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Premium Member
06 2.5 XT MT
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242 Posts
Discussion Starter · #1 ·
One to do on your tea-break.

Id like to more accurately know how many times, using the turkey baster method on the power steering reservoir, it would take to replenish the old fluid with maybe 80 or 90% new.
The guides say "do this a few times with a drive in between" or similar but I was after an answer that was derived using actual data rather than gut feel.

The following is an assumption for this game.
The total power steering system holds iro 650ml (?). Of that, 250ml (?) is held in the reservoir.
250 as a % of 650 comes out at approx 40% new fluid replaced but then mixed in with the old.

How do you then work out what another 40% change of new, to the now old/new fluid mix, would give you? I really hope this isn't something you learn at school and I've just forgotten how to work it out haha.

I realise its a game of diminishing returns with every change once you hit a certain volume because you're taking out a good chunk of the new fluid just to get a small amount of the old out. I also realise this isn't the best method. Again, I know it doesn't matter and I should just do it 3 or 4 times and be done with it. Not the point I'm trying to get at though. I'd like to know the maths for one, and satisfy my curiosity second. Only one guy in the office has worked it out so far but we dunno if it's correct. Be a nice ego boost for him if he's right :icon_wink:

By the way, my pump was making a horrendous noise from cold. Bought a new belt, but the one ECP supplied was 5mm too short and no amount of levering while turning pulley was gonna get that on. So I put the old on because it was visibly perfect and re-tightened. Performed the above method on the fluid and now its squeal free. Happy boy again :)

This is just for fun :)
 

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Registered
2008 GRB 20th anniversary 6 MT
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6,575 Posts
I am not a mathematician but maths was my favourite subject.
Initially we have 650ml of fluid, 250ml of which have been replaced with fresh fluid.
The volumetric distribution after the first change is V1F=250/650=0.3846

On the 2nd change we remove another 250ml of fluid to replace with fresh.
From the 400ml of fluid that is left in the system 38.46% of it is fresh from the first change, i.e. 400ml x 0.3846 = 153.85ml in the system are fresh from the 1st change + 250ml of fresh fluid from the second change leaves us with 403.85ml of fresh fluid after the second change.
The volumetric distribution after the second change is V2F=403.85/650=0.62

On the 3rd change we remove another 250ml of fluid to replace with fresh.
From the 400ml of fluid remaining in the system from the 1st and 2nd change, (400ml x 0.62) = 248ml are fresh.
The volumetric distribution of fresh fluid after the 3rd change is V3F=(248+250)/650=0.77

On the 4th change we remove another 250ml of fluid from the system to replace with new.
From the 400ml of fluid remaining in the system from the 1st, 2nd and 3rd changes, 400ml x 0.76 = 304ml are fresh.
The new volumetric distribution of fresh fluid after the 4th change is V4F=(304+250)/650=0.85

If we define as Vt the total volume of fluid in the whole system (reservoir+pipes), R the volume of fluid in the reservoir and Vp the volume of fluid in the pipes we have: Vt=R+Vp

We then have the following volumetric distributions after every change :

V1F = R/Vt = 250/650 = 0.3846
V2F = [(V1F x Vp) + R] / Vt = [(R/Vt x Vp + R] / Vt = R(Vp+Vt)/(Vt2 = [ (0.3846 x 400) + 250 ] / 650 = 0.62
V3F = [(V2F x Vp) + R] / Vt = [R(Vp + Vt) x Vp + R x Vt2] / Vt3 = [ (0.62 x 400) +250 ] / 650 = 0.77
V4F= [ (V3F x Vp) + R ] / Vt = 0.85 i.e. after 4 changes and using 1lt of fresh fluid you will only have replenished 85% of fluid with fresh in the system.

and so on

where Vt2= Vt square, Vt3=Vt cube
P.S. I hate writing maths in a word editor
 

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Premium Member
2015 Highlander AWD XLE 6AT
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4,255 Posts
A shortcut, if you will, is as follows:

400 mL "old" / 650 ml total = 61.5% "old" after change #1
61.5% x 61.5% = 37.8% "old" after change #2
61.5% ^ 3 = 23.3% "old" after change #3
61.5% ^ 4 = 14.3% "old" after change #4
61.5% ^ 5 = 8.8% "old" after change #5​

HTH,
Jim / crewzer
 

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Emeritus Forum Staff
2015 Triumph Tiger XCx 1 Down. 5 Up.
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5,242 Posts
Sounds like a bad way to do a power steering change to me.
 

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Don, King of the parts diagram
MY11 WRX STi Sedan 6MT
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5,910 Posts
Sounds like a bad way to do a power steering change to me.
The easiest way :icon_wink:

As above 3 or 4 refills and its almost as good as new with no bleeding to be done etc.
 

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Premium Member
06 2.5 XT MT
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242 Posts
Discussion Starter · #7 ·
Ah thanks for the responses! I appreciate the time you took to work it out, and type it up so I could understand it!

I've already done three partial changes, problem is gone, fluid is much more cherryade than ribena. The fluid and procedure is so cheap that I'm going to pick up another litre, do a couple more and be thoroughly satisfied that I'm better off than I was before.

Thanks again guys
 

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Premium Member
06 2.5 XT MT
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242 Posts
Discussion Starter · #8 ·
The easiest way :icon_wink:

As above 3 or 4 refills and its almost as good as new with no bleeding to be done etc.
Yeah I've read plenty of horror stories of introducing air into the system, cant get rid of bubbles and people wishing they'd never started. At the moment I dont have the time to get involved in that and this is pretty much the most highly recommended way to tackle it. Works for me :cool:
 

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Registered
2004 looking 6 MT
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262 Posts
OMG I'm getting flashbacks of quadratic equations and OTT algebraic sums from school that i have never had to use since even though i'm an electrical engineer by trade.
Going to sit in a darkened room now to recover.phew
 

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Registered
2008 GRB 20th anniversary 6 MT
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6,575 Posts
A shortcut, if you will, is as follows:

400 mL "old" / 650 ml total = 61.5% "old" after change #1
61.5% x 61.5% = 37.8% "old" after change #2
61.5% ^ 3 = 23.3% "old" after change #3
61.5% ^ 4 = 14.3% "old" after change #4
61.5% ^ 5 = 8.8% "old" after change #5​

HTH,
Jim / crewzer
Nice one, I know I always like to think complicated!
 

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Registered
2004 Forester XT manual
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247 Posts
My personal experience is that after popping the low pressure hose off the rack, letting it drain (going inside to have lunch while it drains out), refilling, and working the steering *lock to lock* with the engine off for a dozen cycles or so, and then topping off the reservoir. Doing the same with the engine running, next, I was able to get all the air out, and I didn't go through 4 liters of fluid in the process.

I have a surplus shop near home, too, where I can occasionally get large 150-200ml 'cardiac' syringes that make removing the old fluid from the reservoir a snap.
 

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Premium Member
06 2.5 XT MT
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242 Posts
Discussion Starter · #12 ·
I didn't go through 4 liters of fluid in the process.
Congratulations. I didn't either :icon_wink: but seriously, you might be surprised how many people don't find it so easy. I'm strapped for time and don't have the inclination to try to resolve any problem that might crop up while this was such an easy thing to try first.

Glad I got my answer before it turned into "you're doing it wrong thread". I said in post #1 it wasn't the best method, might be a bit of a bodge, but its worked for many and it worked for me :icon_cool:
 

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Emeritus Forum Staff
2015 Triumph Tiger XCx 1 Down. 5 Up.
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5,242 Posts
The easiest way :icon_wink:

As above 3 or 4 refills and its almost as good as new with no bleeding to be done etc.
I'll rephrase... sounds like a long winded way to change the fluid :biggrin:
 

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2015 Fiat van Man stick
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4,808 Posts
Pay a nice man to do it.

Even if you did it bit by bit like you say the reverse will also happen. The bit you leave in will still wcontinue to degrade so at first 10% or so of the fluid is shot, then 20, 30 and so on. Before you know it the fluids being changed all the time and you pumps wrecked.

Do it once and do it well. You'd not change your engine oil this way would you?
 

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Registered
2008 GRB 20th anniversary 6 MT
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6,575 Posts
Btw, how often does the power steering fluid need changing?
I don't remember it being listed in the service manual.
 

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Don, King of the parts diagram
MY11 WRX STi Sedan 6MT
Joined
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5,910 Posts
Btw, how often does the power steering fluid need changing?
I don't remember it being listed in the service manual.
Did it once in 8 years/120K when it started to look a bit tired - colour wise.
 

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Premium Member
06 2.5 XT MT
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242 Posts
Discussion Starter · #18 ·
Pay a nice man to do it.

Even if you did it bit by bit like you say the reverse will also happen. The bit you leave in will still wcontinue to degrade so at first 10% or so of the fluid is shot, then 20, 30 and so on. Before you know it the fluids being changed all the time and you pumps wrecked.

Do it once and do it well. You'd not change your engine oil this way would you?
Fair point about the 10% still in there. You wouldn't see a degradation of 20% then 30% etc though. That new fluid has been put in there within minutes/hours/maybe a day max. That 90% would all go bad at the pretty much the same time so you wont forever be changing fluid.

I do wonder how much oil you can remove from the system though. Can you ever remove 100% of the old? I know that the turkey baster I used has been standing upright and its still brown on the inside. I guarantee the pipes will always hang on to this old too. Same goes for any nooks and crannies within the pump. I doubt a change at a garage does anything more than a gravity drain, and probably leaves it all unhooked for less time than "doing it right" would even entail. I could be wrong though. Maybe they do an air flush through the whole lot. But then you'd be running the pump dry while you empty the crap from in there too? Which might not even matter since its belt driven...I dunno...but wet pumps should be lubricated in my mind. I'm happy with how its gone so far.
 

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Emeritus Forum Staff
2015 Triumph Tiger XCx 1 Down. 5 Up.
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5,242 Posts
10% old + 90% new is better than 100% old.
 

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2006 STI
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6,971 Posts
Btw, how often does the power steering fluid need changing?
I don't remember it being listed in the service manual.
Dan's got a nifty little tester thing I think that lets you know. Sounds like the sort of thing you could do with in your tool box Fivos... assuage your nervousness. :biggrin:
 
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