if it were really filthy it might have started to burn in the silencers and the fresh air being pumped down the exhaust when going downhill would keep the fire going. Once everything had burnt out no further fumes would be produced.