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Fuels & Lubricants Discussion all about Fuels & Lubricants. synthetic oil, conventional oil, brands, change intervals, diesel grades, gelling and such debated items like that. Non TDI related postings will be moved or removed. This forum is NOT for the discussion of biodiesel and other alternative fuels.

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Old February 11th, 2000, 09:38   #1
Turbo Steve
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Default Oil Change 101 - Determining Oil & Filter Change Intervals

So, How Often Should I Change My TDI's Engine Oil?

Let me start out by saying that "you can never change your engine oil too frequently!" The more you do it as needed, the longer the engine will last.

The whole debate about exactly "when you change your oil" is somewhat of a grey area. Manufacturers tell you every "7,500 / 10,000 miles" or so. Your neighbor with a classic or luxury car tells you every 3,000 miles. Your other friend who drives a truck tells you he's "never once changed the oil."

The bottom line here, is: large quantities of water are produced by the normal combustion process of diesels, and, depending on engine wear, some of it gets into the crank case. If you have a good crankcase breathing system, the water is removed or burned off. Even so, in cold weather a lot of condensation will take place.

This is bad enough in itself, since water is not noted for its lubrication qualities in an engine. Even worse, water disolves any nitrates formed during the combustion process and leaves you with a mixture of Nitric (HNO3) and / or Nitrous (HNO2) acid circulating round your engine! So not only do you suffer a high rate of wear at start-up and when the engine is cold, you suffer a high rate of subsequent corrosion during normal running or even when stationary.

The point I'm trying to make is that the optimum time for changing oil ought to be related to a number of factors, of which distance traveled or time on the hour meter is probably one of the least important in most cases.

Consider, for example, my selection in rough order, the areas of greater importance:

1 - Number of Cold Starts and city versus highway driving (can produce more condensation and acidic buildup in a cold engine)
2 - Outside Temperature (how long before the TDI engine warms up enough to stop serious condensation)
3 - Effectiveness of Crankcase Ventilation System (e.g. PCV)
4 - Condition / Wear of the TDI Engine (excessive piston blow-by multiplies this problem)
5 - Efficiecy of Fuel Injectors and Quality of Fuel (a clean spray pattern can create less soot)
6 - Distance Traveled or Time on an Engine Hour Meter

The first three items should be given equal top billing or weighting. They need to be taken together since a given number of "cold" starts in the hot summer are not the same as an equal number conducted in cold January. The effect in either case will be modifed by how much "blowby" gets past the pistons.

What we are really after here folks is the severity and duration of the initial condensation period. All other things being equal, that will tell you how much condensation / acid is probably being produced in your crankcase.

More than anything else, these key six areas should also provide a good guideline to follow, for matching oil & filter change intervals to your own "unique style of driving."

Example, we know that VW "recommends" changing the oil in a TDI engine every 10,000 miles. Most people forget or haven't noticed that this is for "NORMAL Conditions." Jon Bartlett's type of highway driving is a perfect example of operating under "Normal Conditions."

Obviously, 90% of us do not fall into that same category and need to be reminded that VW recommends "more frequent oil changes ... [for] SEVERE Conditions," one of which is described by them as "short trips or operating under extremely low temperatures."

I suggest that we consider evaluating our percentage of city versus highway driving (taking into account the number of cold starts) and then outside ambient temperature, in order to establish or estimate how long we can go before changing our diesel's oil.

Obviously, the quality of oil comes into the picture here (another story ), but this should not be as important as confirming your own "unique plan" (frequency of oil changes) through oil analysis on an occasional basis.

[This message has been edited by Turbo Steve (edited February 11, 2000).]
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Old February 11th, 2000, 11:26   #2
TwoSlick
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Default Re: Oil Change 101 - Determining Oil & Filter Change Intervals

This is why we've been recommending 5000 mile change intervals if you use a CF rated synthetic or a petroleum diesel oil - it's conservative.
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Old February 11th, 2000, 11:40   #3
mickey
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Default Re: Oil Change 101 - Determining Oil & Filter Change Intervals

That's also why it pays to run a few oil analyses, until you're comfortable with your chosen drain interval. No sense in changing it too soon, since you'll waste money.

-mickey
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Old February 11th, 2000, 11:43   #4
SkyPup
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Default Re: Oil Change 101 - Determining Oil & Filter Change Intervals

With the slow to warm up TDI the water condesation situation is even more critical than in a gasoline engine, especially if the TDI is engaged in short haul trips in and around town, most especially after sitting cold overnight.

The TDI needs to be driven a minimum of five miles just to get up to minimum operational temperature and then at least another 5 miles to forst begin to burn off the water condensed in the crank through the PCV valve. This kind of driving cycle operation is very demanding of the oil. In addition to the nitric acid formation resulting from the natural 80% concentration in the atomsphere, sulfuric acid from the diesel fuel is also formed. Sulfuric Acid (H2SO4) has twice the proton equilivancy mass per mass of Nitric Acid (HN03) and therefore chemically depletes the TBN reserve buffering capacity of even the world's best oils rapidly. Complex sludge formation is the result of this with corresponding heat increases throughout the engine, blocked oil lines, blocked oil jets, and blocked oilfilters, all in a rapid downward sprial of no return.

High speed passenger car diesel engines are entirely different than low speed large displacement truck diesels and should not be treated the same.
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Old February 11th, 2000, 13:26   #5
TwoSlick
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Default Re: Oil Change 101 - Determining Oil & Filter Change Intervals

Skypup,

You are entirely correct about the low oil temps ...this is the reason why the oil analysis results show very low rates of oxidation. My experience with gas engine VW's and oil analysis shows quite the opposite; reduced nitration and more oxidation of the basestock ....

TS
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Old February 11th, 2000, 14:41   #6
ThinkDiesel
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Default Re: Oil Change 101 - Determining Oil & Filter Change Intervals

Turbosteve said: "The bottom line here, is: large quantities of water are produced by the normal combustion process of diesels..."

Really? I'm no expert, but this doesn't sound right to me. Can you explain it in layman's terms?
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Old February 11th, 2000, 15:03   #7
TwoSlick
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Default Re: Oil Change 101 - Determining Oil & Filter Change Intervals

A small amount of water vapor is produced by combustion and most goes out the tailpipe. The part about the acid forming reactions is simplified, but largely correct ....This process also requires oxygen from the air and the addition of heat.

TS
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