Some advice for drain and fillers

[Sam2000]

So I thought I would take a break from arguing with antiqueshell and look into a question that had been on my mind ever since I got my Mityvac and had been wondering about partial ATF changes.

Namely, if I do drain and fills, what frequency makes them the equivalent to complete exchanges as recommended by the manufacturer.

So I spent some time on spreadsheets and eventually came up with an equation for any scenario.

I will share my findings in an easy to remember way:

If you are going to drain and fill 50% of your system capacity, and the manufacturer recommendation is a complete exchange, you need to divide the maintenance interval by 3 and do the maintenance at that mileage instead. In other words, 3 drain and fills return an average fluid age equal to the average fluid age of one complete exchange during the same period.

So if you have a 30k mile ATF complete exchange requirement, and your drain and fill is 50%, then if you do drain and fills every 10k, your average fluid age for the lifetime of the vehicle will be the same as it would be with 30k complete exchanges.

One possible advantage of the drain and fills is the narrower range of age of fluid. During a 30k interval you go from all new to all deteriorated at 30k. With drain and fills, you range from 9.1k old after a drain and fill to 19.1k old when it is time to do the drain and fill again.

I'm using my model to look at other things eg if you do a 30% drain and fill of power steering fluid every 12 months or 12k, then that's the equivalent of a complete change at 68 months / 68k.

Also that coolant drain and fills every 3 years after initial fills of 5 years are not equivalent unless the drain and fill amount is 75%.

 

[Sam2000]

I can also quantify the effect of not changing your oil filter or the effective result of 20% of your old oil staying in the engine during and oil change. If 20% old oil remains, then that is the equivalent of an oil change 50% longer than your actual time / miles.

 

[Eddie]

My OEM oil filter holds only about 3% of the total oil capacity. 20% of a 4 quart system must be an abnormally large filter, that I have never seen on a passenger vehicle. Ed

 

[johnachak]

Originally Posted By: Eddie
My OEM oil filter holds only about 3% of the total oil capacity. 20% of a 4 quart system must be an abnormally large filter, that I have never seen on a passenger vehicle. Ed


In many cars I have done, oil is trapped in the passages above the filter when the engine is off too. When I pull my filter a bunch of oil comes out of the oil passage above the filter. At least 1/3- 1/2 qt It appears. That could be the remainder of the extra qt. with the filter change. Just MHO.

 

[Sawdusted]

Originally Posted By: Sam2000

If you are going to drain and fill 50% of your system capacity, and the manufacturer recommendation is a complete exchange, you need to divide the maintenance interval by 3 and do the maintenance at that mileage instead. In other words, 3 drain and fills return an average fluid age equal to the average fluid age of one complete exchange during the same period.


Not true. 3 changes of 50% each will result in 12-13% of the old oil left in the system. Only 87-88% of the old fluid will be removed.

 

[Sam2000]

Originally Posted By: Sawdusted
Not true. 3 changes of 50% each will result in 12-13% of the old oil left in the system. Only 87-88% of the old fluid will be removed.


I'm not saying what you think I'm saying.

Please reread my original post more precisely to understand the meaning.

 

[NMBurb02]

Originally Posted By: Sawdusted
Originally Posted By: Sam2000

If you are going to drain and fill 50% of your system capacity, and the manufacturer recommendation is a complete exchange, you need to divide the maintenance interval by 3 and do the maintenance at that mileage instead. In other words, 3 drain and fills return an average fluid age equal to the average fluid age of one complete exchange during the same period.


Not true. 3 changes of 50% each will result in 12-13% of the old oil left in the system. Only 87-88% of the old fluid will be removed.

True, but Sam2000's calculations are based on average fluid age throughout the life of the vehicle, not the percentage of old fluid in the transmission at any one point of time, with the idea of maintaining a comfortable amount of active additives (or at least I think that is his reasoning). I think he may be on to something.

 

[Sam2000]

BINGO!!!!

 

[doitmyself]

Thanks, Sam2000. This is some interesting stuff to ponder about and I hope a good discussion follows.

The one question I have is about the possible effect of the old fluid on the new fluid. Does the remaining 50% of old fluid cause the new fluid to break down faster? If this was the case, then your estimates are a bit off.

 

[Sam2000]

Yes I hope more people see the value.

Feel free to send me specific numbers and I'll plug them in.

Am also interested in the effect on new fluid. I wonder how Toyota / Honda factored this in.

 

[supton]

Toyota seems to think you no longer need to change oil. If that is true, or nearly so, then I wonder about just changing what is in the pan every 20k. Not bothering to touch what is in the cooler etc, just what is easy to change.

 

[Sam2000]

Give me the capacity and drain and fill amount and I'll tell you what that 20k drain and fill equates to in terms of a full exchange.