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Generally it is only 4 cylinder engines which have oil consumption issues do to the bore surface roughness from the factory. Normally they don't get worse as long as good oil is used which stops the piston rings clogging up. Quite often, they get markedly better after 100k miles.

Oil can make a significant difference not only in grade but also how reactive the oil is (how easily it burns). There is a test which is part of oil specification/certification which suggests how easily the oil will burn. If memory serves, it is suggested by the NOACK scale.

First thing to check is the oil separator in the camshaft cover is not clogged and doing its job. A tell tale sign is much oil at throttle body and into inlet manifold. Of that is ok, using a different oil may help. Unfortunately oil producers like to hide how the oil actually performs in tests under a load of marketing descriptions. Ideally a TS or JTD would use 5W/40- good low temperature flow but decent film strength when hot. A PAO or Ester based oil is better. Terms like synthetic are meaningless. A 5W/50 would still tend to burn quite quickly (kinematic viscosity not much more than a 40) and the Motul 5W/50 sport is quite expensive. Initially, a change to 10W/60 may work but I'd suggest using a high detergency flushing oil (available under Comma and Granville brands) to give the piston rings a clean. Something like the Motul X-Power 10W/60 which meets the API:SN specification is slow to burn off and by nature of the aforementioned specification, maintains good piston cleanliness.

Low grade oil and urban running can clog up the rings so a flush, decent oil change and filter is a good precursor to the hammer blow; give it a good kicking afterwards. The heat is actually a good thing as it burns off contaminants and actually prevents sludge build up. In my experience, a mollycoddled TS will have more oil issues than an enthusiastically driven or track day car will have.

It may be necessary to run a thicker oil for a couple of oil changes but if conditions are favourable, it should be able to revert back to a 5W/40 as long as no major issues have developed. If they have, expensive surgery is the only cure.

Beware running with the engine management light on/defective O2 sensors/very blocked air filters as they can make the engine run rich- possibly rich enough to cause some bore wash wear.
 

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New rings are the only solution, plus a hone if the bore is polished.

A lot depends on how the car was run in when new. If it was run in too gently by the conscientious new owner, the ring surfaces become polished. This is bad for oil consumption as oil flows past the rings, but good for longevity. More aggressive running in caused micro-welds between the ring surface and bore, which tear as soon as they are formed to create a microscopically rough surface that retains oil and forms a better seal.

This was first noticed on racebike engines during the 1980's. Some riders ignored old-school run-in advice, thrashed newly built engines and ended up with slightly more power, although possibly shorter motor lifespan. That was counterintuitive to team managers like Mick Grant (former TT winner and Suzuki mgr), who had engines torn down to find out what was going on. The only real difference was the ring surface. The conclusion was that the careful run-in procedures of earlier years were no longer as necessary because engineering tolerances, materials and oils had improved.

Grant was equally informative about synthetic oils, but I've posted about that here before. They were too good for effective running in to be possible, so he recommended the first few hundred miles be done on mineral oils to ensure nice rough ring surfaces, then changed to synth.

This MO later became the standard advice for many mfrs on road bikes as synth became more common. Now, nobody bothers, the engineering tolerances are so good and the marketing requirement is for an appliance that needs no attention except once a year. Running in advice persists, but abbreviated to 'don't labour it or thrash it'. However you will still find people who think the 1960's style prescription of carefully graduated run-in over 1,000 miles is best. It isn't. It's a recipe for an engine that will forever drink oil and be slightly down on power.

Links? I've no idea, I haven't looked. I'm sure this is all well-known, now. I freelanced for Bike magazine in the 1980's, mostly photography, but also as a writer. This is from a conversation I had with Mick Grant. We published something about it, but I don't remember when, mid 80's I expect.
 

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Any leaks? I had a 2.0 TS 156 that went through a similar amount. There was a leak from the rear main seal - between engine and gearbox - which was the main culprit. No smoke etc, so I don’t think it was burning much. A combination of high mileage (161k when I got rid) and the leak really.
 
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