Black oil o in Diesel engines - A Macdonald
What is so different about the combustion process in the cylinfder of a diesel engine that turns oil black after very little use.
Even in brand new diesels the oil seem to go black very rapidly.
Does this affect lubrication, ? why doesnt the filter pick it up. Should oil be changed MORE frequwently in diesel that Petrol engines whose oil often remains clear often even in older engined and even after many thousands of milkes.
Re: Black oil o in Diesel engines - Dan J
A diesel engine is dirtier engine than it's petrol counterpart. As heavy oil is burnt in a diesel engine, there is a much greater tendency for soot to be produced than with petrol by virtue of the compression combustion process. This would dirty the oil faster anyway but also given the much higher compression ratios of a diesel, more unburnt fuels and carbon deposits escapes past the piston rings than in a petrol engine leading to a much faster blackening of the oil.

It is very tiny particles that cause this blackening and subsequently they are not picked up at the filter (like you running dyed water through a filter - no matter how good the filter, the water will still come out coloured).

Diesels generally have shorter service intervals than petrols anyway and should certainly receive more frequent oil changes than a petrol. I know several people here (David W!) change the oil of their diesels every 3000 miles but I am sure 5-6000 would be adequate as well.

Re: Black oil o in Diesel engines - Moosh
"Diesels generally have shorter service intervals than petrols anyway"
Not anymore!
The latest modern direct injection diesel engines service intervals are the same as their petrol counterparts at least 10.000 miles.
Re: Black oil o in Diesel engines - David W
Perhaps that ought to have said...

If you want them to last.... "Diesels generally have shorter service intervals than petrols anyway"

Yes it is true they are pushing the diesels out to 10,000mls but in turn the petrols are being pushed up to 20,000mls.

Re: Black oil o in Diesel engines - Dan J
Only for the benefit of UK fleet managers though? I can't see anyone with a good knowledge of their car (and a heart!) leaving 20000 miles between an oil change whether the car runs off petrol, diesel or vegetable oil!

It'd be a chilly day in hell the day you let one of your Xantias go that long without an oil change huh? :~)
Extended intervals - David W
You know I don't approve of excessive service intervals, merely mention the fact that is the way the makers are moving.

I've seen a couple of engines around 70,000mls that have been trashed by leaving the oil in from 40,000mls, plus running low all the while.

Five litres of oil is the cheapest maintenence you can get.

Re: Extended intervals - Dan J
Absolutely, and the benefit will be the engines you maintain will last years (fingers crossed!).

I have little doubt that anyone in the motoring trade gets to see these awful cases of neglect all too often. 30k is a horrific time to go between an oil change. Independent who services my car when I'm not doing it myself says he gets people in who treat their cars in this way and when something goes wrong expect him to "wave a magic wand" and sort it all out but don't want the car serviced afterwards!

Do you find cars you see generally tend towards the better or worse maintained than you think appropriate?
Extended intervals - David Lacey
15K Service intervals are fine aslong as the recomended oil is used and the driver checks the oil at 2/3 weekly intervals.
The dangers come when nobody lifts the bonnet between service intervals and the trainee technician uses the wrong grade oil.
We have seen many cases of driver neglect - engines running dry. Then the headaches start......

The Rover 75 Diesel has 12000 mile service intervals - BMW technology, as some people will say.....this car uses the 2 litre M47R BMW common rail unit.

30K between services is too long though.

Re: Extended intervals - Stuart B
"the driver checks the oil at 2/3 weekly intervals"

David, if I did that on the Vectra it really "would" be off the bottom of the dipstick. dip twice a week, top up roughly 2 weeks in three, and I am definitely NOT overfilling.

see previous threads.
Re: Extended intervals - me
had a vectra 3 years and it never needed any oil, other than at service intervals, i did check...
Re: Extended intervals - Peter
Too true, my neighbour runs a company car, a T reg BMW 318. It failed recently on the M4. The engine seized at high speed due to a lack of oil. The oil low level warning light did not come on to tell him the engine needed oil and he does not consider it to be his job to lift the bonnet and check such trivia it is hardly surprising. He told me the repair bill was around £3K.

I do not suppose he is alone in such thinking.
Re: Extended intervals - ROBIN
The so-called indirect injection diesels have some version or other of the ricardo Comet V combustion,or pre-combustion chamber.This is grossly inefficient and produces quantities of unoxidised carbon,this is wasted energy and gets into the oil.
Since no manufacturer can be arsed to fit a simple bypass filter to remove it there it stays.These particles are abrasive and thicken the oil,thus making it suitable as a grinding paste but less ideal as a lubricant.
So, on these,thankfully nearly extinct, engines you change the oil as often as you can.If it makes it more likely that you will do it why not change the oil at 5000 and the filters at 10000?.
Direct injection engines,which have become available since car manufacturers suddenly realised that the electronic injection systems used for petrol engines worked just as well with diesel,paraffin,draft Guinness and pretty well any other liquid,are more efficient so burn more of the carbon.Indeed,the oil in my HDi 110 still looks like oil at 1500 miles.
What should happen next is that some manufacturer reads Dr Diesels original work and realises how we should have been doing it all along.
From memory I think its Plasma Charge......
Re: Extended intervals - Darcy Kitchin
Until I read your post I thought graphite (carbon) was a lubricant. Soot is largely carbon so how is it suddenly abrasive?
carbon types - Ian L

The carbon atoms in graphite are aranged in sheets which easily slide over each other...hence a lubricant. Conversely the carbon atoms in diamond are arranged in a tetrahedron....definitely not a lubricant! Not sure of the atomic arrangement of carbon in soot particles but I would hazard a guess that it is more like a loose diamond structure than graphite.


Re: carbon types - Darcy Kitchin
Thanks Ian
Fuel versus Oil Prices - Brian
We all know that UK petrol/diesel is around 70p per litre and that 75 percent of that is tax.

Does anyone know why decent oil is between £3 and £5 a litre and how much of that is tax?
Re: Fuel versus Oil Prices - Moosh
I don't think it attracts the big tax.......excise duty, just VAT @ 17.5%
Re: Fuel versus Oil Prices - peter todd
we use texaco 15w-40 detergent oil in all our vehicles & tractors-205L is currently £145 +vat. because I needed 5w-30 for my mondeo & my supplier did me 25L texaco havoline @ 29.95 + vat both being delivered to the door by weekly truck from suppliers agent in east anglia!
Re: Fuel versus Oil Prices - Paul
I had a Montego, leaked oil everywhere - seals, sump plus piston slap / valve wear.

After it had been left standing for a couple a days, it blew black smoke out for a few miles.

I just topped it up with about a pint of oil every week {did 400 miles per week} - used £2.99 ASDA oil - 12 gallons of fuel to 1 pint of oil - a two stroke Montego.

At least the engine always had fresh oil in it.
Re: Fuel versus Oil Prices - ChrisR
Larger diesel engines (buses, trucks) have always been direct injection. The electronics just make it possible with the tiny half litre (or less) pots we have in our cars. Hence the new VW 1.4 diesel is a 3 cylinder unit. Someone will correct me but isn't 300cc about the minimum viable size for a diesel cylinder? Yet that size gives maximum efficiency for a petrol engine, hence F1 cars have ten cylinders per three litres.

Re: Fuel versus Oil Prices - Moosh
Times are changing for diesel car engines. VW are planning a V10 diesel engine for their future luxury car the D1 to go on sale next year.
[From article in motoring section of today' Financial Times 17/11/01].
Re: Fuel versus Oil Prices - Brian
And there was me thinking that the multiple cylinders in F1 engines was to make them quieter and smooth running!


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